CHAPTER IX.

THE DEVELOPMENT OF THE ORGANS IN THE HEAD.

_The Development of the Brain._

_General History._ In stage G the brain presents a very simple constitution (Pl. 8, fig. G), and is in fact little more than a dilated termination to the cerebro-spinal axis. Its length is nearly one-third that of the whole body, being proportionately very much greater than in the adult.

It is divided by very slight constrictions into three lobes, the posterior of which is considerably the largest. These are known as the fore-brain, the mid-brain, and the hind-brain. The anterior part of the brain is bent slightly downwards about an axis passing through the mid-brain. The walls of the brain, composed of several rows of elongated columnar cells, have a fairly uniform thickness, and even the roof of the hind-brain is as thick as any other part. Towards the end of stage G the section of the hind-brain becomes somewhat triangular with the apex of the triangle directed downwards.

In Pristiurus during stage H no very important changes take place in the constitution of the brain. In Scyllium, however, indications appear in the hind-brain of its future division into a cerebellum and medulla oblongata. The cavity of the anterior part dilates and becomes rounded, while that of the posterior part assumes in section an hour-glass shape, owing to an increase in the thickness of the lateral parts of the walls. At the same time the place of the original thick roof is taken by a very thin layer, which is formed not so much through a change in the character and arrangements of the cells composing the roof, as by a divarication of the two sides of the hind-brain, and the simultaneous introduction of a fresh structure in the form of a thin sheet of cells connecting dorsally the diverging lateral halves of this part of the brain. By stage I, the hind-brain in Pristiurus also acquires an hour-glass shaped section, but the roof has hardly begun to thin out (Pl. 15, figs. 4_a_ and 4_b_).

During stages I and K the cranial flexure becomes more and more pronounced, and causes the mid-brain definitely to form the termination of the long axis of the embryo (Pl. 15, figs. 1, 2, etc.), and before the close of stage K a thin coating of white matter has appeared on the exterior of the whole brain, but no other histological changes of interest have occurred.

During stage L an apparent rectification of the cranial flexure commences, and is completed by stage Q. The changes involved in this process may be advantageously studied by comparing the longitudinal sections of the brain during stages L, P, and Q, represented in Pl. 16, figs. 1_a_, 5 and 7_a_.

It will be seen, first of all, that so far from the flexure of the brain itself being diminished, it is increased, and in P (fig. 5) the angle in the floor of the mid-brain becomes very acute indeed; in other words, the anterior part of the brain has been bent upon the posterior through nearly two right angles, and the infundibulum, or primitive front end of the brain, now points nearly directly backwards. At the same time the cerebral hemispheres have grown directly forwards, and if figures 1_a_ and 5 in Pl. 16 be compared it will be seen that in the older brain of the two the cerebral hemispheres have assumed a position which might be looked on as the result of their having been pushed dorsalwards and forwards against the mid-brain, and having in the process pressed in and nearly obliterated the original thalamencephalon. The thalamencephalon in fig. 1_a_, belonging to stage L, is relatively large, but in fig. 5, belonging to stage P, it only occupies a very small space between the front wall of the mid-brain and the hind wall of the cerebral hemispheres. It is therefore in part by the change in position of the cerebral hemispheres that the angle between the trabeculæ and parachordals becomes increased, _i.e._ their flexure _diminished_, while at the same time the flexure of the brain itself is _increased_. More important perhaps in the apparent rectification of the cranial flexure than any of the previously mentioned points, is the appearance of a bend in the hind-brain which tends to correct the original cranial flexure. The gradual growth of this fresh flexure can be studied in the longitudinal sections which have been represented. It is at its maximum in stage Q. This short preliminary sketch of the development of the brain as a whole will serve as an introduction to the history of the individual divisions of the brain.

_Fore-brain._ In its earliest condition the fore-brain forms a single vesicle without a trace of separate divisions, but buds off very early the optic vesicles, whose history is described with that of the eye (Pl. 15, fig. 3, _op.v_). Between stages I and K the posterior part of the fore-brain sends outwards a papilliform process towards the exterior, which forms the rudiment of the pineal gland (Pl. 15, fig. 1, _pn_). Immediately in front of the rudiment a constriction appears, causing a division of the fore-brain into a large anterior and a small posterior portion. This constriction is shallow at first, but towards the close of stage K becomes much deeper (Pl. 15, fig. 2 and fig. 16_a_), leaving however the two cavities of the two divisions of the fore-brain united ventrally by a somewhat wide canal.

The posterior of the two divisions of the fore-brain forms the thalamencephalon. Its anterior wall adjoining the cerebral rudiment becomes excessively thin (Pl. 15, fig. 11); and its base till the close of stage K is in close contact with the mouth involution, and presents but a very inconspicuous prominence which marks the eventual position of the infundibulum (Pl. 15, figs. 9_a_, 12, 16_a_, _in_). The anterior and larger division of the fore-brain forms the rudiment of the cerebral hemispheres and olfactory lobes. Up to stage K this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction into two lobes. From the canal uniting the two divisions of the fore-brain (which eventually forms part of the thalamencephalon) there spring the hollow optic nerves. A slight ventral constriction separating the cerebral rudiment from that part of the brain where these are attached appears even before the close of stage K (Pl. 15, fig. 11, _op.n_).

During stage L the infundibulum becomes much produced, and forms a wide sack in contact with the pituitary body, and its cavity communicates with that of the third ventricle by an elongated slit-like aperture. This may be seen by comparing Pl. 16, figs. 1_a_ and 1_c_. In fig. 1_c_ taken along the middle line, there is present a long opening into the infundibulum (_in_), which is shewn to be very narrow by being no longer present in fig. 1_a_ representing a section slightly to one side of the middle line. During the same stage the pineal gland grows into a sack-like body, springing from the roof of the thalamencephalon, fig. 1_b_, _pn_. This latter (the thalamencephalon) is now dorsally separated from the cerebral rudiment by a deep constriction, and also ventrally by a less well marked constriction. At its side also a deep constriction is being formed in it, immediately behind the pineal gland. The cerebral rudiment is still quite unpaired and exhibits no sign of becoming constricted into two lobes.

During the next two stages the changes in the fore-brain are of no great importance, and I pass at once to stage O. The infundibulum is now nearly in the same condition as during stage L, though (as is well shewn in the figure of a longitudinal section of the next stage) it points more directly backwards than before. The remaining parts of the thalamencephalon have however undergone considerable changes. The more important of these are illustrated by a section of stage O, Pl. 16, fig. 3, transverse to the long axis of the embryo, and therefore, owing to the cranial flexure, cutting the thalamencephalon longitudinally and horizontally; and for stage P in a longitudinal and vertical section through the brain (Pl. 16, fig. 5). In the first place the roof of the thalamencephalon has become very much shortened by the approximation of the cerebral rudiment to the mid-brain. The pineal sack has also become greatly elongated, and its somewhat dilated extremity is situated between the cerebral rudiment and the external skin. It opens into the hind end of the third ventricle, and its posterior wall is continuous with the front wall of the mid-brain. The sides of the thalamencephalon have become much thickened, and form distinct optic thalami (_op._) united by a very well marked posterior commissure (_pc._). The anterior wall of the thalamencephalon as well as its roof are very thin. The optic nerves have become by stage O quite solid except at their roots, into which the ventricles of the fore-brain are for a short distance prolonged. This solidification is arrived at, so far as I have determined, without the intervention of a fold. The nerves are fibrous, and a commencement of the chiasma is certainly present. From the chiasma there appears to pass out on each side a band of fibres, which runs near the outer surface of the brain to the base of the optic lobes (mid-brain), and here the fibres of the two sides again cross.

By stage O important changes are perceptible in the cerebral rudiment. In the first place there has appeared a slight fold at its anterior extremity (Pl. 16, fig. 3, _x_), destined to form a vertical septum dividing it into two hemispheres, and secondly, lateral outgrowths (vide Pl. 16, fig. 2, _ol.l_), to form the olfactory lobes. Its thin posterior wall presents on each side a fold which projects into the central cavity. From the peripheral end of each olfactory lobe a nerve similar in its histological constitution to any other cranial nerve makes its appearance (Pl. 16, fig. 2); this divides into a number of branches, one of which passes into the connective tissue between the two layers of epithelium in each Schneiderian fold. On the root of this nerve there is a large development of ganglionic cells. I have not definitely observed its origin, but have no reason to doubt that it is a direct outgrowth from the olfactory lobe, exactly similar _in its mode of development_ to any other nerve of the body.

The cerebral rudiment undergoes great changes during stage P. In addition to a great increase in the thickness of its walls, the fold which appeared in the last stage has grown backwards, and now divides it in front into two lobes, the rudiments of the cerebral hemispheres. The greater and posterior section is still however quite undivided, and the cavities of the lobes (lateral ventricles) though separated in front are still quite continuous behind. At the same time, the olfactory lobes, each containing a prolongation of the ventricle, have become much more pronounced (vide Pl. 16, figs. 4_a_ and 4_c_, _ol.l_). The root of the olfactory nerve is now very thick, and the ganglion cells it contains are directly prolonged into the ganglionic portion of the olfactory bulb; in consequence of which it becomes rather difficult to fix on the exact line of demarcation between the bulb and the nerve.

Stage Q is the latest period in which I have investigated the development of the brain. Its structure is represented for this stage in general view in Pl. 16, figs. 6_a_, 6_b_, 6_c_, in longitudinal section in Pl. 16, figs. 7_a_, 7_b_, and in transverse section Pl. 16, figs. 8_a-d_. The transverse sections are taken from a somewhat older embryo than the longitudinal. In the thalamencephalon there is no fresh point of great importance to be noticed. The pineal gland remains as before, and has become, if anything, longer than it was, and extends further forwards over the summit of the cerebrum. It is situated, as might be expected, in the connective tissue within the cranial cavity (fig. 8_a_, _pn_), and does not extend outside the skull, as it appears to do, according to Götte's investigations, in Amphibians. Götte[269] compares the pineal gland with the long persisting pore which leads into the cavity of the brain in the embryo of Amphioxus, and we might add the Ascidians, and calls it "ein Umbildungsprodukt einer letzten Verbindung des Hirns mit der Oberhaut." This suggestion appears to me a very good one, though no facts have come under my notice which confirm it. The sacci vasculosi are perhaps indicated at this stage in the two lateral divisions of the trilobed ventricle of the infundibulum (fig. 8_c_).

Footnote 269: _Ent. d. Unke_, p. 304.

The lateral ventricles (fig. 8_a_) are now quite separated by a median partition, and a slight external constriction marks the lobes of the two hemispheres; these, however, are still united by nervous structures for the greater part of their extent. The olfactory lobes are formed of a distinct bulb and stalk (fig. 8_a_, _ol.l_), and contain, as before, prolongations of the lateral ventricles. The so-called optic chiasma is very distinct (fig. 8_b_, _op.n_), but the fibres from the optic nerves appear to me simply to cross and not to intermingle.

_The mid-brain._ The mid-brain is at first fairly marked off from both the fore and hind brains, but less conspicuously from the latter than from the former. Its roof becomes progressively thinner and its sides thicker up to stage P, its cavity remaining quite simple. The thinness of the roof gives it, in isolated brains of stage P, a bilobed appearance (vide Pl. 16, fig. 4_b_, _mb_, in which the distinctness of this character is by no means exaggerated): During stage Q it becomes really bilobed through the formation in its roof of a shallow median furrow (Pl. 16, fig. 8_b_). Its cavity exhibits at the same time the indication of a division into a central and two lateral parts.

_The hind-brain._ The hind-brain has at first a fairly uniform structure, but by the close of stage I, the anterior part becomes distinguished from the remainder by the fact, that its roof does not become thin as does that of the posterior part. This anterior, and _at first very insignificant portion_, forms the rudiment of the cerebellum. Its cavity is quite simple and is continued uninterruptedly into that of the remainder of the hind-brain. The cerebellum assumes in the course of development a greater and greater prominence, and eventually at the close of stage Q overlaps both the optic lobes in front and the medulla behind (Pl. 16, fig. 7_a_). It exhibits in surface-views of the hardened brain of stages P and Q the appearance of a median constriction, and the portion of the ventricle contained in it is prolonged into two lateral outgrowths (Pl. 16, figs. 8_c_ and 8_d_, _cb_).

The posterior section of the hind-brain which forms the medulla undergoes changes of a somewhat complicated character. In the first place its roof becomes in front very much extended and thinned out. At the raphe, where the two lateral halves of the brain originally united, a separation, as it were, takes place, and the two sides of the brain become pushed apart, remaining united by only a very thin layer of nervous matter (Pl. 15, fig. 6, _iv.v._). As a result of this peculiar growth in the brain, the roots of the nerves of the two sides which were originally in contact at the dorsal summit of the brain become carried away from one another, and appear to rise at the sides of the brain (Pl. 15, figs. 6 and 7). Other changes also take place in the walls of the brain. Each lateral wall presents two projections towards the interior (Pl. 15, fig. 5_a_). The ventral of these vanish, and the dorsal approximate so as nearly to divide the cavity of the hind-brain, or fourth ventricle, into a large dorsal and a small ventral channel (Pl. 15, fig. 6), and this latter becomes completely obliterated in the later stages. The dorsal pair, while approximating, also become more prominent, and stretch into the dorsal moiety of the fourth ventricle (Pl. 15, fig. 6). They are still very prominent at stage Q (Pl. 16, fig. 8_d_, _ft_), and correspond in position with the fasciculi teretes of human anatomy. Part of the root of the seventh nerve originates from them. They project freely in front into the cavity of the fourth ventricle (Pl. 16, fig. 7_a_, _ft_).

By stage Q restiform tracts are indistinctly marked off from the remainder of the brain, and are anteriorly continued into the cerebellum, of which they form the peduncles. Near their junction with the cerebellum they form prominent bodies (Pl. 16, fig. 7_a_, _rt_), which are regarded by Miklucho-Maclay[270] as representing the true cerebellum.

Footnote 270: _Das Gehirn d. Selachier_, Leipzig, 1870.

By stage O the medulla presents posteriorly, projecting into its cavity, a series of lobes which correspond with the main roots (not the branches) of the vagus and glosso-pharyngeal nerves (Pl. 17, fig. 5). There appear to me to be present seven or eight projections: their number cannot however be quite certainly determined. The first of them belongs to the root of the glosso-pharyngeal, the next one is interposed between the glosso-pharyngeal and the first root of the vagus, and is without any corresponding nerve-root. The next five correspond to the five main roots of the vagus. For each projection to which a nerve pertains there is a special nucleus of nervous matter, from which the root springs. These nuclei do not stain like the remainder of the walls of the medulla, and stand out accordingly very conspicuously in stained sections.

The coating of white matter which appeared at the end of stage K, on the exterior of each lateral half of the hind-brain, extends from a point just dorsal to the attachment of the nerve-roots to the ventral edge of the medulla, and is specially connected with the tissue of the upper of the two already described projections into the fourth ventricle.

A rudiment of the tela vasculosa makes its appearance during stage Q, and is represented by the folds in the wall of the fourth ventricle in my figure of that stage (Pl. 16, fig. 7_a_, _tv_).

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The development of the brain in Elasmobranchii has already been worked out by Professor Huxley, and a brief but in many respects very complete account of it is given in his recent paper on Ceratodus[271]. He says, pp. 30 and 31, "The development of the cerebral hemispheres in Plagiostome Fishes differs from the process by which they arise in the higher Vertebrata. In a very early stage, when the first and second visceral clefts of the embryo Scyllium are provided with only a few short branchial filaments, the anterior cerebral vesicle is already distinctly divided into the thalamencephalon (from which the large infundibulum proceeds below, and the small tubular peduncle of the pineal gland above, while the optic nerve leaves its sides) and a large single oval vesicle of the hemispheres. On the ventral face of the integument covering these are two oval depressions, the rudimentary olfactory sacs.

Footnote 271: _Proceedings of the Zoological Society_, 1876, Pt. 1. pp. 30 and 31.

"As development proceeds the vesicle of the hemispheres becomes divided by the ingrowth of a median longitudinal septum, and the olfactory lobes grow out from the posterior lateral regions of each ventricle thus formed, and eventually rise on to the dorsal faces of the hemispheres, instead of, as in most Vertebrata, remaining on their ventral sides. I may remark, that I cannot accept the views of Miklucho-Maclay, whose proposal to alter the nomenclature of the parts of the Elasmobranch's brain, appears to me to be based upon a misinterpretation of the facts of development."

The last sentence of the paragraph brings me to the one part on which it is necessary to say a few words, viz. the views of Miklucho-Maclay. His views have not received any general acceptance, but the facts narrated in the preceding pages shew, beyond a doubt, that he has 'misinterpreted' the facts of development, and that the ordinary view of the homology of the parts is the correct one. A comparison of the figures I have given of the embryo brain with similar figures of the brain of higher Vertebrates shews this point conclusively. Miklucho-Maclay has been misled by the large size of the cerebellum, but, as we have seen, this body does not begin to be conspicuous till late in embryonic life. Amongst the features of the embryonic brain of Elasmobranchii, the long persisting unpaired condition of the cerebral hemisphere, upon which so much stress has already been laid by Professor Huxley, appears to me to be one of great importance, and may not improbably be regarded as a real ancestral feature. Some observations have recently been published by Professor B. G. Wilder[272] upon this point, and upon the homologies and development of the olfactory lobes. Fairly good figures are given to illustrate the development of the cerebral hemispheres, but the conclusions arrived at are in part opposed to my own results. Professor Wilder says: "The true hemispheres are the lateral masses, more or less completely fused in the middle line, and sometimes developing at the plane of union a bundle of longitudinal commissural fibres. The hemispheres retain their typical condition as anterior protrusions of the anterior vesicle; but they lie mesiad of the olfactory lobes, _and in Mustelus at least seem to be formed after them_." The italics are my own. From what has been said above, it is clear that the statement italicised, for Scyllium at least, completely reverses the order of development. Still more divergent from my conclusions are Professor Wilder's statements on the olfactory lobes. He says: "The true olfactory lobe, or rhinencephalon, seems, therefore, to embrace only the hollow base of the crus, more or less thickened, and more or less distinguishable from the main mass as a hollow process. The olfactory bulb, with the more or less elongated crus of many Plagiostomes, seems to be developed independently, or in connection with the olfactory sack, as are the general nerves;" and again, "But the young and adult brains since examined shew that the ventricle (_i.e._ the ventricle of the olfactory lobe) ends as a rounded cul-de-sac before reaching the 'lobe.'"

Footnote 272: "Anterior brain-mass with Sharks and Skates," _American Journal of Science and Arts_, Vol. XII. 1876.

The majority of the statements contained in the above quotations are not borne out by my observations. Even the few preparations of which I have given figures, appear to me to prove that (1) the olfactory lobes (crura and bulbs) are direct outgrowths from the cerebral rudiment, and develop quite independently of the olfactory sack; (2) that the ventricle of the cerebral rudiment does not stop short at the base of the crus; (3) that from the bulb a nerve grows out which has a centrifugal growth like other nerves of the body, and places the central olfactory lobe in communication with the peripheral olfactory sack. In some other Vertebrates this nerve seems hardly to be developed, but it is easily intelligible, that if in the ordinary course of growth the olfactory sack became approximated to the olfactory lobe, the nerve which grew out from the latter to the sack might become so short as to escape detection.

_Organs of Sense._

_The olfactory organ._ The olfactory pit is the latest formed of the three organs of special sense. It appears during a stage intermediate between _I_ and _K_, as a pair of slight thickenings of the external epiblast, in the normal vertebrate position on the under side of the fore-brain immediately in front of the mouth (Pl. 15, figs. 1 and 2, _ol_).

The epiblast cells which form this thickening are very columnar, but present no special peculiarities. Each thickened patch of skin soon becomes involuted as a shallow pit, which remains in this condition till the close of the stage _K_. The epithelium very early becomes raised into a series of folds (Schneiderian folds). These are bilaterally symmetrical, and diverge like the barbs of a feather from a median line (Pl. 15, fig. 14). The nasal pits at the close of stage K are still separated by a considerable interval from the walls of the brain, and no rudiment of an olfactory lobe arises till a later period; but a description of the development of this as an integral part of the brain has already been given, p. 401.

_Eye._ The eye does not present in its early development any very special features of interest. The optic vesicles arise as hollow outgrowths from the base of the fore-brain (Pl. 15, fig. 3, _op.v_), from which they soon become partially constricted, and form vesicles united to the base of the brain by comparatively narrow hollow stalks, the rudiments of the optic nerves. The constriction to which the stalk or optic nerve is due takes place from above and backwards, so that the optic nerves open into the base of the front part of the thalamencephalon (Pl. 15, fig. 13_a_, _op.n_). After the establishment of the optic nerves, there take place the formation of the lens and the pushing in of the anterior wall of the optic vesicle towards the posterior.

The lens arises in the usual vertebrate fashion. The epiblast in front of the optic vesicle becomes very much thickened, and then involuted as a shallow pit, which eventually deepens and narrows. The walls of the pit are soon constricted off as a nearly spherical mass of cells enclosing a very small central cavity, in some cases indeed so small as to be barely recognizable (Pl. 15, fig. 7, _l_). The pushing in of the anterior wall of the optic vesicle towards the posterior takes place in quite the normal manner; but, as has been already noticed by Götte[273] and others, is not a simple mechanical result of the formation of the lens, as is shewn by the fact that the vesicle assumes a flattened form even before the appearance of the lens. The whole exterior of the optic cup becomes invested by mesoblast, but _no mesoblastic cells grow in between the lens and the adjoining wall of the optic cup_.

Footnote 273: _Entwicklungsgeschichte d. Unke._

Round the exterior of the lens, and around the exterior and interior of the optic cup, there appear membrane-like structures, similar to those already described round the spinal cord and other organs. These membrane-like structures appear with a varying distinctness, but at the close of stage _K_ stand out with such remarkable clearness as to leave no doubt that they are not artificial products (Pl. 15, fig. 13_a_)[274]. They form the rudiments of the hyaloid membrane and lens capsule. Similar, though less well marked membranes, may often be seen lining the central cavity of the lens and the space between the two walls of the optic cup. The optic cup is at first very shallow, but owing to the rapid growth of the free edge of its walls soon becomes fairly deep. The growth extends to the whole circumference of the walls except the point of entrance of the optic nerve (Pl. 15, fig. 13_a_), where no growth takes place; here accordingly a gap is left in the walls which forms the well-known choroid slit. While this double walled cup is increasing in size, the wall lining the cavity of the cup becomes thick, and the outer wall very thin (fig. 13_a_). No further differentiations arise before the close of stage K.

Footnote 274: The engraver has not been very successful in rendering these membranes.

The lens is carried outwards with the growth of the optic cup, leaving the cavity of the cup quite empty. It also grows in size, and its central cavity becomes larger. Still later its anterior wall becomes very thin, and its posterior wall thick, and doubly convex (fig. 13_a_). Its changes, however, so exactly correspond to those already known in other Vertebrates, that a detailed description of them would be superfluous.

_No mesoblast passes into the optic cup round its edge_, but a process of mesoblast, accompanied by a blood-vessel, passes into the space between the lens and the wall of the optic cup through the choroid slit (fig. 13_a_, _ch_). This process of tissue is very easily seen, and swells out on entering the optic cup into a mushroom-like expansion. It forms the processus falciformis, and from it is derived the vitreous humour.

About the development of the parts of the eye, subsequently to stage _K_, I shall not say much. The iris appears during stage O, as an ingrowing fold of both layers of the optic cup with a layer of mesoblast on its outer surface, which tends to close over the front of the lens. Both the epiblast layers comprising the iris are somewhat atrophied, and the outer one is strongly pigmented. At stage O the mesoblast first also grows in between the external skin and the lens to form the rudiment of the mesoblastic structures of the eye in front of the lens. The layer, when first formed, is of a great tenuity.

The points in my observations, to which I attach the greatest importance, are the formation of the lens capsule and the hyaloid membrane; with the development of these may be treated also that of the vitreous humour and rudimentary _processus falciformis_. The development of these parts in Elasmobranchii has recently been dealt with by Dr Bergmeister[275], and his observations with reference to the vitreous humour and processus falciformis, the discovery of which in embryo Elasmobranchii is due to him, are very complete. I cannot, however, accept his view that the hyaloid membrane is a mesoblastic product. Through the choroid slit there grows, as has been said, a process of mesoblast, the processus falciformis, which on entering the optic cup dilates, and therefore appears mushroom-shaped in section. At the earliest stage (K) a blood-vessel appeared in connection with it, but no vascular structure came under my notice in the later stages. The structure of this process during stage P is shewn in Pl. 17, fig. 6, _p.fal._; it is there seen to be composed of mesoblast-cells with fibrous prolongations. The cells, as has been noticed by Bergmeister, form a special border round its dilated extremity. This process is formed much earlier than the vitreous humour, which is first seen in stage O. In hardened specimens this latter appears either as a gelatinous mass with a meshwork of fibres or (as shewn in Pl. 17, fig. 6) with elongated fibres proceeding from the end of the processus falciformis. These fibres are probably a product of the hardening reagent, but perhaps represent some preformed structure in the vitreous humour. I have failed to detect in it any cellular elements. It is more or less firmly attached to the hyaloid membrane.

Footnote 275: "Embryologie d. Coloboms," _Sitz. d. k. Akad. Wien_, Bd. LXXI. 1875.

On each side of the processus falciformis in stage P a slight fold of the optic cup is to be seen, but folds so large as those represented by Bergmeister have never come under my notice, though this may be due to my not having cut sections of such late embryos as he has. The hyaloid membrane appears long before the vitreous humour as a delicate basement membrane round the inner surface of the optic cup (Pl. 15, fig. 13_a_), which is perfectly continuous with a similar membrane round the outer surface. In the course of development the hyaloid membrane becomes thicker than the membrane outside the optic cup, with which however it remains continuous. This is very clear in my sections of stage M. By stage O the membrane outside the cup has ceased to be distinguishable, but the hyaloid membrane may nevertheless be traced to the very edge of the cup round the developing iris; but does not unite with the lens capsule. It can also be traced quite to the junction of the two layers of the optic cup at the side of the choroid slit (Pl. 17, fig. 6, _hy.m_). When the vitreous humour becomes artificially separated from the retina, the hyaloid membrane sometimes remains attached to the former, but at other times retains in preference its attachment to the retina. My observations do not throw any light upon the junction of the hyaloid membrane and lens capsule to form the suspensory ligament, nor have I ever seen (as described by Bergmeister) the hyaloid membrane extending across the free end of the processus falciformis and separating the latter from the vitreous humour. This however probably appears at a period subsequent to the latest one investigated by me. The lens capsule arises at about the same period as the hyaloid membrane, and is a product of the cells of the lens. It can be very distinctly seen in all the stages subsequent to its first formation. The proof of its being a product of the epiblastic lens, and not of the mesoblast, lies mainly in the fact of there being no mesoblast at hand to give rise to it at the time of its formation, vide Pl. 15, fig. 13_a_. If the above observations are correct, it is clear that the hyaloid membrane and lens capsule are respectively products of the retina and lens; so that it becomes necessary to go back to the older views of Kölliker and others in preference to the more modern ones of Lieberkühn and Arnold. It would take me too far from my subject to discuss the arguments used by the later investigators to maintain their view that the hyaloid membrane and lens capsule are mesoblastic products; but it will suffice to say that the continuity of the hyaloid membrane over the pecten in birds is no conclusive argument against its retinal origin, considering the great amount of apparently independent growth which membranes, when once formed, are capable of exhibiting.

Bergmeister's and my own observations on the vitreous humour clearly prove that this is derived from an ingrowth through the choroid slit. On the other hand, the researches of Lieberkühn and Arnold on the Mammalian Eye appear to demonstrate that a layer of mesoblast becomes in Mammalia involuted with the lens, and from this the vitreous humour (including the _membrana capsulo-pupillaris_) is said to be in part formed. Lieberkühn states that in Birds the vitreous humour is formed in a similar fashion. I cannot, however, accept his results on this point. It appears, therefore, that, so far as is known, all groups of Vertebrata, with the exception of Mammalia, conform to the Elasmobranch type. The differences between the types of Mammalia and remaining Vertebrata are, however, not so great as might at first sight appear. They are merely dependent on slight differences in the manner in which the mesoblast enters the optic cup. In the one case it grows in round one specialized part of the edge of the cup, _i.e._ the choroid slit; in the other, round the whole edge, including the choroid slit. Perhaps the mode of formation of the vitreous humour in Mammalia may be correlated with the early closing of the choroid slit.

_Auditory Organ._ With reference to the development of the organ of hearing I have very little to say. Opposite the interval between the seventh and the glosso-pharyngeal nerves the external epiblast becomes thickened, and eventually involuted as a vesicle which remains however in communication with the exterior by a narrow duct. Towards the close of stage K the auditory sack presents three protuberances--one pointing forwards, a second backwards, and a third outwards. These are respectively the rudiments of the anterior and posterior vertical and external horizontal semicircular canals. These rudiments are easily visible from the exterior (Pl. 15, fig. 2).

* * * * *

As has been already pointed out, the epiblast of Elasmobranchii during the early periods of development exhibits no division into an epidermic and a nervous layer, and in accordance with its primitive undifferentiated condition, those portions of the organs of sense which are at this time directly derived from the external integument are formed indiscriminately from the whole, and not from an inner or so-called nervous part of it only. In the Amphibians the auditory sack and lens are derived from the nervous division of the epiblast only, while the same division of the layer plays the major part in forming the olfactory organ. It is also stated that in Birds and Mammals the part of the epiblast corresponding to the nervous layer is alone concerned in the formation of the lens, though this does not appear to be the case with the olfactory or auditory organs in these groups of Vertebrates.

_Mouth involution and Pituitary body._

The development of the mouth involution and the pituitary body is closely related to that of the brain, and may conveniently be dealt with here. The epiblast in the angle formed by the cranial flexure becomes involuted as a hollow process situated in close proximity to the base of the brain. This hollow process is the mouth involution, and it is bordered on its posterior surface by the front wall of the alimentary tract, and on its anterior by the base of the fore-brain.

The uppermost end of this does not till near the close of stage K become markedly constricted off from the remainder, but is nevertheless the rudiment of the pituitary body. Pl. 15, figs. 9_a_ and 12, _m_ shew in a most conclusive manner the correctness of the above account, and demonstrate that it is from the mouth involution, and not, as has usually been stated, from the alimentary canal, that the pituitary body is derived.

This fact was mentioned in my preliminary account of Elasmobranch development[276]; and has also been shewn to be the case in Amphibians by Götte[277]; and in Birds by Mihalkowics[278]. The fact is of considerable importance with reference to speculations as to the meaning of this body.

Footnote 276: _Quarterly Journal of Microscopic Science_, Oct. 1874.

Footnote 277: _Entwicklungsgeschichte der Unke._ Götte was the first to draw attention to this fact. His observations were then shewn to hold true for Elasmobranchii by myself, and subsequently for Birds by Mihalkowics.

Footnote 278: _Arch. f. micr. Anat._ Vol. XI.

Plate 15, fig. 7 represents a transverse section through the head during a stage between I and K; but, owing to the cranial flexure, it cuts the fore part of the head longitudinally and horizontally, and passes through both the fore-brain (_fb_) and the hind-brain (_iv.v._). Close to the base of the fore-brain are seen the mouth (_m_), and the pituitary involution from this (_pt._). In contact with the pituitary involution is the blind anterior termination of the throat, which a little way back opens to the exterior by the first visceral cleft (I. _v.c._). This figure alone suffices to demonstrate the correctness of the above account of the pituitary body; but the truth of this is still further confirmed by other figures on the same plate (figs. 9_a_ and 12, _m_); in which the mouth involution is in contact with, but still separated from, the front end of the alimentary tract. By the close of stage K, the septum between the mouth and throat becomes pierced, and the two are placed in communication. This condition is shewn in Pl. 15, fig. 16_a_, and Pl. 16, figs. 1_a_, 1_c_, _pt_ In these figures the pituitary involution has become very partially constricted off from the mouth involution, though still in direct communication with it. In later stages the pituitary involution becomes longer and dilated terminally, while the passage connecting it with the mouth becomes narrower and narrower, and is finally reduced to a solid cord, which in its turn disappears. The remaining vesicle then becomes divided into lobes, and connects itself closely with the infundibulum (Pl. 16, figs. 5 and 6 _pt_). The later stages for Elasmobranchii are fully described by W. Müller in his important memoir on the Comparative Anatomy and development of this organ[279].

Footnote 279: W. Müller, "Ueber Entwicklung and Bau d. Hypophysis u. d. Processus infundibuli cerebri," _Jenaische Zeitschrift_, Bd. VI.

_Development of the Cranial Nerves._

The present section deals with the whole development (so far as I have succeeded in elucidating it) of the cranial nerves (excluding the optic and olfactory nerves and the nerves of the eye-muscles) from their first appearance to their attainment of the adult condition. My description commences with the first development of the nerves, to this succeeds a short description of the nerves in the adult Scyllium, and the section is completed by an account of the gradual steps by which the adult condition is attained.

_Early Development of the Cranial Nerves._--Before the close of stage H the more important of the cranial nerves make their appearance. The fifth and the seventh are the first to be formed. The fifth arises by stage G (Pl. 15, fig. 3, V), near the anterior end of the hind-brain, as _an outgrowth from the extreme dorsal summit of the brain, in identically the same way as the dorsal root of a spinal nerve_.

The roots of the two sides sprout out from the summit of the brain, in contact with each other, and grow ventralwards, one on each side of the brain, in close contact with its walls. I have failed to detect more than one root for the two embryonic branches of the fifth (ophthalmic and mandibular), _and no trace of an anterior or ventral root has been met with in any of my sections_.

The seventh nerve is formed nearly simultaneously with or shortly after the fifth, and some little distance behind and independently of it, opposite the anterior end of the thickening of the epiblast to form the auditory involution. It arises precisely like the fifth, from the extreme dorsal summit of the neural axis (Pl. 15, fig. 4_a_, VII). So far as I have been able to determine, the auditory nerve and the seventh proper possess only a single root common to the two. There is no anterior root for the seventh any more than for the fifth.

Behind the auditory involution, at a stage subsequent to that in which the fifth and seventh nerves appear, there arise a series of roots from the dorsal summit of the hind-brain, which form the rudiments of the glosso-pharyngeal and vagus nerves. These roots are formed towards the close of stage H, but are still quite short at the beginning of stage I. Their manner of development resembles that of the previously described cranial nerves. The central ends of the roots of the opposite sides are at first in contact with each other, and there is nothing to distinguish the roots of the glosso-pharyngeal and of the vagus nerves from the dorsal roots of spinal nerves. Like the dorsal roots of the spinal nerves, they appear as a series of ventral prolongations of a continuous outgrowth from the brain, which outgrowth is moreover continuous with that for the spinal nerves[280]. The outgrowth of the vagus and glosso-pharyngeal nerves is not continuous with that of the seventh nerve. This is shewn by Pl. 15, figs. 4_a_ and 4_b_. The outgrowth of the seventh nerve though present in 4_a_ is completely absent in 4_b_ which represents a section just behind 4_a_.

Footnote 280: In the presence of this continuous outgrowth of the brain from which spring the separate nerve stems of the vagus, may perhaps be found a reconciliation of the apparently conflicting statements of Götte and myself with reference to the vagus nerve. Götte regards the vagus as a single nerve, from its originating as an undivided rudiment; but it is clear from my researches that, for Elasmobranchii at least, this method of arguing will not hold good, since it would lead to the conclusion that all the spinal nerves were branches of one single nerve, since they too spring as processes from a continuous outgrowth from the brain!

Thus, by the end of stage I, there have appeared the rudiments of the 5th, 7th, 8th, 9th and 10th cranial nerves, all of which spring from the hind-brain. These nerves all develop precisely as do the posterior roots of the spinal nerves, and it is a remarkable fact _that hitherto I have failed to find a trace in the brain of a root of any cranial nerve arising from the ventral corner of the brain as do the anterior roots of the spinal nerves_[281].

Footnote 281: The conclusion here arrived at with reference to the anterior roots, is opposed to the observations of both Gegenbaur on Hexanchus, _Jenaische Zeitschrift_, Vol. VI., and of Jackson and Clarke on Echinorhinus, _Journal of Anatomy and Physiology_, Vol. X. These morphologists identify certain roots springing from the medulla below and behind the main roots of the vagus as true anterior roots of this nerve. The existence of these roots is not open to question, but without asserting that it is impossible for me to have failed to detect such roots had they been present in the embryo, I think I may maintain if these anterior roots are not present in the embryo, their identification as vagus roots must be abandoned; and they must be regarded as belonging to spinal nerves. This point is more fully spoken of at p. 428.

It is admittedly difficult to prove a negative, and it may still turn out that there are anterior roots of the brain similar to those of the spinal cord; in the mean time, however, the balance of evidence is in favour of there being none such. This at first sight appears a somewhat startling conclusion, but a little consideration shews that it is not seriously opposed to the facts which we know. In the first place it has been shewn by myself[282] that in Amphioxus (whose vertebrate nature I cannot doubt) only dorsal nerve-roots are present. Yet the nerves of Amphioxus are clearly mixed motor and sensory nerves, and it appears to me far more probable that Amphioxus represents a phase of development in which the nerves had not acquired two roots, rather than one in which the anterior root has been lost. In other words, the condition of the nerves in Amphioxus appears to me to point to the conclusion _that primitively the cranio-spinal nerves of vertebrates were nerves of mixed function with one root only, and that root a dorsal one; and that the present anterior or ventral root is a secondary acquisition_. This conclusion is further supported by the fact that the posterior roots develop in point of time before the anterior roots. If it be admitted that the vertebrate nerves primitively had only a single root, then the retention of that condition in the brain implies that this became differentiated from the remainder of the nervous system at a very early period before the acquirement of anterior nerve-roots, and that these eventually become developed only in the case of spinal nerves, and not in the case of the already highly modified cranial nerves.

Footnote 282: _Journal of Anatomy and Physiology_, Vol. X. [This Edition, No. IX.]

* * * * *

_Subsequent Changes of the Nerves._ To simplify my description of the subsequent growth of the cranial nerves, I have inserted a short description of their distribution in the adult. This is taken from a dissection of Scyllium stellare, which like other species has some individualities of its own not found in the other Elasmobranchii. For points not touched on in this description I must refer the reader to the more detailed accounts of my predecessors, amongst whom may specially be mentioned Stannius[283] for Carcharias, Spinax, Raja, Chimæra, &c.; Gegenbaur[284] for Hexanchus; Jackson and Clarke[285] for Echinorhinus.

Footnote 283: _Nervensystem d. Fische_, Rostock, 1849.

Footnote 284: _Jenaische Zeitschrift_, Vol. VI.

Footnote 285: _Journal of Anatomy and Physiology_, Vol. X.

The ordinary nomenclature has been employed for the branches of the fifth and seventh nerves, though embryological data to be adduced in the sequel throw serious doubts upon it. Since I am without observations on the origin of the nerves to the muscles of the eyes, all account of these is omitted.

The fifth nerve arises from the brain by three roots[286]: (1) an anterior more or less ventral root; (2) a root slightly behind, but close to the former[287], formed by the coalescence of two distinct strands, one arising from a dorsal part of the medulla, and a second and larger from the ventral; (3) a dorsal and posterior root, in its origin quite distinct and well separated from the other two, and situated slightly behind the dorsal strand of the second root. This root a little way from its attachment becomes enclosed for a short distance in the same sheath as the dorsal part of the second root, and a slight mixture of fibres seems to occur, but the majority of its fibres have no connection with those of the second root. The first and second roots of the fifth appear to me partially to unite, but before their junction the ramus ophthalmicus profundus is given off from the first of them.

Footnote 286: My results with reference to these roots accord exactly, so far as they go, with the more carefully worked out conclusions of Stannius, _loc. cit._ pp. 29 and 30.

Footnote 287: The root of the seventh nerve cannot properly be distinguished from this root.

The fifth nerve, according to the usual nomenclature, has three main divisions. The first of these is the ophthalmic. It is formed by the coalescence of two entirely independent branches of the fifth, which unite on leaving the orbit. The dorsalmost of these, or ramus ophthalmicus superficialis, originates from the third and posterior of the roots of the fifth, nearly the whole of which appears to enter into its formation. This root is situated on the dorsal part of the "lobi trigemini," _at a point posterior to that of the other roots of the fifth or even of the seventh nerve_. The branch itself enters the orbit by a separate foramen, and, keeping on the dorsal side of it, reenters the cartilage at its anterior wall, and is there joined by the _ramus ophthalmicus profundus_. This latter nerve arises from the anterior root of the fifth, separately pierces the wall of the orbit, and takes a course slightly ventral to the superior ophthalmic nerve, but does not (as is usual with Elasmobranchii) run below the superior rectus and superior oblique muscles of the eye. The nerve formed by the coalescence of the superficial and deep ophthalmic branches courses a short way below the surface, and supplies the mucous canals of the front of the snout. It is a purely sensory nerve. Strong grounds will be adduced in the sequel for regarding the _ramus ophthalmicus superficialis_, though not the _ophthalmicus profundus_, as in reality a branch of the seventh, and not of the fifth nerve.

The second division of the fifth nerve is the superior maxillary, which appears to me to arise from both the first and second roots of the fifth, though mainly from the first. It divides once into two main branches. The first of these--the buccal nerve of Stannius--after passing forwards along the base of the orbit takes its course obliquely across the palatine arch and behind and below the nasal sack, supplying by the way numerous mucous canals, and dividing at last into two branches, one of these passing directly forwards on the ventral surface of the snout, and the second keeping along the front border of the mouth. The second division of the superior maxillary nerve (superior maxillary of Stannius), after giving off a small branch, which passes backwards in company with a branch from the inferior maxillary nerve to the levator maxillæ superioris, itself keeps close to the buccal nerve, and eventually divides into numerous fine twigs to the mucous canals of the skin at the posterior region of the upper jaw. It anastomoses with the buccal nerve. The inferior maxillary nerve arises mainly from the second root of the fifth. After sending a small branch to the levator maxillæ superioris, it passes outwards along the line separating the musculus adductor mandibulæ from the musculus levator labii superioris, and after giving branches to these muscles takes a course forward along the border of the lower jaw. It appears to be a mixed motor and sensory nerve.

The seventh or facial nerve arises by a root close to, but behind and below the second root of the fifth, and is intimately fused with this. It divides almost at once into a small anterior branch and large posterior.

The anterior branch is the palatine nerve. It gives off at first one or two very small twigs, which pursue a course towards the spiracle, and probably represent the spiracular nerves of other Elasmobranchii. Immediately after giving off these branches it divides into two stems, a posterior smaller and an anterior larger one. The former eventually takes a course which tends towards the angle of the jaw, and is distributed to the mucous membrane of the roof of the mouth, while the larger one bends forwards and supplies the mucous membrane at the edge of the upper jaw. The main stem of the seventh, after giving off a branch to the dorsal section of the musculus constrictor superficialis, passes outwards to the junction of the upper and lower jaws, where it divides into two branches, an anterior superficial branch, which runs immediately below the skin on the surface of the lower jaw, and a second branch, which takes a deep course along the posterior border of the lower jaw, between it and the hyoid, and sends a series of branches backwards to the ventral section of the musculus constrictor superficialis. The main stem of the facial is mixed motor and sensory. I have not noticed a dorsal branch, similar to that described by Jackson and Clarke.

The auditory nerve arises immediately behind the seventh, but requires no special notice here. A short way behind the auditory is situated the root of the glossopharyngeal nerve. This nerve takes an oblique course backwards through the skull, and gives off in its passage a very small dorsal branch, which passes upwards and backwards through the cartilage towards the roof of the skull. At the point where the main stem leaves the cartilage it divides into two branches, an anterior smaller branch to the hinder border of the hyoid arch, and a posterior and larger one to anterior border of the first branchial arch. It forks, in fact, over the first visceral cleft.

The vagus arises by a great number of distinct strands from the sides of the medulla. In the example dissected there were twelve in all. The anterior three of these were the largest; the middle one having the most ventral origin. The next four were very small and in pairs, and were separated by a considerable interval from the next four, also very small, and these again by a marked interval from the hindermost strand.

The common stem formed by the junction of these gives off immediately on leaving the skull a branch which forks on the second branchial cleft; a second for the third cleft is next given off; the main stem then divides into a dorsal branch--the lateral nerve--and a ventral one--the branchio-intestinal nerve--which, after giving off the branches for the two last branchial clefts, supplies the heart and intestinal tract. The lateral nerve passes back towards the posterior end of the body, internal to the lateral line, and between the dorso-lateral and ventro-lateral muscles. It gives off at its origin a fine nerve, which has a course nearly parallel to its own. The main stem of the vagus, at a short distance from its central end, receives a nerve which springs from the ventral side of the medulla, on about a level with the most posterior of the true roots of the vagus. This small nerve corresponds with the ventral or anterior roots of the vagus described by Gegenbaur, Jackson, and Clarke (though in the species investigated by the latter authors these roots did not join the vagus, but the anterior spinal nerves). Similar roots are also mentioned by Stannius, who found two of them in the Elasmobranchii dissected by him; it is possible that a second may be present in Scyllium, but have been overlooked by me, or perhaps may have been exceptionally absent in the example dissected.

_The Fifth Nerve._ The thinning of the roof of the brain, in the manner already described, produces a great change in the apparent position of the roots of all the nerves. The central ends of the rudiments of the two sides are, as has been mentioned, at first in contact dorsally but, when by the growth of the roof of the brain its two lateral halves become pushed apart, the nerves also shift their position and become widely separated. The roots of the fifth nerve are so influenced by these changes that they spring from the brain about half way up its sides, and a little ventral to the border of its thin roof. While this change has been taking place in the point of attachment of the fifth nerve, it has not remained in other respects in a stationary condition.

During stage H it already exhibits two distinct branches known as the mandibular and ophthalmic. These branches first lie outside a section of the body-cavity which exists in the front part of the head. The ophthalmic branch of the fifth being situated near the anterior end of this, and the mandibular near the posterior end.

In stage I the body-cavity in this part becomes divided into two parts one behind the other, the posterior being situated in the mandibular arch. The bifurcation of the nerve then takes place over the summit of the posterior of the two divisions of the body-cavity, Pl. 15, figs. 9_b_, V, and 10, V, &c., and at first both branches keep close to the sides of this.

The anterior or ophthalmic branch of the fifth soon leaves the walls of the cavity just spoken of and tends towards the eye, and there comes in close contact with the most anterior section of the body-cavity which exists in the head. These relations it retains unchanged till the close of stage K. Between stages I and K it may easily be seen from the surface; but, before the close of stage K, the increased density of the tissues renders it invisible in the living embryo.

The posterior branch of the fifth extends downwards into the mandibular arch in close contact with the posterior and outer wall of the body space already alluded to. At first no branches from it can be seen, but I have detected by the close of stage K, by an examination of the living embryo, a branch springing from it a short way from its central extremity, and passing forwards, Pl. 15, fig. 2, V This branch I take to be the rudiment of the superior maxillary division of the fifth nerve. It is shewn in section, Pl. 15, fig. 15_a_, V.

In the stages after K the anatomy of the nerves becomes increasingly difficult to follow, and accordingly I must plead indulgence for the imperfections in my observations on all the nerves subsequently to this date. In the fifth I find up to stage O a single ophthalmic branch (Pl. 17, fig. 4_b_, V._op.th._), which passes forwards slightly dorsal to the eye and parallel and ventral to a branch of the seventh, which will be described when I come to that nerve. I have been _unable_ to observe that this branch divides into a ramus superficialis and ramus profundus, and subsequently to stage O I have no observations on it.

By stage O the fifth may be observed to have two very distinct roots, and a large ganglionic mass is developed close to their junction (Gasserian ganglion), Pl. 17, fig. 4_a_. But in addition to this ganglionic enlargement, all of the branches have special ganglia of their own, Pl. 17, fig. 4_b_.

_Summary._ The fifth nerve has almost from the beginning two branches, the ophthalmic (probably the inferior ophthalmic of the adult) and the inferior maxillary. The superior maxillary nerve arises later than the other two as a branch from the inferior, originating comparatively far from its root. There is at first but a single root for the whole nerve, which subsequently becomes divided into two. Ganglionic swellings are developed on the common stem and main branches of the nerve.

A general view of the nerve is shewn in the diagram in Pl. 17, fig. 1.

* * * * *

_Seventh and Auditory Nerves._ There appears in my earliest sections a single large rudiment in the position of the seventh and auditory nerves; but in longitudinal sections of an embryo somewhat older than stage I, in which the auditory organ forms a fairly deep pit, still widely open to the exterior, there are to be seen immediately in front of the ear the rudiments of two nerves, which come into contact where they join the brain and have their roots still closely connected at the end of stage K (Pl. 15, figs. 10 and 15_a_ and 15_b_). The anterior of these pursues a straight course to the hyoid arch (Pl. 15, fig. 10, VII), the second of the two (Pl. 15, fig. 10, _au.n._), which is clearly the rudiment of the auditory nerve, develops a ganglionic enlargement and, turning backward, closely hugs the ventral wall of the auditory involution.

The observation just recorded appears to lead to the following conclusions with reference to the development of the auditory nerve. A single rudiment arises from the brain for the auditory and seventh nerves. This rudiment subsequently becomes split into two parts, an anterior to form the seventh nerve, and a posterior to form the auditory nerve. The ganglionic part of the auditory nerve is derived from the primitive outgrowths from the brain, and not from the auditory involution. I do not feel perfectly confident that an independent origin of the auditory nerve might not have escaped my notice; but, admitting the correctness of the view which attributes to the seventh and auditory a common origin, it follows that the auditory nerve primitively arose in connection with the seventh, of which it may either, as Gegenbaur believes, be a distinct part--the ramus dorsalis--or else may possibly have formed part of a commissure, homologous with that uniting the dorsal roots of the spinal nerves, connecting the seventh with the glossopharyngeal nerve. In either case it must be supposed secondarily to have become separate and independent in consequence of the development of the organ of hearing.

My sections of embryos of stage K and the subsequent stages do not bring to light many new facts with reference to the auditory nerve: they demonstrate however that its ganglionic part increases greatly in size, and in stage O there is a distinct root for the auditory nerve in contact with that for the seventh.

The history of the seventh nerve in its later stages presents points of great interest. Near the close of stage K there may be observed, in the living embryos and in sections, two branches of the seventh in addition to the original trunk to the hyoid arch, both arising from its anterior side; one passes straight forwards close to the external skin, but is at first only traceable a short way in front of the fifth, and a second passes downwards into the mandibular arch in such a fashion, that the seventh nerve forks over the hyomandibular cleft (vide Pl. 15, fig. 2, VII.; 15_a_, VII.). My sections shew both these branches with great clearness. A third branch has also come under my notice, whose course leads me to suppose that it supplies the roof of the palate.

In the later stages my attention has been specially directed to the very remarkable anterior branch of the seventh. This may, in stages L to O, be traced passing on a level with the root of the fifth nerve above the eye, and apparently terminating in branches to the skin in front of the eye (Pl. 17, figs. 3, VII.; 4_a_, VII,_a_). It courses close beneath the skin (though this does not appear in the sections represented on account of their obliqueness), and runs parallel and dorsal to the ophthalmic branch of the fifth nerve, and may easily be seen in this position in longitudinal sections belonging to stage O; but its changes after this stage have hitherto baffled me, and its final fate is therefore, to a certain extent, a matter of speculation.

The two other branches of the seventh, viz., the hyoid or main branch and mandibular branch, retain their primitive arrangement till the close of stage O.

The fate of the remarkable anterior branch of the seventh nerve is one of the most interesting points which has started up in the course of my investigations on the development of the cranial nerves, and it is a matter of very great regret to me that I have not been able to clear up for certain its later history.

Its primitive distribution leads to the supposition that it becomes the nerve known in the adult as the _ramus ophthalmicus superficialis of the fifth nerve_, and this is the view which I admit myself to be inclined to adopt. There are several points in the anatomy of this nerve in the adult which tell in favour of accepting this view with reference to it. In the first place, the ramus ophthalmicus superficialis rises from the brain (vide description above, p. 417), quite independently of the ramus ophthalmicus profundus, and not in very close connection with the other branches of the fifth, and also considerably behind these, quite as far back indeed as the ventral root of the seventh. There is therefore nothing in the position of its root opposed to its being regarded as a branch of the seventh nerve. Secondly, its distribution, which might at first sight be regarded as peculiar, presents no very strange features if it is looked on as a ramus dorsalis of the seventh, whose apparent anterior instead of dorsal course is due to the cranial flexure. If, however, the distribution of the ramus ophthalmicus superficialis is used as an argument against my view, a satisfactory reply is to be found in the fact that a branch of the seventh nerve certainly has the distribution in question _in the embryo_, and that there is no reason why it should not retain it _in the adult_.

Finally, the junction of the two rami ophthalmici, most remarkable if they are branches of a single nerve, would present nothing astonishing when they are regarded as branches of two separate nerves.

If this view be adopted, certain modifications of the more generally accepted views of the morphology of the cranial nerves will be necessitated; but this subject is treated of at the end of this section.

Some doubt hangs over the fate of the other branches of the seventh nerve, but their destination is not so obscure as that of the anterior branch. The branch to the roof of the mouth can be at once identified as the 'palatine nerve', and it only remains to speak of the mandibular branch.

It may be noticed first of all with reference to this branch, that the seventh behaves precisely like the less modified succeeding cranial nerves. It forks in fact over a visceral cleft (the hyomandibular) the two sides of which it supplies; the branch at the anterior side of the cleft is the later developed and smaller of the two. There cannot be much doubt that the mandibular branch must be identified with the spiracular nerve (præ-spiracular branch Jackson and Clarke) of the adult, and if the chorda tympani of Mammals is correctly regarded as the mandibular branch of the seventh nerve, then the spiracular nerve must represent it. Jackson and Clarke[288] take a different view of the homology of the chorda tympani, and regard it as equivalent to the ramus mandibularis internus (one of the two branches into which the seventh eventually divides), because this nerve takes its course over the ligament connecting the mandible with the hyoid. This view I cannot accept so long as it is admitted that the chorda tympani is the branch of a cranial nerve supplying the anterior side of a cleft. The ramus mandibularis internus, instead of forming with the main branch of the seventh a fork over the spiracle, passes to its destination completely behind and below the spiracle, and therefore fails to fulfil the conditions requisite for regarding it as a branch to the anterior wall of a visceral cleft. It is indeed clear that the ramus mandibularis internus cannot be identified with the embryonic mandibular branch of the seventh (which passes above the spiracle or hyomandibular cleft) when there is present in the adult another nerve (the spiracular nerve), which exactly corresponds in distribution with the embryonic nerve in question. My view accords precisely with that already expressed by Gegenbaur in his masterly paper on the nerves of Hexanchus, in which he distinctly states that he looks upon the spiracular nerve as the homologue of an anterior branchial branch of a division of the vagus. In the adult the spiracular nerve is sometimes represented by one or two branches of the palatine, _e.g._ Scyllium, but at other times arises independently from the main stem of the seventh[289]. The only difficulty in my identification of the embryonic mandibular branch with the adult spiracular nerve, is the extremely small size of the latter in the adult, compared with the size of mandibular in the embryo; but it is hardly surprising to find an atrophy of the spiracular nerve accompanying an atrophy of the spiracle itself. The palatine appears to me to have been rightly regarded by Jackson and Clarke as the great superficial petrosal of Mammals.

Footnote 288: _Loc. cit._

Footnote 289: Hexanchus, Gegenbaur, _Jenaische Zeitschrift_, Vol. VI.

On the common root of the branches of the seventh nerve, as well as on its hyoid branch, ganglionic enlargements are present at an early period of development.

_The Glossopharyngeal and Vagus Nerves._ Behind the ear there are formed a series of five nerves which pass down to respectively the first, second, third, fourth and fifth visceral arches.

For each arch there is thus one nerve, whose course lies close to the posterior margin of the preceding cleft, a second anterior branch being developed later. These nerves are connected with the brain (as I have determined by transverse sections) by roots at first attached to the dorsal summit, but eventually situated about half-way down the sides (Pl. 15, fig. 6) nearly opposite the level of the process which divides the ventricle of the hind-brain into a dorsal and a ventral moiety. The foremost of these nerves is the glossopharyngeal. The next four are, as has been shewn by Gegenbaur[290], equivalent to four independent nerves, but form, together with the glossopharyngeal, a compound nerve, which we may briefly call the vagus.

Footnote 290: _Loc. cit._

This compound nerve by stage K attains a very complicated structure, and presents several remarkable and unexpected features. Since it has not been possible for me completely to elucidate the origin of all its various parts, it will conduce to clearness if I give an account of its structure during stage K or L, and then return to what facts I can mention with reference to its development. Its structure during these stages is represented on the diagram, Pl. 17, fig. 1. There are present five branches, viz. the glossopharyngeal and four branches of the vagus, arising probably by a considerably greater number of strands from the brain[291]. All the strands from the brain are united together by a thin commissure, _Vg.com._, continuous with the commissure of the posterior roots of the spinal nerves, and from this commissure the five branches are continued obliquely ventralwards and backwards, and each of them dilates into a ganglionic swelling. They all become again united together by a second thick commissure, which is continued backwards as the intestinal branch of the vagus nerve _Vg.in._ The nerves, however, are continued ventralwards each to its respective arch. From the hinder part of the intestinal nerve springs the lateral nerve _n.l._, at a point whose relations to the branches of the vagus I have not certainly determined.

Footnote 291: In the diagram there are only five strands represented. This is due to the fact that I have not certainly made out their true number.

The whole nerve-complex formed by the glossopharyngeal and the vagus nerves cannot of course be shewn in any single section. The various roots are shewn in Pl. 17, fig. 5. The dorsal commissure is represented in longitudinal section in Pl. 15, fig. 15_b_, _com._, and in transverse section in Pl. 17, fig. 2, _Vg.com_ The lower commissure continued as the intestinal nerve is shewn in Pl. 15, fig. 15_a_, _Vg._, and as seen in the living embryo in Pl. 15, figs. 1 and 2. The ganglia are seen in Pl. 15, fig. 6, _Vg_. The junction of the vagus and glossopharyngeal nerves is shewn in Pl. 15, fig. 10. My observations have not taught me much with reference to the origin of the two commissures, viz. the dorsal one and the one which forms the intestinal branch of the vagus. Very possibly they originate as a single commissure which becomes longitudinally segmented. It deserves to be noticed that the dorsal commissure has a long stretch, from the last branch of the vagus to the first spinal nerve, during which it is not connected with the root of any nerve; vide fig. 15_b_, _com_. This space probably contained originally the now lost branches of the vagus. In many transverse sections where the dorsal commissure might certainly be expected to be present it cannot be seen, but this is perhaps due to its easily falling out of the sections. I have not been able to prove that the commissure is continued forwards into the auditory nerve.

The relation of the branches of the vagus and glossopharyngeal to the branchial clefts requires no special remark. It is fundamentally the same in the embryo as in the adult. The branches at the posterior side of the clefts are the first to appear, those at the anterior side of the clefts being formed subsequently to stage K.

One of the most interesting points with reference to the vagus is the number of separate strands from the brain which unite to form it. The questions connected with these have been worked out in a masterly manner, both from an anatomical and a theoretical standpoint, by Professor Gegenbaur[292]. It has not been possible for me to determine the exact number of these in my embryos, nor have I been able to shew whether they are as numerous at the earliest appearance of the vagus as at a later embryonic period. The strands are connected (Pl. 17, fig. 5) with separate ganglionic centres in the brain, though in several instances more than one strand is connected with a single centre. In an embryo between stage O and P more than a dozen strands are present. In an adult Scyllium I counted twelve separate strands, but their number has been shewn by Gegenbaur to be very variable. It is possible that they are remnants of the roots of the numerous primary branches of the vagus which have now vanished; and this perhaps is the explanation of their variability, since in the case of all organs which are on the way to disappear variability is a precursor of disappearance.

Footnote 292: _Loc. cit._

A second interesting point is the presence of the two connecting commissures spoken of above. It was not till comparatively late in my investigations that I detected the dorsal one. This has clearly the same characters as the dorsal commissure already described as connecting the roots of all the spinal nerves, and is indeed a direct prolongation of this. It becomes gradually thinner and thinner, and finally ceases to be observable by about the close of stage L. It is of importance as shewing the similarity of the branches of the vagus to the dorsal roots of the spinal nerves. The ventral of the two commissures persists in the adult as the common stem from which all the branches of the vagus successively originate, and is itself continued backwards as the intestinal branch of the vagus. The glossopharyngeal nerve alone becomes eventually separated from the succeeding branches. Stannius and Gegenbaur have, as was mentioned above, detected in adult Elasmobranchii roots which join the vagus, and which resemble the anterior or ventral roots of spinal nerves; and I have myself described one such root in the adult Scyllium. I have searched for these in my embryos, but without obtaining conclusive results. In the earliest stages I can find no trace of them, but I have detected in stage L one anterior root on debatable border-land, which may conceivably be the root in question, but which I should naturally have put down for the root of a spinal nerve. Are the roots in question to be regarded as proper roots of the vagus, or as ventral roots of spinal nerves whose dorsal roots have been lost? The latter view appears to me the most probable one, partly from the embryological evidence furnished by my researches, which is clearly opposed to the existence of anterior roots in the brain, and partly from the condition of these roots in Echinorhinus, in which they join the succeeding spinal nerves and not the vagus[293]. The similar relations of the apparently homologous branch or branches in many Osseous Fish may also be used as an argument for my view.

Footnote 293: Vide Jackson and Clarke, _loc. cit._ The authors take a different view to that here advocated, and regard the ventral roots described by them as having originally belonged to the vagus.

If, as seems probable, the roots in question become the hypoglossal nerve, this nerve must be regarded as formed from the anterior roots of one or more spinal nerves. Without embryological evidence it does not however seem possible to decide whether the hypoglossal nerve contains elements only of anterior roots or of both anterior and posterior roots.

_Mesoblast of the Head._

_Body-Cavity and Myotomes of the Head._--During stage F the appearance of a cavity on each side in the mesoblast of the head was described. (Vide Pl. 10, figs. 3_b_ and 6, _pp_.) These cavities end in front opposite the blind anterior extremity of the alimentary canal; behind they are continuous with the general body-cavity. I propose calling them the _head-cavities_. The cavities of the two sides have no communication with each other.

Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft (vide Pl. 15, figs. 4_a_ and 4_b_, _pp._); and during stage H it becomes, owing to the formation of a second cleft, divided into three sections: (1) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft; (3) a section behind the first branchial cleft.

The section in front of the hyomandibular cleft stands in a peculiar relation to the two branches of the fifth nerve. The ophthalmic branch of the fifth lies close to the outer side of its anterior part, the mandibular branch close to the outer side of its posterior part. During stage I this front section of the head-cavity grows forward, and becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution, and is connected with the ophthalmic branch of the fifth nerve. The posterior part lies completely within the mandibular arch, and is closely connected with the mandibular division of the fifth nerve.

As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole head-cavity becomes on each side divided into (1) a premandibular section; (2) a mandibular section; (3) a hyoid section; (4) sections in the branchial arches.

The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells. It is situated close to the eye, and presents a rounded or sometimes triangular figure in sections (Pl. 15, figs. 7, 9_b_ and 16_b_, 1_pp._). The ophthalmic branch of the fifth nerve passes close to its superior and outer wall.

Between stages I and K the anterior cavities of the two sides are prolonged ventralwards and meet below the base of the fore-brain (Pl. 15, fig. 8, 1_pp._). The connection between the two cavities appears to last for a considerable time, and still persists at the close of stage L. The anterior or premandibular pair of cavities are the only parts of the body-cavity within the head which unite ventrally. In the trunk, however, the primitively independent lateral halves of the body-cavity always unite in this way. The section of the head-cavity just described is so similar to the remaining posterior sections that it must be considered as equivalent to them.

The next division of the head-cavity, which from its position may be called the mandibular cavity, presents during the stages I and K a spatulate shape. It forms a flattened cavity, dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft, Pl. 15, fig. 1_pp._ and fig. 7, 9_b_ and 15_a_, 2_pp_. Like the previous space it is lined by a short columnar epithelium.

The fifth nerve, as has already been mentioned, bifurcates over its dorsal summit, and the mandibular branch of that nerve passes down on its posterior and outer side. The mandibular aortic arch is situated close to its inner side, Pl. 15, fig. 7. Towards the close of this period the upper part of the cavity atrophies. Its lower part also becomes much narrowed, but its walls of columnar cells persist and lie close to one another. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. This thicker wall is on its inner side separated from the surrounding tissue by a small space lined by a membrane-like structure. In each of the remaining arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch. A dorsal dilated portion appears, however, to be present in the third or hyoid section alone, and even there disappears by the close of stage K. The cavities in the posterior parts of the head become much reduced like those in its anterior part, though at rather a later period. Their walls however persist, and become more columnar. In Pl. 15, fig. 13_b_, _pp._, is represented the cavity in the last arch but one, at a period when the cavity in the mandibular arch has become greatly reduced. It occupies the same position on the outer side of the aortic trunk of its arch as does the cavity in the mandibular arch (Pl. 15, fig. 7, 2 _pp_). In Torpedo embryos the head-cavity is much smaller, and atrophies earlier than in the embryos of Pristiurus and Scyllium.

It has been shewn that, with the exception of the most anterior, the divisions of the body-cavity in the head become atrophied, _not so however their walls_. The cells forming these become elongated, and by stage N become distinctly developed into muscles. Their exact history I have not followed in its details, but they almost unquestionably become the musculus constrictor superficialis and musculus interbranchialis[294]; and probably also musculus levator mandibuli and other muscles of the front part of the head.

Footnote 294: Vide Vetter, "Die Kiemen und Kiefermusculatur d. Fische." _Jenaische Zeitschrift_, Vol. VII.

The most anterior cavity close to the eye remains unaltered much longer than the remaining cavities, and its two halves are still in communication at the close of stage L. I have not yet succeeded in tracing the subsequent fate of its walls, _but think it probable that they develop into the muscles of the eye_. The morphological importance of the sections of the body-cavity in the head cannot be over-estimated, and the fact that the walls become developed into the muscular system of the head renders it almost certain _that we must regard them as equivalent to the muscle-plates of the body, which originally contain, equally with those of the head, sections of the body-cavity_. If this determination is correct, there can be no doubt that they ought to serve as valuable guides to the number of segments which have coalesced to form the head. This point is, however, discussed in a subsequent section.

_General mesoblast of the head._--In stage G no mesoblast is present in the head, except that which forms the walls of the head-cavity.

During stage H a few cells of undifferentiated connective tissue appear around the stalk of the optic vesicle, and in the space between the front end of the alimentary tract and the base of the brain in the angle of the cranial flexure. They are probably budded off from the walls of the head-cavities. Their number rapidly increases, and they soon form an investment surrounding all the organs of the head, and arrange themselves as a layer, between the walls of the roof of the fore and mid-brain and the external skin. At the close of stage K they are still undifferentiated and embryonic, each consisting of a large nucleus surrounded by a very delicate layer of protoplasm produced into numerous thread-like processes. They form a regular meshwork, the spaces of which are filled up by an intercellular fluid.

I have not worked out the development of the cranial and visceral skeleton; but this has been made the subject of an investigation by Mr Parker, who is more competent to deal with it than any other living anatomist. His results were in part made known in his lectures before the Royal College of Surgeons[295], and will be published in full in the _Transactions of the Zoological Society_.

Footnote 295: A report of the lectures appeared in _Nature_.

All my efforts have hitherto failed to demonstrate any segmentation in the mesoblast of the head, other than that indicated by the sections of the body-cavity before-mentioned; but since these, as above stated, must be regarded as equivalent to muscle-plates, any further segmentation of mesoblast could not be anticipated. To this statement the posterior part of the head forms an apparent exception. Not far behind the auditory involution there are visible at the end of period K a few longitudinal muscles, forming about three or four muscle-plates, the ventral part of which is wanting. I have not the means of deciding whether they properly belong to the head, or may not really be a part of the trunk system of muscles which has, to a certain extent, overlapped the back part of the head, but am inclined to accept the latter view. These cranial muscle-plates are shewn in Pl. 15, fig. 15_b_, and in Pl. 17, fig. 2.

_Notochord in the Head._

The notochord during stage G is situated for its whole length close under the brain, and terminates opposite the base of the mid-brain. As the cranial flexure becomes greater and mesoblast is collected in the angle formed by this, the termination of the notochord recedes from the base of the brain, but remains in close contact with the front end of the alimentary canal. At the same time its terminal part becomes very much thinner than the remainder, ends in a point, and exhibits signs of a retrogressive metamorphosis. It also becomes bent upon itself in a ventral direction through an angle of 180°; vide Pl. 15, figs. 9_a_ and 16_a_. In some cases this curvature is even more marked than is represented in these figures.

The bending of the end of the notochord is not directly caused by the cranial flexure, as is proved by the fact that the end of the notochord becomes bent through a far greater angle than does the brain. During the stages subsequent to K the ventral flexure of the notochord disappears, and its terminal part acquires by stage O a distinct dorsal curvature.

_Hypoblast of the Head._

The only feature of the alimentary tract in the head which presents any special interest is the formation of the gill-slits and of the thyroid body. In the present section the development of the former alone is dealt with; the latter body will be treated in the section devoted to the general development of the alimentary tract.

The gill-slits arise as outgrowths of the lining of the throat towards the external skin. In the gill-slits of Torpedo I have observed a very slight ingrowth of the external skin towards the hypoblastic outgrowth in one single case. In all other cases observed by me, the outgrowth from the throat meets the passive external skin, coalesces with it, and then, by the dissolution of the wall separating the lumen of the throat from the exterior, a free communication from the throat outwards is effected; vide Pl. 15, figs. 5_a_ and _b_, and 13_b_. Thus it happens that the walls lining the clefts are entirely formed of hypoblast. The clefts are formed successively[296], the anterior appearing first, and it is not till after the rudiments of three have appeared, that any of them become open to the exterior.

Footnote 296: Vide Plate 8.

In stage K, four if not five are open to the exterior, and the rudiments of six, the full number, have appeared[297]. Towards the close of stage K there arise, from the walls of the 2nd, 3rd and 4th clefts, very small knob-like processes, the rudiments of the external gills. These outgrowths are formed both by the lining of the gill-cleft and by the adjoining mesoblast[298].

Footnote 297: The description of stage K and L, pp. 292 and 293, is a little inaccurate with reference to the number of the visceral clefts, though the number visible in the hardened embryos is correctly described.

Footnote 298: Vide on the development of the gills, Schenk, _Sitz. d. k. Akad. Wien_, Vol. LXXI, 1875.

From the mode of development of the gill-clefts, it appears that their walls are lined externally by hypoblast, and therefore that the external gills are processes of the walls of the alimentary tract, _i.e._ are covered by an hypoblastic, and not an epiblastic layer. It should be remembered, however, that after the gill-slits become open, the point where the hypoblast joins the epiblast ceases to be determinable, so that some doubt hangs over the above statement.

The identification of the layer to which the gills belong is not without interest. If the external gills have an epiblastic origin, they may be reasonably regarded[299] as homologous with the external gills of Annelids; but, if derived from the hypoblast, this view becomes, to say the least, very much less probable.

Footnote 299: Vide Dohrn, _Ursprung d. Wirbelthiere_.

_Segmentation of the Head._

The nature of the vertebrate head and its relation to the trunk forms some of the oldest questions of Philosophical Morphology.

The answers of the older anatomists to these questions are of a contradictory character, but within the last few years it has been more or less generally accepted that the head is, in part at least, merely a modified portion of the trunk, and composed, like that, of a series of homodynamous segments[300]. While the researches of Huxley, Parker, Gegenbaur, Götte, and other anatomists, have demonstrated in an approximately conclusive manner that the head is composed of a series of segments, great divergence of opinion still exists both as to the number of these segments, and as to the modifications which they have undergone, especially in the anterior part of the head. The questions involved are amongst the most difficult in the whole range of morphology, and the investigations recorded in the preceding pages do not, I am very well aware, go far towards definitely solving them. At the same time my observations on the nerves and on the head-cavities appear to me to throw a somewhat new light upon these questions, and it has therefore appeared to me worth while shortly to state the results to which a consideration of these organs points. There are three sets of organs, whose development has been worked out, each of which presents more or less markedly a segmental arrangement:--(1) The cranial nerves; (2) the visceral clefts; (3) the divisions of the head-cavity.

Footnote 300: Semper, in his most recent work, maintains, if I understand him rightly, that the head is in no sense a modified part of the trunk, but admits that it is segmented in a similar fashion to the trunk.

The first and second of these have often been employed in the solution of the present problem, while the third, so far as is known, exists only in the embryos of Elasmobranchii.

The development of the cranial nerves has recently been studied with great care by Dr Götte, and his investigations have led him to adopt very definite views on the segments of head. The arrangement of the cranial nerves _in the adult_ has frequently been used in morphological investigations about the skull, but there are to my mind strong grounds against regarding it as affording a safe basis for speculation. The most important of these depends on the fact that nerves are liable to the greatest modification on any changes taking place in the organs they supply. On this account it is a matter of great difficulty, amounting in many cases to actual impossibility, to determine the morphological significance of the different nerve-branches, or the nature of the fusions and separations which have taken place at the roots of the nerves. It is, in fact, only in those parts of the head which have, relatively speaking, undergone but slight modifications, and which require no special elucidation from the nerves, that these sufficiently retain in the adult their primitive form to serve as trustworthy morphological guides.

I propose to examine separately the light thrown on the segmentation of the head by the development of (1) the nerves, (2) the visceral clefts, (3) the head-cavities; and then to compare the three sets of results so obtained.

The post-auditory nerves present no difficulties; they are all organized in the same fashion, and, as was first pointed out by Gegenbaur, form five separate nerves, each indicating a segment. A comparison of the post-auditory nerves of Scyllium and other typical Elasmobranchii with those of Hexanchus and Heptanchus proves, however, that other segments were originally present behind those now found in the more typical forms. And the presence in Scyllium of numerous (twelve) strands from the brain to form the vagus, as well as the fact that a large section of the commissure connecting the vagus roots with the posterior roots of the spinal nerves is not connected with the brain, appear to me to shew that all traces of the lost nerves have not yet vanished.

Passing forwards from the post-auditory nerves, we come to the seventh and auditory nerves. The embryological evidence brought forward in this paper is against regarding these nerves as representing two segments. Although it must be granted that my evidence is not conclusive against an independent formation of these two nerves, yet it certainly tells in favour of their originating from a common rudiment, and Marshall's results on the origin of the two nerves in Birds (published in the _Journal of Anatomy and Physiology_, Vol. XI. Part 3) support, I have reason to believe, the same conclusion. Even were it eventually to be proved that the auditory nerve originated independently of the seventh, the general relations of this nerve, embryological and otherwise, are such that, provisionally at least, it could not be regarded as belonging to the same category as the facial or glossopharyngeal nerves, and it has therefore no place in a discussion on the segmentation of the head.

The seventh nerve of the embryo (Pl. 17, fig. 1, VII) is formed by the junction of three conspicuous branches, (1) an anterior dorsal branch which takes a more or less horizontal course above the eye (VII. _a_); (2) a main branch to the hyoid arch (VII. _hy_); (3) a smaller branch to the posterior edge of the mandibular arch (VII. _mn_). The first of these branches can clearly be nothing else but the typical "ramus dorsalis," of which however the auditory may perhaps be a specialized part. The fact that this branch pursues an anterior and not a directly dorsal course is probably to be explained as a consequence of the cranial flexure. The two other branches of the seventh nerve are the same as those present in all the posterior nerves, viz. the branches to the two sides of a branchial cleft, in the present instance the spiracle; the seventh nerve being clearly the nerve of the hyoid arch.

The fifth nerve presents in the arrangement of its branches a similarity to the seventh nerve so striking that it cannot be overlooked. This similarity is at once obvious from an inspection of the diagram of the nerves on Pl. 17, fig. 1, V, or from an examination of the sections representing these nerves (Pl. 17, figs. 3 and 4). It divides like the seventh nerve into three main branches: (1) an anterior and dorsal branch (_r._ ophthalmicus profundus), whose course lies parallel to but ventral to that of the dorsal branch of the seventh nerve; (2) a main branch to the mandibular arch (_r._ maxillæ inferioris); and (3) an anterior branch to the palatine arcade (_r._ maxillæ superioris). I was at first inclined to regard the anterior branch of the fifth (ophthalmic) as representing a separate nerve, and was supported in this view by its relation to the most anterior of the head-cavities; but the unexpected discovery of an exactly _similar branch_ in the seventh nerve has induced me to modify this view, and I am now constrained to view the fifth as a single nerve, whose branches exactly correspond with those of the seventh. The anterior branch of the fifth is, like the corresponding branch of the seventh, the _ramus dorsalis_, and the two other branches are the equivalent of the branches of the seventh, which fork over the spiracle, though in the case of the fifth nerve no distinct cleft is present unless we regard the mouth as such. Embryology thus appears to teach us that the fifth nerve is a single nerve supplying the mandibular arch, and not, as has been usually thought, a complex nerve resulting from the coalescence of two or three distinct nerves. My observations do not embrace the origin or history of the third, fourth, and sixth nerves, but it is hardly possible to help suspecting that in these we have the nerve of one or more segments in front of that supplied by the fifth nerve; a view which well accords with the most recent morphological speculations of Professor Huxley[301].

Footnote 301: Preliminary note upon the brain and skull of Amphioxus, _Proc. of the Royal Society_, Vol. XXII.

From this enumeration of the nerves the optic nerve is excluded for obvious reasons, and although it has been shewn above that the olfactory nerve develops like the other nerves as an outgrowth from the brain, yet its very late appearance and peculiar relations are, at least for the present, to my mind sufficient grounds for excluding it from the category of segmental cranial nerves.

The nerves then give us indications of seven cranial segments, or, if the nerves to the eye-muscles be included, of _at the least_ eight segments, but to these must be added a number of segments now lost, but which once existed behind the last of those at present remaining.

The branchial clefts have been regarded as guides to segmentation by Gegenbaur, Huxley, Semper, etc., and this view cannot I think be controverted. In Scyllium there are six clefts which give indications of seven segments, viz., the segments of the mandibular arch, hyoid arch, and of the five branchial arches. If, following the views of Dr Dohrn[302], we regard the mouth as representing a cleft, we shall have seven clefts and eight segments; and it is possible, as pointed out in Dr Dohrn's very suggestive pamphlet, that remnants of a still greater number of præoral clefts may still be in existence. Whatever may be the value of these speculations, such forms as Hexanchus and Heptanchus and Amphioxus make it all but certain that the ancestors of Vertebrates had a number of clefts behind those now developed.

Footnote 302: _Ursprung d. Wirbelthiere._

The last group of organs to be dealt with for our present question is that of the Head-Cavities.

The walls of the spaces formed by the cephalic prolongations of the body-cavity develop into muscles and resemble the muscle-plates of the trunk, and with these they must be identified, as has been already stated. As equivalent to the muscle-plates, they clearly are capable of serving as very valuable guides for determining the segmentation of the head. There are then a pair of these in front of the mandibular arch, a pair in the mandibular arch, and a pair in each succeeding arch. In all there are eight pairs of these cavities representing eight segments, the first of them præoral. As was mentioned above, each of the sections of the head-cavity (except perhaps the first) stands in a definite relation to the nerve and artery of the arch in which it is situated.

The comparative results of these three independent methods of determining the segmentation of the head are in the subjoined table represented in a form in which they can be compared:--

_Table of the Cephalic Segments as determined by the Nerves, Visceral Arches, and Head-Cavities._

+----------+---------------------+------------------+----------------+ | Segments | Nerves | Visceral Arches | Head-Cavities | | | | | or Cranial | | | | | Muscle-Plates | |----------+---------------------+------------------+----------------+ |Præoral 1 |3rd and 4th and | (?) |1st head cavity | | |? 6th nerves (perhaps| |(in my figures | | |representing more | | 1_pp._) | | |than one segment) | | | | | | | | |Postoral 2|5th nerve |Mandibular |2nd head-cavity | | | | |(in my figures | | | | | 2_pp._) | | | | | | | ---- 3|7th nerve |Hyoid |3rd head-cavity | | | | | | | ---- 4|Glossopharyngeal |1st branchial arch|4th head-cavity | | |nerve | | | | | | | | | ---- 5|1st branch of vagus |2nd branchial arch|5th head-cavity | | | | | | | ---- 6|2nd branch of vagus |3rd branchial arch|6th head-cavity | | | | | | | ---- 7|3rd branch of vagus |4th branchial arch|7th head-cavity | | | | | | | ---- 8|4th branch of vagus |5th branchial arch|8th head-cavity | +----------+---------------------+------------------+----------------+

In the above table the first column denotes the segments of the head as indicated by a comparison of the three sets of organs employed. The second column denotes the segments as obtained by an examination of the nerves; the third column is for the visceral arches (which lead to the same results as, but are more convenient for our table than, the visceral clefts), and the fourth column is for the head-cavities. It may be noticed that from the second segment backwards the three sets of organs lead to the same results. The head-cavities indicate one segment in front of the mouth, and now that the ophthalmic branch of the fifth has been dethroned from its position as a separate nerve, the eye-nerves, or one of them, may probably be regarded as belonging to this segment. If the suggestion made above (p. 431), that the walls of the first cavity become the eye-muscles, be correct, the eye-nerves would perhaps after all be the most suitable nerves to regard as belonging to the segment of the first head-cavity.

EXPLANATION OF PLATES 15, 16, 17.

PLATE 15. (THE HEAD DURING STAGES G--K.)

COMPLETE LIST OF REFERENCE LETTERS.

1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal vein. _al._ Alimentary canal. _ao._ Aorta. _au._ Thickening of epiblast to form the auditory pit. _aun._ Auditory nerve. _aup._ Auditory pit. _auv._ Auditory vesicle. _b._ Wall of brain. _bb._ Base of brain. _cb._ Cerebellum. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._ Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._ Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._ Olfactory pit. _op._ Eye. _opn._ Optic nerve. _opv._ Optic vesicle. _opth_V. Ophthalmic branch of fifth. _p._ Posterior root of spinal nerve. _pn._ Pineal gland. 1, 2 etc. _pp._ First, second, etc. section of body-cavity in the head. _pt._ Pituitary body. _so._ Somatopleure. _sp._ Splanchnopleure. _spc._ Spinal cord. _Th._ Thyroid body. _v._ Blood-vessel. iv._v._ Fourth ventricle. v. Fifth nerve. _Vc._ Visceral cleft. _Vg._ Vagus. vii. Seventh or facial nerve.

Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent object.

The points which deserve special attention are: (1) The sections of the body-cavity in the head (_pp_): the first or premandibular section being situated close to the eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. The third at the summit of the hyoid arch.

The cranial nerves and the general appearance of the brain are well shewn in the figure.

The notochord cannot be traced in the living embryo so far forward as it is represented. It has been inserted according to the position which it is seen to occupy in sections.

Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K, viewed as a transparent object.

The figure shews the condition of the brain; the branches of the fifth and seventh nerves (v. vii.); the rudiments of the semicircular canals; and the commencing appearance of the external gills as buds on both walls of 2nd, 3rd, and 4th clefts. The external gills have not appeared on the first cleft or spiracle.

Fig. 3. Section through the head of a Pristiurus embryo during stage G. It shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the brain. (2) The optic vesicles not yet constricted off from the fore-brain.

Figs. 4_a_ and 4_b_. Two sections through the head of a Pristiurus embryo of stage I. They shew (1) the appearance of the seventh nerve. (2) The portion of the body-cavity belonging to the first and second visceral arches. (3) The commencing thickening of epiblast to form the auditory involution.

In 4_b_, the posterior of the two sections, no trace of an auditory nerve is to be seen.

Figs. 5_a_ and 5_b_. Two sections through the head of a Torpedo embryo with 3 visceral clefts. Zeiss A, ocul. 1.

5_a_ shews the formation of the thin roof of the fourth ventricle by a divarication of the two lateral halves of the brain.

Both sections shew the commencing formation of the thyroid body (_th_) at the base of the mandibular arch.

They also illustrate the formation of the visceral clefts by an outgrowth from the alimentary tract without any corresponding ingrowth of the external epiblast.

Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. Zeiss A, ocul. 1.

The section shews (1) the attachment of a branch of the vagus to the walls of the hind-brain. (2) The peculiar form of the hind-brain.

Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to a stage intermediate between I and K, passing through both the fore-brain and the hind-brain. Zeiss A, ocul. 1.

The section illustrates (1) the formation of the pituitary body (_pt_) from the mouth involution (_m_), and proves that, although the wall of the throat (_al_) is in contact with the mouth involution, there is by this stage no communication between the two. (2) The eye. (3) The sections of the body-cavity in the head (1_pp_, 2_pp_). (4) The fifth nerve (v.) and the seventh nerve (vii).

Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. It shews the ventral junction of the anterior sections of the body-cavity in the head (1_pp_).

Figs. 9_a_ and 9_b_. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. 1.

9_a_ is taken through the median line, but is reconstructed from two sections. It shews (1) The divisions of the brain--The cerebrum and thalamencephalon in the fore-brain; the mid-brain; the commencing cerebellum in the hind-brain. (2) The relation of the mouth involution to the infundibulum. (3) The termination of the notochord.

9_b_ is a section to one side of the same brain. It shews (1) The divisions of the brain. (2) The point of outgrowth of the optic nerves (_opn_). (3) The sections of the body-cavity in the head and the bifurcation of the optic nerve over the second of these.

Fig. 10. Longitudinal section through the head of a Pristiurus embryo somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of the nerves and the junction of the fifth, seventh, and auditory nerves with the brain.

Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo of stage K, slightly to one side of the middle line. It shews the deep constriction separating the thalamencephalon from the cerebral hemispheres.

Fig. 12. Longitudinal section through the base of the brain of an embryo of a stage intermediate between I and K.

It shews (1) the condition of the end of the notochord; (2) the relation of the mouth involution to the infundibulum.

Fig. 13_a_. Longitudinal and horizontal section through part of the head of a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.

The figure contains the eye cut through in the plane of the choroid slit. Thus the optic nerve (_opn_) and choroid slit (_ch_) are both exhibited. Through the latter is seen passing mesoblast accompanied by a blood-vessel (_v_). _Op_ represents part of the optic vesicle to one side of the choroid slit.

No mesoblast can be seen passing round the outside of the optic cup; and the only mesoblast which enters the optic cup passes through the choroid slit.

Fig. 13_b_. Transverse section through the last arch but one of the same embryo as 13_a_. Zeiss A, ocul. 1.

The figure shews (1) The mode of formation of a visceral cleft without any involution of the external skin. (2) The head-cavity in the arch and its situation in relation to the aortic arch.

Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13, considerably magnified. The specimen was prepared by removing the nasal pit, flattening it out and mounting in glycerine after treatment with chromic acid. It shews the primitive arrangement of the Schneiderian folds. One side has been injured.

Figs. 15_a_ and 15_b_. Two longitudinal and vertical sections through the head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.

15_a_ is the most superficial section of the two. It shews the constitution of the seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior branch of the seventh nerve deserves a special notice.

15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_) continuous with the dorsal commissures of the posterior root of the spinal nerves.

Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus embryo belonging to the end of stage K. Zeiss a, ocul. 1.

16_a_ passes through the median line of the brain and shews the infundibulum, notochord and pituitary body, etc.

The pituitary body still opens into the mouth, though the septum between the mouth and the throat is broken through.

16_b_ is a more superficial section shewing the head-cavities _pp_ 1, 2, 3, and the lower vagus commissure.

PLATE 16.

COMPLETE LIST OF REFERENCE LETTERS.

_auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres. _ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._ Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes. _md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._ Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus. _pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland. _pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve. _x._ Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres.

Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a, ocul. 1.

1_a_ is taken slightly to one side of the middle line, and shews the general features of the brain, and more especially the infundibulum (_in_) and pituitary body (_pt_).

1_b_ is through the median line of the pineal gland.

1_c_ is through the median line of the base of the brain, and shews the notochord (_ch_) and pituitary body (_pt_); the latter still communicating with the mouth. It also shews the wide opening of the infundibulum in the middle line into the base of the brain.

Fig. 2. Section through the unpaired cerebral rudiment during stage O, to shew the origin of the olfactory lobe and the olfactory nerve. The latter is seen to divide into numerous branches, one of which passes into each Schneiderian fold. At its origin are numerous ganglion cells represented by dots. Zeiss a, ocul. 2.

Fig. 3. Horizontal section through the three lobes of the brain during stage O. Zeiss a, ocul. 2.

The figure shews (1) the very slight indications which have appeared by this stage of an ingrowth to divide the cerebral rudiment into two lobes (_x_): (2) the optic thalami united by a posterior commissure, and on one side joining the base of the mid-brain, and behind them the pineal gland: (3) the thin posterior wall of the cerebral rudiment with folds projecting into the cerebral cavity.

Figs. 4_a_, 4_b_, 4_c_. Views from the side, from above, and from below, of a brain of Scyllium canicula during stage P. In the view from the side the eye (_op_) has not been removed.

The bilobed appearance both of the mid-brain and cerebellum should be noticed.

Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage P. Zeiss a, ocul. 2.

There should be noticed (1) the increase in the flexure of the brain accompanying a rectification of the cranial axis; (2) the elongated pineal gland, and (3) the structure of the optic thalamus.

Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q.

Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a, ocul. 2.

7_a_ cuts the hind part of the brain nearly through the middle line; while 7_b_ cuts the cerebral hemispheres and pineal gland through the middle.

In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform tracts (_rt_) into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the cerebral hemispheres (1), the pineal gland (2), and the relations of the optic thalami (3) are shewn.

Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a, ocul. 1.

8_a_ passes through the cerebral hemispheres at their junction with the olfactory lobes. On the right side is seen the olfactory nerve coming off from the olfactory lobe. At the dorsal side of the hemispheres is seen the pineal gland (_pn_).

8_b_ passes through the mid-brain now slightly bilobed, and the opening into the infundibulum (_in_). At the base of the section are seen the optic nerves and their chiasma.

8_c_ passes through the opening from the ventricle of the mid-brain into that of the cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of the sacci vasculosi.

8_d_ passes through the front end of the medulla, and shews the roots of the seventh pair of nerves, and the overlapping of the medulla by the cerebellum.

PLATE 17.

COMPLETE LIST OF REFERENCE LETTERS.

vii. _a._ Anterior branch of seventh nerve. _ar._ Anterior root of spinal nerve. _auv._ Auditory vesicle. _cer._ Cerebrum. _ch._ Notochord. _ch._ Epithelial layer of choroid membrane. _gl._ Glossopharyngeal nerve. vii. _hy._ Hyoid branch of seventh nerve. _hym._ Hyaloid membrane. _ll._ Lateral line. v. _mn._ Ramus mandibularis of fifth nerve. vii. _mn._ Mandibular (spiracular) branch of seventh nerve. v. _mx._ Ramus maxillæ superioris of fifth nerve. _nl._ Nervus lateralis. _ol._ Olfactory pit. _op._ Eye. v. _opth._ Ramus ophthalmicus of fifth nerve. _pch._ Parachordal cartilage. _pfal._ Processus falciformis. _pp._ Head cavity. _pr._ Posterior root of spinal nerve. _rt._ Retina. _sp._ Spiracle. v. Fifth nerve. vii. Seventh nerve. _vc._ Visceral cleft. _vg._ Vagus nerve. _vg.br._ Branchial branch of vagus. _vgcom._ Commissure uniting the roots of the vagus, and continuous with commissure uniting the posterior roots of the spinal nerves. _vgr._ Roots of vagus nerves in the brain. _vgin._ Intestinal branch of vagus. _vh._ Vitreous humour.

Fig. 1. Diagram of cranial nerves at stage L.

A description of the part of this referring to the vagus and glossopharyngeal nerves is given at p. 426. It should be noticed that there are only five strands indicated as springing from the spinal cord to form the vagus and glossopharyngeal nerves. It is however probable that there are even from the first a greater number of strands than this.

Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A, ocul. 2.

It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.

Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a, ocul. 2.

It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v. _oth_).

Figs. 4_a_ and 4_b_. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.

4_a_ is the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.

4_b_ is a slightly more ventral section, and shews the course of the fifth nerve.

Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B, ocul. 2.

There appears to be one root in the brain for the glossopharyngeal, and at least six for the vagus. The fibres from the roots divide in many cases into two bundles before leaving the brain. Swellings of the brain towards the interior of the fourth ventricle are in connection with the first five roots of the vagus, and the glossopharyngeal root; and a swelling is also intercalated between the first vagus root and the glossopharyngeal root.

Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B, ocul. 2.

The figure shews (1) the rudimentary processus falciformis (_pfal_) giving origin to the vitreous humour; and (2) the hyaloid membrane (_hym_) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis.