CHAPTER VI.

GANOIDEI[33].

[33] The following classification of the Ganoidei is employed in the present chapter:

I. Selachoidei. {Acipenseridæ. {Polyodontidæ.

{Polypteridæ. II. Teleostoidei. {Amiidæ. {Lepidosteidæ.

It is only within quite recent times that any investigations have been made on the embryology of this heterogeneous, but primitive group of fishes. Much still remains to be done, but we now know the main outlines of the development of Acipenser and Lepidosteus, which are representatives of the two important subdivisions of the Ganoids. Both types have a complete segmentation, but Lepidosteus presents in its development some striking approximations to the Teleostei. I have placed at the end of the chapter a few remarks with reference to the affinities indicated by the embryology.

ACIPENSER[34].

[34] Our knowledge of the development of Acipenser is in the main derived from Salensky's valuable observations. His full memoir is unfortunately published in Russian, and I have been obliged to satisfy myself with the abstract (No. 90), and with what could be gathered from his plates. Prof. Salensky very kindly supplied me with some embryos; and I have therefore been able to some extent to work over the subject myself. This is more especially true for the stages after hatching. The embryos of the earlier stages were not sufficiently well preserved for me to observe more than the external features and a few points with reference to the formation of the layers.

The freshly laid ovum is 2 mm. in diameter and is invested by a two-layered shell, covered by a cellular layer derived from the follicle[35]. The segmentation, though complete, approaches the meroblastic type more nearly than the segmentation of the frog's egg. The first furrow appears at the formative pole, at which the germinal vesicle was situated. The earlier phases of the segmentation are like those of meroblastic ova, in that the furrows only penetrate for a certain distance into the egg. Eight vertical furrows appear before the first equatorial furrow; which is somewhat irregular, and situated close to the formative pole.

[35] Seven micropylar apertures, six of which form a circle round the seventh, are stated by Kowalevsky, Wagner, and Owsjannikoff (No. 89) to be present at one of the poles of the inner egg membrane. They are stated by Salensky to vary in number from five to thirteen.

In the later stages the vertical furrows extend through the whole egg, and a segmentation cavity appears between the small and the large spheres. The segmentation is thus in the main similar to that of a frog, from which it diverges in the fact that there is a greater difference in size between the small and the large segments.

[FIG. 50. EMBRYOS OF ACIPENSER VIEWED FROM THE DORSAL SURFACE. (After Salensky.)

A. Stage before the appearance of the mesoblastic somites. B. Stage with five somites.

_Mg._ medullary groove; _bl.p._ blastopore; _s.d._ segmental duct; _Fb._ fore-brain; _Hb._ hind-brain; _m.s._ mesoblastic somite.]

In the final stages of the segmentation the cells become distinctly divided into two layers. A layer of small cells is placed at the formative pole, and constitutes the epiblast. The cells composing it are divided, like those of Teleostei, etc., into a superficial epidermic and a deeper nervous layer. The remaining cells constitute the primitive hypoblast (the eventual hypoblast and mesoblast); they form a great mass of yolk-cells at the lower pole, and also spread along the roof of the segmentation cavity, on the inner side of the epiblast.

A process of unsymmetrical invagination now takes place, which is in its essential features exactly similar to that in the frog or the lamprey, and I must refer the reader for the details of the process to the chapter on the Amphibia. The edge of the cap of epiblast forms an equatorial line. For the greater extent of this line the epiblast cells grow over the hypoblast, as in an epibolic gastrula, but for a small arc they are inflected. At the inflected edge an invagination of cells takes place, underneath the epiblast, towards the segmentation cavity, and gives rise to the dorsal wall of the mesenteron and the main part of the dorsal mesoblast. The slit below the invaginated layer gradually dilates to form the alimentary cavity; the ventral wall of which is at first formed of yolk-cells. The epiblast along the line of the invaginated cells soon becomes thickened, and forms a medullary plate, which is not very distinct in surface views. The cephalic extremity of this plate, which is furthest removed from the edge, dilates, and the medullary plate then assumes a spatula form (fig. 50 A, _Mg_).

By the continued extension of the epiblast the uncovered part of the hypoblast has in the meantime become reduced to a small circular pore--the blastopore--and in surface views of the embryo has the form represented in fig. 50 A, _bl.p_. The invagination of the mesenteron has in the meantime extended very far forwards, and the segmentation cavity has become obliterated. The lip of the blastopore has moreover become inflected for its whole circumference.

The invaginated cells forming the dorsal wall of the mesenteron soon become divided into a pigmented hypoblastic epithelium adjoining the lumen of the mesenteron (fig. 51, _En_) and a mesoblastic layer (_Sgp_), between the hypoblast and the epiblast. The mesoblast is divided into two plates, between which is placed the notochord[36] (_Ch_).

[36] Salensky believes that the notochord is derived from the mesoblast. I could not satisfy myself on this point.

With the completion of the medullary plate and the germinal layers, the first embryonic period may be considered to come to a close. The second period ends with the hatching of the embryo. During it the rudiments of the greater number of organs make their appearance. The general form of the embryo during this period is shewn in figs. 50 B and 52 A and B.

One of the first changes to take place is the conversion of the medullary plate into the medullary canal. This, as shewn in fig. 51, is effected in the usual vertebrate fashion, by the establishment of a medullary groove which is then converted into a closed canal by the folding over of the sides.

The uncovered patch of yolk in the blastoporic area soon becomes closed over; and on the formation of the medullary canal the usual neurenteric canal becomes established.

[FIG. 51. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER EMBRYO. (After Salensky.)

_Rf._ medullary groove; _Mp._ medullary plate; _Wg._ segmental duct; _Ch._ notochord; _En._ hypoblast; _Sgp._ mesoblastic somite; _Sp._ parietal part of mesoblastic plate.]

The further changes which take place are in the main similar to those in other Ichthyopsida, but in some ways the appearance of the embryo is, as may be gathered from fig. 52, rather strange. This is mainly due to the fact that the embryo does not become folded off from the yolk in the manner usual in Vertebrates; and as will be shewn in the sequel, the relation of the yolk to the embryo is unlike that in any other known Vertebrate. The appearance of the embryo is something like that of an ordinary embryo slit open along the ventral side and then flattened out. Organs which properly belong to the ventral side appear on the lateral parts of the dorsal surface. Owing to the great forward extension of the yolk the heart (fig. 52 B) appears to be placed directly in front of the head.

Even before the formation of the medullary canal the cephalic portion of the nervous system becomes marked out. This part, after the closure of the medullary groove, becomes divided into two (fig. 50 B), and then three lobes--the fore-, the mid-, and the hind-brain (fig. 52, A and B). From the lateral parts of the at first undivided fore-brain the optic vesicles (fig. 52 B, _Op_) soon sprout out; and in the hind-brain a dilatation to form the fourth ventricle appears in the usual fashion.

[FIG. 52. EMBRYOS OF ACIPENSER BELONGING TO TWO STAGES VIEWED FROM THE DORSAL SURFACE. (After Salensky.)

_Fb._ fore-brain; _Mb._ mid-brain; _Hb._ hind-brain; _cp._ cephalic plate; _Op._ optic vesicle; _Auv._ auditory vesicle; _Olp._ olfactory pit; _Ht._ heart; _Md._ mandibular arch; _Ha._ hyoid arch; _Br´._ first branchial arch; _Sd._ segmental duct.]

The epiblast at the sides of the brain constitutes a more or less well-defined structure, which may be spoken of as a cephalic plate (fig. 52 A, _cp_). From this plate are formed the essential parts of the organs of special sense. Anteriorly the olfactory pits arise (fig. 52 B, _Olp_) as invaginations of both layers of the epiblast. The lens of the eye is formed as an ingrowth of the nervous layer only, and opposite the hind-brain the auditory sack (fig. 52 A and B, _Auv_) is similarly formed from the nervous layer of the epiblast. At the sides of the cephalic plate the visceral arches make their appearance; and in fig. 52 A and B there are shewn the mandibular (_Md_), hyoid (_Ha_) and first branchial (_Br´_) arches, with the hyomandibular (spiracle) and hyobranchial clefts between them. They constitute peculiar concentric circles round the cephalic plate; their shape being due to the flattened form of the embryo, already alluded to.

While the above structures are being formed in the head the changes in the trunk have also been considerable. The mesoblastic plates at the junction of the head and trunk become very early segmented, the segments being formed from before backwards (fig. 50 B). With their formation the trunk rapidly increases in length. At their outer border the segmental duct (fig. 50 B, and fig. 52 A, _Sd_) is very early established. It is formed, as in Elasmobranchs, as a solid outgrowth of the mesoblast (fig. 51, _Wg_); but its anterior extremity becomes converted into a pronephros (fig. 57, _pr.n_).

Before hatching, the embryo has to a small extent become folded off from the yolk both anteriorly and posteriorly; and has also become to some extent vertically compressed. As a result of these changes, the general form of its body becomes much more like that of an ordinary Teleostean embryo.

The general features of the larva after hatching are illustrated by figs. 53, 54 and 55. Fig. 53 represents a larva of about 7 mm. and fig. 54 a lateral and fig. 55 a ventral view of the head of a larva of about 11 mm.

[FIG. 53. LARVA OF ACIPENSER OF 7 MM., SHORTLY AFTER HATCHING.

_ol._ olfactory pit; _op._ optic vesicle; _sp._ spiracle; _br.c._ branchial clefts; _an._ anus.]

There are only a few points which call for special attention in the general form of the body. In the youngest larva figured the ventral part of the hyomandibular cleft is already closed: the dorsal part of the cleft is destined to form the spiracle (_sp_). The arch behind is the hyoid: on its posterior border is a membranous outgrowth, which will develop into the operculum. In older larvæ, a very rudimentary gill appears to be developed on the front walls of the spiracular cleft (Parker), but I have not succeeded in satisfying myself about its presence; and rows of gill papillæ appear on the hyoid and the true branchial arches (figs. 54 and 55, _g_). The biserially-arranged gill papillæ of the true branchial arches are of considerable length, and are not at first covered by the operculum; but they do not form elongated thread-like external gills similar to those of the Elasmobranchii.

The oral cavity is placed on the ventral side of the head; it has at first a more or less rhomboidal form. It soon however (fig. 55) becomes narrowed to a slit with projecting lips, and eventually becomes converted into the suctorial mouth of the adult. The most remarkable feature connected with the mouth is the development of provisional teeth (fig. 55) on both jaws.

These teeth were first discovered by Knock (No. 88). They do not appear to be calcified, and might be supposed to be of the same nature as the horny teeth of the Lamprey. They are however developed like true teeth, as a deposit between a papilla of subepidermic tissue and an epidermic cap. The substance of which they are formed corresponds morphologically to the enamel of ordinary teeth. As they grow they pierce the epidermis, and form hollow spine-like structures with a central axis filled with subepidermic (mesoblastic) cells. They disappear after the third month of larval life.

In front of the mouth two pairs of papillæ grow out, which appear to be of the same nature as the papillæ on the suctorial disc in the embryo of Lepidosteus (_vide_ p. 115). They are very short in the embryo represented in fig. 53; soon however they grow in length (figs. 54 and 55, _st_); and it is probable that they become the barbels, since these occupy a precisely similar position[37].

[37] If these identifications are correct the barbels of fishes must be phylogenetically derived from the papillæ of a suctorial disc adjoining the mouth.

[FIG. 54. SIDE VIEW OF A LARVA OF ACIPENSER OF 11 MILLIMETRES.

_op._ eye; _ol._ olfactory pit; _st._ suctorial (?) processes; _m._ mouth; _sp._ spiracle; _g._ gills.]

The openings of the nasal pits are at first single; but the opening of each becomes gradually divided into two by the growth of a flap on the outer side (fig. 54, _ol_). It is probable that this flap is equivalent to the fold of the superior maxillary process of the Amniota, which by its growth roofs over the open groove which originally leads from the external to the internal nares; so that the two openings of each nasal sack, so established in these and in other fishes, correspond to the external and internal nares of higher Vertebrata.

[FIG. 55. VENTRAL VIEW OF A LARVA OF ACIPENSER OF 11 MILLIMETRES.

_m._ mouth; _st._ suctorial (?) processes; _op._ eye; _g._ gills.]

At the time of hatching there is a continuous dorso-ventral fin, which, by atrophy in some parts, and hypertrophy in other parts, gives rise to all the unpaired fins of the adult, except the first dorsal and the abdominal. The caudal part of the fin is at first symmetrical, and the heterocercal tail is produced by the special growth of the ventral part of the fin.

[FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF A LARVA OF ACIPENSER TO SHEW THE POSITION OCCUPIED BY THE YOLK.

_in._ intestine; _st._ stomach filled with yolk; _oes._ oesophagus; _l._ liver; _ht._ heart; _ch._ notochord; _sp.c._ spinal cord.]

Of the internal features of development in the Sturgeon the most important concern the relation of the yolk to the alimentary tract. In most Vertebrata the yolk-cells form a protuberance of the part of the alimentary canal, immediately behind the duodenum. The yolk may either, as in the lamprey or frog, form a simple thickening of the alimentary wall in this region, or it may constitute a well-developed yolk-sack as in Elasmobranchii and the Amniota. In either case the liver is placed in front of the yolk. In the Sturgeon on the contrary the yolk is placed almost entirely in front of the liver, and the Sturgeon appears to be also peculiar in that the yolk, instead of constituting an appendage of the alimentary tract, is completely enclosed in a dilated portion of the tract which becomes the stomach (figs. 56 and 57). It dilates this portion to such extent that it might be supposed to form a true external yolk-sack. In the stages before hatching the glandular hypoblast, which was established on the dorsal side of the primitive mesenteron, envelops the yolk-cells, which fuse together into a yolk-mass, and lose all trace of their original cellular structure.

The peculiar flattening out of the embryo over the yolk (_vide_ p. 105) is no doubt connected with the mode in which the yolk becomes enveloped by the hypoblast.

[FIG. 57. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A LARVA OF ACIPENSER 5 MM. IN LENGTH.

_st._ epithelium of stomach; _yk._ yolk; _ch._ notochord, below which is a subnotochordal rod; _pr.n._ pronephros; _ao._ aorta; _mp._ muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres; _sp.c._ spinal cord; _b.c._ body cavity.]

As the posterior part of the trunk, containing the intestine, becomes formed, the yolk is gradually confined to the anterior part of the alimentary tract, which, as before stated, becomes the stomach. The epithelial cells of the stomach, as well as those of the intestine, are enormously dilated with food-yolk (fig. 57, _st_). Behind the stomach is formed the liver. The subintestinal vein bringing back the blood to the liver appears to have the same course as in Teleostei, in that the blood, after passing through the liver, is distributed to the walls of the stomach and is again collected into a venous trunk which falls into the sinus venosus. As the yolk becomes absorbed, the liver grows forwards underneath the stomach till it comes in close contact with the heart. The relative position of the parts at this stage is shewn diagrammatically in fig. 56. At the commencement of the intestine there arises in the larva of about 14 mm. a great number of diverticula, which are destined to form the compact glandular organ, which opens at this spot in the adult. At this stage there is also a fairly well developed pancreas opening into the duodenum at the same level as the liver.

No trace of the air-bladder was present at the stage in question.

The spiral valve is formed, as in Elasmobranchii, as a simple fold in the wall of the intestine.

There is a well developed subnotochordal rod (fig. 57) which, according to Salensky, becomes the subvertebral ligament of the adult; a statement which confirms an earlier suggestion of Bridge. The pronephros (head-kidney) resembles in the main that of Teleostei (fig. 57); while the front end of the mesonephros, which is developed considerably later than the pronephros, is placed some way behind it. In my oldest larva (14 mm.) the mesonephros did not extend backwards into the posterior part of the abdominal cavity.

BIBLIOGRAPHY.

(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung." _Bull. Soc. Nat._ Moscow, 1871.

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick. d. Störe." Vorläuf. Mittheilung. _Mélanges Biologiques tirés du Bulletin d. l'Acad. Imp. St Pétersbourg_, Vol. VII. 1870.

(90) W. Salensky. "Development of the Sterlet (Acipenser ruthenus)." 2 Parts. _Proceedings of the Society of Naturalists in the imperial University of Kasan._ 1878 and 9 (Russian). Part I., abstracted in Hoffmann and Schwalbe's _Jahresbericht_ for 1878.

(91) W. Salensky. "Zur Embryologie d. Ganoiden (Acipenser)." _Zoologischer Anzeiger_, Vol. I., Nos. 11, 12, 13.

LEPIDOSTEUS[38].

[38] Alexander Agassiz was fortunate enough to succeed in procuring and rearing a batch of eggs of this interesting form. He has given an adequate account of the external characters of the post-embryonic stages, and very liberally placed his preserved material of the stages both before and after hatching at Prof. W. K. Parker's and my disposal. The account of the stages prior to hatching is the result of investigations carried on by Professor Parker's son, Mr W. N. Parker, and myself on the material supplied to us by Agassiz. This material was not very satisfactorily preserved, but I trust that our results are not without some interest.

The ova of Lepidosteus are spherical bodies of about 3 mm. in diameter. They are invested by a tough double membrane, composed of (1) an outer layer of somewhat pyriform bodies, radiately arranged, which appear to be the remains of the follicular cells; and (2) of an inner zona radiata, the outer part of which is radiately striated, while the inner part is homogeneous.

[FIG. 58. SURFACE VIEW OF THE OVUM OF LEPIDOSTEUS WITH THE MEMBRANES REMOVED ON THE THIRD DAY AFTER IMPREGNATION.]

The segmentation, as in the Sturgeon, is complete, but approaches closely the meroblastic type. It commences with a vertical furrow at the animal pole, extending through about one-fifth of the circumference. Before this furrow has proceeded further a second furrow is formed at right angles to it. The next stages have not been observed, but on the third day after impregnation (fig. 58), the animal pole is completely divided into small segments, which form a disc similar to the blastoderm of meroblastic ova; while the vegetative pole, which subsequently forms a large yolk-sack, is divided by a few vertical furrows, four of which nearly meet at the pole opposite the blastoderm. The majority of the vertical furrows extend only a short way from the edge of the small spheres, and are partially intercepted by imperfect equatorial furrows.

The stages immediately following the segmentation are still unknown, and in the next stage satisfactorily observed, on the fifth day after impregnation, the body of the embryo is distinctly differentiated. The lower pole of the ovum is then formed of a mass in which no traces of segments or segmentation furrows can be detected.

The embryo (fig. 59) has a dumbbell-shaped outline, and is composed of (1) an outer area, with some resemblance to the area pellucida of an avian embryo, forming the lateral part of the body; and (2) a central portion consisting of the vertebral plates and medullary plate. The medullary plate is dilated in front to form the brain (_br_). Two lateral swellings in the brain are the commencing optic vesicles. The caudal extremity of the embryo is somewhat swollen.

[FIG. 59. SURFACE VIEW OF A LEPIDOSTEUS EMBRYO ON THE FIFTH DAY AFTER IMPREGNATION.

_br._ dilated extremity of medullary plate which forms the rudiment of the brain.]

Sections of this stage (fig. 60) are interesting as shewing a remarkable resemblance between Lepidosteus and Teleostei.

The three layers are fully established. The epiblast (_ep_) is formed of a thicker inner nervous stratum, and an outer flattened epidermic stratum. Along the axial line there is a solid keel-like thickening of the nervous layer of the epidermis, which projects towards the hypoblast. This thickening (_MC_) is the medullary cord; and there is no evidence of the epidermic layer being at this or any subsequent period concerned in its formation (_vide_ chapter on Teleostei, p. 72). In the region of the brain the medullary cord is so thick that it gives rise, as in Teleostei, to a projection of the whole body of the embryo towards the yolk. Posteriorly it is flatter. The mesoblast (_Me_) in the trunk has the form of two plates, which thin out laterally. The hypoblast (_hy_) is a single layer of cells, and is nowhere folded in to form a closed alimentary canal. The hypoblast is separated from the neural cord by the notochord (_Ch_), which throughout the greater part of the embryo is a distinct structure.

[FIG. 60. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY AFTER IMPREGNATION.

_MC._ medullary cord; _Ep._ epiblast; _Me._ mesoblast; _hy._ hypoblast; _Ch._ notochord.]

In the region of the tail, the axial part of the hypoblast, the notochord, and the neural cord fuse together, the fused part so formed is the homologue of the neurenteric canal of other types. Quite at the hinder end of the embryo the mesoblastic plates cease to be separable from the axial structures between them.

In a somewhat later stage the embryo is considerably more elongated, embracing half the circumference of the ovum. The brain is divided into three distinct vesicles.

Anteriorly the neural cord has now become separated from the epidermis. The whole of the thickened nervous layer of the epiblast appears to remain united with the cerebrospinal cord, so that the latter organ is covered dorsally by the epidermic layer of the epiblast only. The nervous layer soon however grows in again from the two sides.

Where the neural cord is separated from the epidermis, it is already provided with a well-developed lumen. Posteriorly it remains in its earlier condition.

In the region of the hind-brain traces of the auditory vesicles are present in the form of slightly involuted thickenings of the nervous layer of the epidermis.

[FIG. 61. EMBRYO OF LEPIDOSTEUS ON THE SIXTH DAY AFTER IMPREGNATION.

_op._ optic vesicles; _br.c._ branchial clefts (?); _s.d._ segmental duct.

N.B. The branchial clefts and segmental duct are somewhat too prominent.]

The mesoblast of the trunk is divided anteriorly into splanchnic and somatic layers.

In the next stage, on the sixth day after impregnation (fig. 61), there is a great advance in development. The embryo is considerably longer, and a great number of mesoblastic somites are visible. The body is now laterally compressed and raised from the yolk.

The region of the head is more distinct, and laterally two streaks are visible (_br.c_), which, by comparison with the Sturgeon, would seem to be the two first visceral clefts[39]: they are not yet perforated. In the lateral regions of the trunk the two segmental ducts are visible in surface views (fig. 61, _sd_) occupying the same situation as in the Sturgeon. Their position in section is shewn in fig. 62, _sg_.

[39] I have as yet been unable to make out these structures in section.

[FIG. 62. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION.

_mc._ medullary cord; _ms._ mesoblast; _sg._ segmental duct; _ch._ notochord; _x._ subnotochordal rod; _hy._ hypoblast.]

With reference to the features in development, visible in sections, a few points may be alluded to.

The optic vesicles are very prominent outgrowths of the brain, but are still solid, though the anterior cerebral vesicle has a well-developed lumen. The auditory vesicles are now deep pits of the nervous layer of the epiblast, the openings of which are covered by the epidermic layer. They are shewn for a slightly later stage in fig. 63 (_au.v_).

There is now present a subnotochordal rod, which develops as in other types from a thickening of the hypoblast (fig. 62, _x_).

[FIG. 63. SECTION THROUGH THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION.

_au.v._ auditory vesicle; _au.n._ auditory nerve; _ch._ notochord; _hy._ hypoblast.]

In an embryo of the seventh day after impregnation, the features of the preceding stage become generally more pronounced.

The optic vesicles are now provided with a lumen (fig. 64), and have approached close to the epidermis. Adjoining them a thickening (_l_) of _the nervous layer_ of the epidermis has appeared, which will form the lens. The cephalic extremity of the segmental duct, which, as shewn in fig. 61, is bent inwards towards the middle line, has now become slightly convoluted, and forms the rudiment of a pronephros (head-kidney).

[FIG. 64. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION.

_al._ alimentary tract; _fb._ thalamencephalon; _l._ lens of eye; _op.v._ optic vesicle. The mesoblast is not represented.]

During the next few days the folding off of the embryo from the yolk commences, and proceeds till the embryo acquires the form represented in fig. 65.

Both the head and tail are quite free from the yolk; and the embryo presents a general resemblance to that of a Teleostean.

On the ventral surface of the front of the head there is a disc (figs. 65, 66, _sd_), which is beset with a number of processes, formed as thickenings of the epiblast. As shewn by Agassiz, these eventually become short suctorial papillæ[40]. Immediately behind this disc is placed a narrow depression which forms the rudiment of the mouth.

[40] These papillæ are very probably sensitive structures; but I have not yet investigated their histological characters.

The olfactory pits are now developed, and are placed near the front of the head.

A great advance has taken place in the development of the visceral clefts and arches. The oral region is bounded behind by a well-marked mandibular arch, which is separated by a shallow depression from a still more prominent hyoid arch (fig. 65, _hy_). Between the hyoid and mandibular arches a double lamella of hypoblast, which represents the hyomandibular cleft, is continued from the throat to the external skin, but does not, at this stage at any rate, contain a lumen.

The hyoid arch is prolonged backwards into a considerable opercular fold, which to a great extent overshadows the branchial clefts behind. The hyobranchial cleft is widely open.

Behind the hyobranchial cleft are four pouches of the throat on each side, not yet open to the exterior. They are the rudiments of the four branchial clefts of the adult.

The trunk has the usual compressed piscine form, and there is a well-developed dorsal fin continuous round the end of the tail, with a ventral fin. There is no trace of the paired fins.

The anterior and posterior portions of the alimentary tract are closed in, but the middle region is still open to the yolk. The circulation is now fully established, and the vessels present the usual vertebrate arrangement. There is a large subintestinal vein.

[FIG. 65. EMBRYO OF LEPIDOSTEUS SHORTLY BEFORE HATCHING.

_ol._ olfactory pit; _sd._ suctorial disc; _hy._ hyoid arch.]

The first of Agassiz' embryos was hatched about ten days after impregnation. The young fish on hatching immediately used its suctorial disc to attach itself to the sides of the vessel in which it was placed.

[FIG. 66. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC.

_m._ mouth; _op._ eye; _s.d._ suctorial disc.]

The general form of Lepidosteus shortly after hatching is shewn in fig. 67. On the ventral part of the front of the head is placed the large suctorial disc. At the side of the head are seen the olfactory pit, the eye and the auditory vesicle; while the projecting vesicle of the mid-brain is very prominent above. Behind the mouth follow the visceral arches. The mandibular arch (_md_) is placed on the hinder border of the mouth, and is separated by a deep groove from the hyoid arch (_hy_). This groove is connected with the hyomandibular cleft, but I have not determined whether it is now perforated. The posterior border of the hyoid arch is prolonged into an opercular fold. Behind the hyoid arch are seen the true branchial arches.

[FIG. 67. LARVA OF LEPIDOSTEUS SHORTLY AFTER HATCHING. (After Parker.)

_ol._ olfactory pit; _op._ optic vesicle; _au.v._ auditory vesicle; _mb._ mid-brain; _sd._ suctorial disc; _md._ mandibular arch; _hy._ hyoid arch with operculum; _br._ branchial arches; _an._ anus.]

There is still a continuous dorso-ventral fin, in which there are as yet no fin-rays, and the anterior paired fins are present.

The yolk-sack is very large, but its communication with the alimentary canal is confined to a narrow vitelline duct, which opens into the commencement of the intestine _immediately behind_ the duct of the liver, which is now a compact gland. The yolk in Lepidosteus thus behaves very differently from that in the Sturgeon. In the first place it forms a special external yolk-sack, instead of an internal dilatation of part of the alimentary tract; and in the second place it is placed behind instead of in front of the liver.

I failed to find any trace of a pancreas. There is however, _opening on the dorsal side of the throat_, a well-developed appendage continued backwards beyond the level of the commencement of the intestine. This appendage is no doubt the air-bladder.

In the course of the further growth of the young Lepidosteus, the yolk-sack is rapidly absorbed, and has all but disappeared after three weeks. A rich development of pigment early takes place; and the pigment is specially deposited on the parts of the embryonic fin which will develop into the permanent fins.

The notochord in the tail bends slightly upwards, and by the special development of a caudal lobe an externally heterocercal tail like that of Acipenser is established. The ventral paired fins are first visible after about the end of the third week, and by this time the operculum has grown considerably, and the gills have become well developed.

The most remarkable changes in the later periods are those of the mouth.

[FIG. 68. HEAD OF AN ADVANCED LARVA OF LEPIDOSTEUS. (After Parker.)

_ol._ openings of the olfactory pit; _sd._ remains of the larval suctorial disc.]

The upper and lower jaws become gradually prolonged, till they eventually form a snout; while at the end of the upper jaw is placed the suctorial disc, which is now considerably reduced in size (fig. 68, _sd_). The "fleshy globular termination of the upper jaw of the adult Lepidosteus is the remnant of this embryonic sucking disc." (Agassiz, No. 92.)

The fin-rays become formed as in Teleostei, and parts of the continuous embryonic fin gradually undergo atrophy. The dorsal limb of the embryonic tail, as has been shewn by Wilder, is absorbed in precisely the same manner as in Teleostei, leaving the ventral lobe to form the whole of the permanent tail-fin.

BIBLIOGRAPHY.

(92) Al. Agassiz. "The development of Lepidosteus." _Proc. Amer. Acad. of Arts and Sciences_, Vol. XIII. 1878.

_General observations on the Embryology of the Ganoids._

The very heterogeneous character of the Ganoid group is clearly shewn both in its embryology and its anatomy. The two known types of formation of the central nervous system are exemplified in the two species which have been studied, and these two species, though in accord in having a holoblastic segmentation, yet differ in other important features of development, such as the position of the yolk etc. Both types exhibit Teleostean affinities in the character of the pronephros; but as might have been anticipated Lepidosteus presents in the origin of the nervous system, the relations of the hypoblast, and other characters, closer approximations to the Teleostei than does Acipenser. There are no very prominent Amphibian characters in the development of either type, other than a general similarity in the segmentation and formation of the layers. In the young of Polypterus an interesting amphibian and dipnoid character is found in the presence of a pair of true external gills covered by epiblast. These gills are attached at the hinder end of the operculum, and receive their blood from the hyoid arterial arch[41]. In the peculiar suctorial disc of Lepidosteus, and in the more or less similar structure in the Sturgeon, these fishes retain, I believe, a very primitive vertebrate organ, which has disappeared in the adult state of almost all the Vertebrata; but it is probable that further investigations will shew that the Teleostei, and especially the Siluroids, are not without traces of a similar structure.

[41] Vide Steindachner, _Polypterus Lapradei_, &c., and Hyrtl, "Ueber d. Blutgefässe, &c." _Sitz. Wiener Akad._, Vol. LX.