PART III.

THE DEVELOPMENT OF PERIPATUS CAPENSIS.

[The remarkable discoveries about the early development of Peripatus, which Balfour made in June last, shortly before starting for Switzerland, have already been the subject of a short communication to the Royal Society (_Proc. Roy. Soc._ No. 222, 1882). They relate (1) to the blastopore, (2) to the origin of the mesoblast.

Balfour left no manuscript account or notes of his discovery in connection with the drawings which he prepared in order to illustrate it, but he spoke about it to Professor Ray Lankester and also to us, and he further gave a short account of the matter in a private letter to Professor Kleinenberg.

In this letter, which by the courtesy of Professor Kleinenberg we have been permitted to see, he describes the blastopore as an elongated slit-like structure extending along nearly the whole ventral surface; and further states, as the result of his examination of the few and ill-preserved embryos in his possession, that the mesoblast appears to originate as paired outgrowths from the lips of the blastopore.

The drawings left by Balfour in connection with the discoveries are four in number: one of the entire embryo, shewing the slit-like blastopore and the mesoblastic somites, the other three depicting the transverse sections of the same embryo.

The first drawing (fig. 37), viz. that of the whole embryo, shews an embryo of an oval shape, possessing six somites, whilst along the middle of its ventral surface there are two slit-like openings, lying parallel to the long axis of the body, and placed one behind the other. The mesoblastic somites are arranged bilaterally in pairs, six on either side of these slits. The following note in his handwriting is attached to this drawing:

"Young larva of _Peripatus capensis_.--I could not make out for certain which was the anterior end. Length 1.34 millimetres."

Balfour's three remaining drawings (figs. 40-42) are, as already stated, representations of transverse sections of the embryo figured by him as a whole. They tend to shew, as he stated in the letter referred to above, that the mesoblast originates as paired outgrowths from the hypoblast, and that these outgrowths are formed near the junction of the hypoblast with the epiblast at the lips of the blastopore.

In fig. 42 the walls of the mesoblastic somites appear continuous with those of the mesenteron near the blastopore.

In fig. 40, which is from a section a little in front of fig. 42, the walls of the mesoblastic somites are independent of those of the mesenteron.

Fig. 41 is from a section made in front of the region of the blastopore.

In all the sections the epiblast lying over the somites is thickened, while elsewhere it is formed of only one layer of cells; and this thickening subsequently appears to give rise to the nervous system. Balfour in his earlier investigations on the present subject found in more advanced stages of the embryo the nerve-cords still scarcely separated from the epiblast[572].

Footnote 572: _Comparative Embryology_; original edition, Vol. I. p. 318. [This edition, Vol. II. p. 385.]

We have since found, in Balfour's material, embryos of a slightly different age to that just described. Of these, three (figs. 34, 35, 36) are younger, while one (fig. 38) is older than Balfour's embryo.

Stage A.--The youngest (fig. 34) is of a slightly oval form, and its greatest length is .48 mm. It possesses a blastopore, which is elongated in the direction of the long axis of the embryo, and is slightly narrower in its middle than at either end. From one end of the blastopore there is continued an opaque band. This we consider to be the posterior end of the blastopore of the embryo. The blastopore leads into the archenteron.

Stage B.--In the next stage (fig. 35) the embryo is elongate-oval in form. Its length is .7 mm. The blastopore is elongated and slightly narrowed in the middle. At the posterior end of the embryo there is a mass of opaque tissue. On each side of the blastopore are three mesoblastic somites. The length of the blastopore is .45 mm.

Stage C.--In the next stage (fig. 36) the features are much the same as in the preceding. The length of the whole embryo is .9 mm.

The following were the measurements of an embryo of this stage with five somites, but slightly younger than that from which fig. 36 was drawn.

Length of embryo .74 mm. " blastopore .46 " Distance between hind end of blastopore and hind end of body .22 " Distance between front end of body and front end of blastopore .06 "

The somites have increased to five, and there are indications of a sixth being budded off from the posterior mass of opaque tissue. The median parts of the lips of the blastopore have come together preparatory to the complete fusion by which the blastopore becomes divided into two parts.

Stage D.--The next stage is Balfour's stage, and has been already described.

The length is 1.34.

It will be observed, on comparing it with the preceding embryos, that while the anterior pair of somites in figs. 35 and 36 lie at a considerable distance from what we have called the anterior end of the embryo (_a_), in the embryo now under consideration they are placed at the anterior end of the body, one on each side of the middle line. We cannot speak positively as to how they come there, whether by a pushing forward of the anterior somites of the previous stage, or by the formation of new somites anteriorly to those of the previous stage.

In the next stage it is obvious that this anterior pair of somites has been converted into the præoral lobes.

The anterior of the two openings to which the blastopore gives rise is placed between the second pair of somites; we shall call it the embryonic mouth. The posterior opening formed from the blastopore is elongated, being dilated in front and continued back as a narrow slit (?) to very near the hind end of the embryo, where it presents a second slight dilatation. The anterior dilatation of the posterior open region of the blastopore we shall call the embryonic anus.

Lately, but too late to be figured with this memoir, we have been fortunate enough to find an embryo of apparently precisely the same stage as fig. 37. We are able, therefore, to give a few more details about the stage.

The measurements of this embryo were:

Length of whole embryo 1.32 mm. Distance from front end of body to front end of mouth .32 " Distance from embryonic mouth to hind end of embryonic anus .52 " Distance from hind end of embryonic anus to hind end of body .45 " Length of embryonic anus .2 " " part of blastopore behind embryonic anus .2 " Greatest width of embryo .64 "

Stage E.--In the next stage (figs. 38 and 39) the flexure of the hind end of the body has considerably increased. The anterior opening of the blastopore, the embryonic mouth, has increased remarkably in size. It is circular, and is placed between the second pair of mesoblastic somites. The anterior dilatation of the posterior opening of the blastopore, the embryonic anus, has, like the anterior opening, become much enlarged. It is circular, and is placed on the concavity of the ventral flexure. From its hind end there is continued to the hind end of the body a groove (shewn in fig. 39 as a dotted line), which we take to be the remains of the posterior slit-like part of the posterior opening of the blastopore of the preceding stage. The posterior dilatation has disappeared. The embryo has apparently about thirteen somites, which are still quite distinct from one another, and apparently do not communicate at this stage with the mesenteron.

The epiblast lying immediately over the somites is, as in the, earlier stages, thickened, and the thickenings of the two sides join each other in front of the embryonic mouth, where the anterior pair of mesoblastic somites (the præoral lobes) are almost in contact.

The median ventral epiblast, _i.e._ the epiblast in the area, bounded by the embryonic mouth and anus before and behind and by the developing nerve-cords laterally, is extremely thin, and consists of one layer of very flat cells. Over the dorsal surface of the body the epiblast cells are cubical, and arranged in one layer.

Measurements of Embryo of Stage E.

Length of embryo 1.12 mm. Greatest width .64 " Distance from front end of embryonic mouth to hind end of embryonic anus .48 " Greatest length of embryonic mouth .16 " Length between hind end of embryonic mouth and front end of embryonic anus .29 "

These measurements were made with a micrometer eyepiece, with the embryo lying on its back in the position of fig. 38, so that they simply indicate the length of the straight line connecting the respective points.

This is the last embryo of our series of young stages. The next and oldest embryo was 3.2 mm. in length. It had ringed antennæ, seventeen (?) pairs of legs, and was completely doubled upon itself, as in Moseley's figure.

The pits into the cerebral ganglia and a mouth and anus were present. There can be no doubt that the mouth and anus of this embryo become the mouth and anus of the adult.

The important question as to the connection between the adult mouth and anus, and the embryonic mouth and anus of the Stage E, must, considering the great gap between Stage E and the next oldest embryo, be left open. Meanwhile, we may point out that the embryonic mouth of Stage E has exactly the same position as that of the adult; but that the anus is considerably in front of the hind end of the body in Stage E, while it is terminal in the adult.

If the embryonic mouth and anus do become the adult mouth and anus, there would appear to be an entire absence of stomodæum and proctodæum in _Peripatus_, unless the buccal cavity represents the stomodæum. The latter is formed, as has been shewn by Moseley, by a series of outgrowths round the simple mouth-opening of the embryo, which enclosing the jaws give rise to the tumid lips of the adult.

For our determination of the posterior and anterior ends of each of these embryos, Stage A to E, we depend upon the opaque tissue seen in each case at one end of the blastopore.

In Stage A it has the form of a band, extending backwards from the blastopore.

In Stages B-D, it has the form of an opaque mass of tissue occupying the whole hind end of the embryo, and extending a short distance on either side of the posterior end of the blastopore.

This opacity is due in each case to a proliferation of cells of the hypoblast, and, perhaps, of the epiblast (?).

There can be no doubt that the mesoblast so formed gives rise to the great majority of the mesoblastic somites.

This posterior opacity is marked in Stage C by a slight longitudinal groove extending backwards from the hind end of the blastopore. This is difficult to see in surface views, and has not been represented in the figure, but is easily seen in sections.

But in Stage D this groove has become very strongly marked in surface views, and looks like a part of the original blastopore of Stage C.

Sections shew that it does not lead into the archenteron, but only into the mass of mesoblast which forms the posterior opacity. It presents an extraordinary resemblance to the primitive streak of vertebrates, and the ventral groove of insect embryos.

We think that there can be but little doubt that it is a part of the original blastopore, which, on account of its late appearance (this being due to the late development of the posterior part of the body to which it belongs), does not acquire the normal relations of a blastopore, but presents only those rudimentary features (deep groove connected with origin of mesoblast) which the whole blastopore of other tracheates presents.

We think it probable that the larval anus eventually shifts to the hind end of the body, and gives rise to the adult anus. We reserve the account of the internal structure of these embryos (Stages A-E) and of the later stages for a subsequent memoir.

We may briefly summarise the more important facts of the early development of _Peripatus capensis_, detailed in the preceding account.

1. The greater part of the mesoblast is developed from the walls of the archenteron.

2. The embryonic mouth and anus are derived from the respective ends of the original blastopore, the middle part of the blastopore closing up.

3. The embryonic mouth almost certainly becomes the adult mouth, _i.e._ the aperture leading from the buccal cavity into the pharynx, the two being in the same position. The embryonic anus is in front of the position of the adult anus, but in all probability shifts back, and persists as the adult anus.

4. The anterior pair of mesoblastic somites gives rise to the swellings of the præoral lobes, and to the mesoblast of the head[573].

Footnote 573: We have seen nothing in any of our sections which we can identify as of so-called mesenchymatous origin.

There is no need for us to enlarge upon the importance of these facts. Their close bearing upon some of the most important problems of morphology will be apparent to all, and we may with advantage quote here some passages from Balfour's _Comparative Embryology_, which shew that he himself long ago had anticipated and in a sense predicted their discovery.

"Although the mesoblastic groove of insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished from development." (_Comparative Embryology_, Vol. I. p. 378, the original edition[574].)

Footnote 574: This edition, Vol. II. p. 457.

"TRACHEATA.--Insecta. It (the mesoblast) grows inwards from the lips of the germinal groove, which probably represents the remains of a blastopore." (_Comparative Embryology_, Vol. II. p. 291, the original edition[575].)

Footnote 575: This edition, Vol. III. p. 352.

"It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been, in the first instance, derived from a pair of archenteric diverticula." (_Comparative Embryology_, Vol. II. p. 294, the original edition[576].)

Footnote 576: This edition, Vol. III. p. 356.

The facts now recorded were discovered in June last, only a short time before Balfour started for Switzerland; we know but little of the new ideas which they called up in his mind. We can only point to passages in his published works which seem to indicate the direction which his speculations would have taken.

After speculating as to the probability of a genetic connection between the circumoral nervous system of the Coelenterata, and the nervous system of Echinodermata, Platyhelminthes, Chætopoda, Mollusca, &c., he goes on to say:

"A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united in front and behind--exactly such a nervous system, in fact, as is present in many Nemertines (the Enopla and Pelagonemertes), in _Peripatus_ and in primitive molluscan types (Chiton, Fissurella, &c.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chætopoda and Arthropoda. It is especially deserving of notice, in connection with the nervous system of the above mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the dorsal side of the intestines. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nerve-ring which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor." (_Comparative Embryology_, Vol. II. pp. 311, 312, the original edition[577].)

Footnote 577: This edition, Vol. III. pp. 378, 379.

The facts of development here recorded give a strong additional support to this latter view, and seem to render possible a considerable extension of it along the same lines.]

LIST OF MEMOIRS ON PERIPATUS.

1. M. Lansdown Guilding. "An Account of a New Genus of Mollusca," _Zoological Journal_, Vol. II. p. 443, 1826.

2. M. Andouin and Milne-Edwards. "Classific. des Annélides et description de celles qui habitent les côtes de France," p. 411, _Ann. Scien. Nat._ ser. I. Vol. XXX. 1833.

3. M. Gervais. "Études p. servir à l'histoire naturelle des Myriapodes," _Ann. Scien. Nat._ ser. II. Vol. VII. 1837, p. 38.

4. Wiegmann. Wiegmann's _Archiv_, 1837.

5. H. Milne-Edwards. "Note sur le _Peripate juluforme_," _Ann. Scien. Nat._ ser. II. Vol. XVIII. 1842.

6. Blanchard. "Sur l'organisation des Vers," chap. IV. pp. 137-141, _Ann. Scien. Nat._ ser. III. Vol. VIII. 1847.

7. Quatrefages. "Anat. des Hermelles, note on," p. 57, _Ann. Scien. Nat._ ser. III. Vol. X. 1848.

8. Quatrefages. _Hist. Nat. des Annelés_, 1865, Appendix, pp. 675-6.

9. De Blainville. _Suppl. au Dict. des Sc. Nat._ Vol. I.

10. Ed. Grube. "Untersuchungen üb. d. Bau von _Peripatus Edwardsii_," _Archiv für Anat. und Physiol._ 1853.

11. Saenger. "Moskauer Naturforscher Sammlung," _Abth. Zool._ 1869.

12. H. N. Moseley. "On the Structure and Development of _Peripatus capensis_," _Proc. Roy. Soc._ No. 153, 1874.

13. H. N. Moseley. "On the Structure and Development of _Peripatus capensis_," _Phil. Trans._ Vol. CLXIV. 1874.

14. H. N. Moseley. "Remarks on Observations by Captain Hutton, Director of the Otago Museum, on _Peripatus novæ zealandiæ_," _Ann. and Mag. of Nat. History_, Jan. 1877.

15. Captain Hutton. "Observations on _Peripatus novæ zealandiæ_," _Ann. and Mag. of Nat. History_, Nov. 1876.

16. F. M. Balfour. "On Certain Points in the Anatomy of _Peripatus capensis_," _Quart. Journ. of Micr. Science_, Vol. XIX. 1879.

17. A. Ernst. _Nature_, March 10th, 1881.

EXPLANATION OF PLATES 46-53[578].

Footnote 578: The explanations of the figures printed within inverted commas are by Professor Balfour, the rest are by the Editors.

COMPLETE LIST OF REFERENCE LETTERS.

A. Anus. _a._ Dorso-lateral horn of white matter in brain. _a.g._ Accessory gland of male (modified accessory leg gland). _at._ Antenna. _at.n._ Antennary nerve. _b._ Ventro-lateral horn of white matter of brain. _b.c._ Body-cavity. _bl._ Blastopore. C. Cutis. _c._ Postero-dorsal lobe of white matter of brain. _c.g._ Supra-oesophageal ganglia. _cl._ Claw. _c.m._ Circular layer of muscles. _co._ Commissures between the ventral nerve-cords. _co.2._ Second commissure between the ventral nerve-cords. _co_{1}. 2. Mass of cells developed on second commissure. _cor._ Cornea. _c.s.d._ Common duct for the two salivary glands. _cu._ Cuticle. _d._ Ventral protuberance of brain. _d.l.m._ Dorsal longitudinal muscle of pharynx. _d.n._ Median dorsal nerve to integument from supra-oesophageal ganglia. _d.o._ Muscular bands passing from the ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity. E. Eye. E. Central lobe of white matter of brain. _e.n._ Nerves passing outwards from the ventral cords. _ep._ Epidermis. _ep.c._ Epidermis cells. F.1, F.2, _&c._ First and second pair of feet, &c. _f._ Small accessory glandular tubes of the male generative apparatus. F._g._ Ganglionic enlargement on ventral nerve-cord, from which a pair of nerves to foot pass off. _f.gl._ Accessory foot-gland. F._n._ Nerves to feet. _g.co._ Commissures between the ventral nerve-cords containing ganglion cells. _g.o._ Generative orifice. H. Heart. _h._ Cells in lateral division of body-cavity. _hy._ Hypoblast. _i.j._ Inner jaw. _j._ Jaw. _j.n._ Nerves to jaws. L. Lips. _l._ Lens. _l.b.c._ Lateral compartment of body-cavity. _le._ Jaw lever (cuticular prolongation of inner jaw lying in a backwardly projecting diverticulum of the buccal cavity). _l.m._ Bands of longitudinal muscles. M. Buccal cavity. M{1}. Median backward diverticulum of mouth or common salivary duct which receives the salivary ducts. _me._ Mesenteron. _mes._ Mesoblastic somite. _m.l._ Muscles of jaw lever. _m.s._ Sheets of muscle passing round the side walls of pharynx to dorsal body-wall. _od._ Oviduct. _oe._ OEsophagus. _oes.co._ OEsophageal commissures. _o.f.g._ Orifice of duct of foot-gland. _o.j._ Outer jaw. _op._ Optic ganglion. _op.n._ Optic nerve. _or.g._ Ganglionic enlargements for oral papillæ. _or.n._ Nerves to oral papillæ. _or.p._ Oral papillæ. _o.s._ Orifice of duct of segmental organ. _ov._ Ovary. P. Pads on ventral side of foot. _p._ Common duct into which the vasa deferentia open. _p.c._ Posterior lobe of brain. _p.d.c._ Posterior commissure passing dorsal to rectum. _p.f._ Internal opening of nephridium into body-cavity. _ph._ Pharynx. _pi._ Pigment in outer ends of epidermic cells. _pi.r._ Retinal pigment. _p.n._ Nerves to feet. _p.p._ Primary papilla. _pr._ Prostate. R. Rectum. _Re._ Retinal rods. R. _m._ Muscle of claw. _s._ Vesicle of nephridium. _s_{1}. Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia. _s.c._1. Region No. 1 of coiled tube of nephridium. _s.c._2. Region No. 2 of ditto. _s.c._3. Region No. 3 of ditto. _s.c._4. Region No. 4 of ditto. _s.d._ Salivary duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland. _sl.g._ Tubules of slime gland. _s.o._1, 2, 3, _&c._ Nephridia of 1st, 2nd, &c., feet. _s.o.f._ Terminal portion of nephridium. _s.p._ Secondary papilla. _st._ Stomach. _st.e._ Epithelium of stomach. _sy._ Sympathetic nerve running in muscles of tongue and pharynx. _sy´_. Origin of pharyngeal sympathetic nerves. T. Tongue. _t._ Teeth on tongue. _te._ Testis. _tr._ Tracheæ. _tr.c._ Cells found along the course of the tracheæ. _tr.o._ Tracheal stigma. _tr.p._ Tracheal pit. _ut._ Uterus. _v.c._ Ventral nerve cord. _v.d._ Vas deferens. _v.g._ Imperfect ganglia of ventral cord.

PLATE 46.

Fig. 1. _Peripatus capensis_, x 4; viewed from the dorsal surface. (From a drawing by Miss Balfour.)

PLATE 47.

Fig. 2. A left leg of _Peripatus capensis_, viewed from the ventral surface; x 30. (From a drawing by Miss Balfour.)

Fig. 3. A right leg of _Peripatus capensis_, viewed from the front side. (From a drawing by Miss Balfour.)

Fig. 4. The last left (17th) leg of a male _Peripatus capensis_, viewed from the ventral side to shew the papilla at the apex of which the accessory gland of the male, or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is appended the following note. "Figure shewing the accessory genital gland of male, which opens on the last pair of legs by a papilla on the ventral side. The papilla has got a slit-like aperture at its extremity."

Fig. 5. Ventral view of head and oral region of _Peripatus capensis_. (From a drawing by Miss Balfour.)

PLATE 48.

Figs. 6 and 7 are from one drawing.

Fig. 6. _Peripatus capensis_ dissected so as to shew the alimentary canal, slime glands, and salivary glands; x 3. (From a drawing by Miss Balfour.)

Fig. 7. The anterior end of Fig. 6 enlarged; x 6. (From a drawing by Miss Balfour.) The dissection is viewed from the ventral side, and the lips, L., have been cut through in the middle line behind and pulled outwards, so as to expose the jaws, _j._, which have been turned outwards, and the tongue, T., bearing a median row of chitinous teeth, which branches behind into two. The junction of the salivary ducts, _s.d._, and the opening of the median duct so formed into the buccal cavity is also shewn. The muscular pharynx, extending back into the space between the 1st and 2nd pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S-shaped ventro-dorsal curve.

A. Anus. _at._ Antenna. F.1, F.2. First and second feet. _j._ Jaws. L. Lips. _oe._ OEsophagus. _or.p._ Oral papilla. _ph._ Pharynx. R. Rectum. _s.d._ Salivary duct. _s.g._ Salivary gland. _sl.d._ Slime reservoir. _sl.g._ Portion of tubules of slime gland. _st._ Stomach. T. Tongue in roof of mouth.

Fig. 8. _Peripatus capensis_, x 4; male. (From a drawing by Miss Balfour.) Dissected so as to shew the nervous system, slime glands, ducts of the latter passing into the oral papilla, accessory glands opening on the last pair of legs (enlarged crural glands), and segmental organs, viewed from dorsal surface. The first three pairs of segmental organs consist only of the vesicle and duct leading to the exterior. The fourth and fifth pairs are larger than the succeeding, and open externally to the crural glands. The ventral nerve-cords unite behind dorsal to the rectum.

A. Anus. _a.g._ Accessory generative gland, or enlarged crural gland of the 17th leg. _at._ Antenna. _c.g._ Supra-oesophageal ganglia with eyes. _co._ Commissures between the ventral nerve-cords. _d.n._ Large median nerve to dorsal integument from hinder part of brain. F.1, 2, &c. Feet. _g.o._ Generative orifice. _oe._ OEsophagus. _oes.co._ OEsophageal commissures. _or.p._ Oral papilla. _p.d.c._ Posterior dorsal commissure between the ventral nerve-cords. _ph._ Pharynx. _p.n._ Nerves to feet, one pair from each ganglionic enlargement. _sl.d._ Reservoir of slime gland. _sl.g._ Tubules of slime gland. _s.o._1, 2, 3, _&c._ Segmental organs. _v.c._ Ventral nerve-cords. _v.g._ Imperfect ganglia of ventral cords.

Figs. 9 and 10. Left jaw of _Peripatus capensis_ (male), shewing reserve jaws. (From a drawing by Miss Balfour.)

Fig. 9. Inner jaw.

Fig. 10. Outer jaw.

PLATE 49.

Figs. 11-16. A series of six transverse sections through the head of _Peripatus capensis._

Fig. 11. The section is taken immediately behind the junction of the supra-oesophageal ganglia, _c.g._, and passes through the buccal cavity, M., and jaws, _o.j._ and _i.j._

Fig. 12. The section is taken through the hinder part of the buccal cavity at the level of the opening of the mouth into the pharynx and behind the jaws. The cuticular rod-like continuation (_le._) of the inner jaw lying in a backwardly directed pit of the buccal cavity is shewn; on the right hand side the section passes through the opening of this pit.

Fig. 13. The section passes through the front part of the pharynx, and shews the opening into the latter of the median backward diverticulum of the mouth (M{1}), which receives the salivary ducts. It also shews the commencement of the ventral nerve-cords, and the backwardly projecting lobes of the brain.

Fig. 14. The section passes through the anterior part of the pharynx at the level of the second commissure (_co._ 2), between the ventral nerve-trunks, and shews the mass of cells developed on this commissure, which is in contact with the epithelium of the backward continuation of the buccal cavity (M{1}).

Fig. 15. Section through the point of junction of the salivary ducts with the median oral diverticulum.

Fig. 16. Section behind the pharynx through the oesophagus.

_b.c._ Body-cavity. C. Cutis. _c.b.c._ Central compartment of body-cavity. _c.g._ Supra-oesophageal ganglia. _c.m._ Layer of circular muscles. _co._ Commissure between ventral nerve-cords. _co._ 2. Second commissure between the ventral nerve-cords. _co{1}._ 2. Mass of cells developed on second commissure (probably sensory). _c.s.d._ Common duct for the two salivary glands. _d.l.m._ Dorsal longitudinal muscles of pharynx. _d.o._ Muscles serving to dilate the opening of the pharynx. _Ep._ Epidermis. _e.n._ Nerve passing outwards from ventral nerve-cord. H. Heart. _i.j._ Inner jaw. _j.p._ Jaw papillæ. _L._ Lips of buccal cavity. _l.b.c._ Lateral compartment of body-cavity. _le._ Rod-like cuticular continuation of inner jaw, lying in a pit of the buccal cavity. _l.m._ Bands of longitudinal muscles. M. Buccal cavity. M{1}. Median backward continuation of buccal cavity. _m.l._ Muscles of jaw lever. _m.s._ Muscular sheets passing from side walls of pharynx to dorsal body-wall. _oe._ OEsophagus. _oes.co._ OEsophageal commissures. _o.j._ Outer jaw. _ph._ Pharynx. _s.d._ Salivary duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland. _sy._ Sympathetic nerves running in muscles of tongue or pharynx. _sy{1}._ Origin of sympathetic nerves to pharynx. T. Tongue. _v.c._ Ventral nerve-cords.

Figs. 17, 18. Two longitudinal horizontal sections through the head of _Peripatus capensis_. Fig. 17 is the most ventral. They are both taken ventral to the cerebral ganglia. In Fig. 17 dorsal tracheal pits are shewn with tracheæ passing off from them. (Zeiss a a, Hartnack's camera.) C. Cutis. _c.s.d._ Common salivary duct. _ep._ Epidermis. _i.j._ Inner jaw. M. Buccal cavity. M{1}. Median backward diverticulum of mouth. _o.j._ Outer jaw. _s.d._ Salivary ducts. T. Tongue. _t._ Teeth on tongue. _tr._ Tracheæ. _tr.p._ Tracheal pits.

PLATE 50.

Fig. 19. "A, B, C, D, E, F, G." Seven transverse sections illustrating the structure of the supra-oesophageal ganglia. (Zeiss A, Hartnack's camera.) _a._ Dorso-lateral horn of white matter. _b._ Ventro-lateral horn of white matter. _c._ Postero-dorsal lobe of white matter. _d._ Ventral protuberance of brain. _e._ Central lobe of white matter. _o.p._ Optic ganglion.

"A. Section through anterior portions of ganglia close to the origin of the antennary nerve. B. Section a little in front of the point where the two ganglia unite. C. Section close to anterior junction of two ganglia. D. Section through origin of optic nerve on the right side. E. Section shewing origin of the optic nerve on the left side. F. Section through the dorso-median lobe of white matter. G. Section near the termination of the dorsal tongue of ganglion cells."

PLATE 51.

Fig. 20. Portion of a transverse section through the hinder part of _Peripatus capensis_ (male). The section passes through a leg, and shews the opening of the segmental organ (_o.s._), and of a crural gland, _o.f.g._, and the forward continuation of the enlarged crural gland of the 17th leg (_f.gl._). (Zeiss a a, Hartnack's camera.) _a.g._ accessory gland of male (modified crural gland of last leg). C. Cutis. _cl._ Claw. _cu._ Cuticle. _ep._ Epidermis. _f.gl._ Crural gland. _h._ Cells in lateral compartment of body-cavity. _o.f.g._ Orifice of accessory foot gland. _o.s._ Opening of segmental organ. P. Three spinous pads on ventral surface of foot. _pr._ Prostate. R.M. Retractor muscle of claw. _s._ Vesicle of nephridium. _s.c.i._ Region No. 1 of coiled part of nephridium. _sl.g._ Tubule of slime gland. _s.o.t._ Terminal portion of nephridium. _st._ Stomach. _st.e._ Epithelium of stomach. _v.c._ Ventral nerve-cord. _v.d._ Vas deferens.

Fig. 21. "Longitudinal vertical section through the supra-oesophageal ganglion and oesophageal commissures of _Peripatus capensis_. (Zeiss a a, Hartnack.)" _at._ Antenna. _e._ Central lobe of white matter. _j._ Part of jaw. _s.g._ Salivary gland.

Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral nerve-cords of _Peripatus capensis_ enlarged and viewed from the ventral surface. The paired appendages (_d_) of the ventral surface of the brain are seen, and the pair of sympathetic nerves (_sy_{1}) arising from the ventral surface of the hinder part.

From the commencement of the oesophageal commissures (_oes.co._) pass off on each side a pair of nerves to the jaws (_j.n._).

The three anterior commissures between the ventral nerve-cords are placed close together; immediately behind them the nerve-cords are swollen, to form the ganglionic enlargements from which pass off to the oral papillæ a pair of large nerves on each side (_or.n._).

Behind this the cords present a series of enlargements, one pair for each pair of feet, from which a pair of large nerves pass off on each side to the feet (_p.n_). _at.n._ Antennary nerves. _co._ Commissures between ventral cords. _d._ Ventral appendages of brain. E. Eye. _e.n._ Nerves passing outwards from ventral cord. _F.g._ Ganglionic enlargements from which nerves to feet pass off. _j.n._ Nerves to jaws. _or.g._ Ganglionic enlargement from which nerves to oral papillæ pass off. _or.n._ Nerves to oral papillæ. _p.c._ Posterior lobe of brain. _p.n._ Nerves to feet. _s.y._ Sympathetic nerves.

Fig. 23. "Longitudinal horizontal section through the head of _Peripatus capensis_, shewing the structure of the brain, the antennary and optic nerves, &c. (Zeiss a a, Hartnack's camera.)" _at._ Antenna. _at.n._ Antennary nerve. _cor._ Cornea. _e._ Central mass of white matter. _l._ Lens. _op.n._ Optic nerve. _ph._ Pharynx. _p.p._ Primary papilla covered with secondary papillæ and terminating in a long spine. _sy._ Pharyngeal sympathetic nerves.

Fig. 24. "Eye of _Peripatus capensis_, as shewn in a longitudinal horizontal section through the head. The figure is so far diagrammatic that the lens is represented as filling up the whole space between the rods and the cornea. In the actual section there is a considerable space between the parts, but this space is probably artificial, being in part caused by the shrinkage of the lens and in part by the action of the razor. (Zeiss C, Hartnack's camera.)" (It appears that the ganglionic region of the eye is covered by a thin capsule, which is omitted in the figure.)

_cor._ Cornea. _l._ Lens. _op._ Optic ganglion. _op.n._ Optic nerve. _pi.r._ Pigment. _Re._ rods. _s.p._ Secondary papillæ.

Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing the peculiar arrangement of the circular muscular fibres. (Zeiss A, Hartnack's camera.)

PLATE 52.

Fig. 26. Portion of ventral cord of _Peripatus capensis_ enlarged, shewing two ganglionic enlargements and the origin of the nerves and commissures. (From a drawing by Miss Balfour.)

_co._ Commissures. E._n._ Nerves passing out from ventral cords. F._n._ Nerves to feet. _g.co._ Commissures between the ventral cords containing ganglion cells. _v.g._ Ganglionic enlargements.

Fig. 27. Segmental organ from the 5th pair of legs of _Peripatus capensis_. This nephridium resembles those of the 4th legs, and differs from all the others in its large size and in the absence of any dilatation giving rise to a collecting vesicle on its external portion (enlarged). The terminal portion has the same histological characters as in the case of the hinder segmental organs. (From a drawing by Miss Balfour.)

Fig. 28. Segmental organ or nephridium from the 9th pair of legs of _Peripatus capensis_, shewing the external opening, the vesicle, the coiled portion and the terminal portion with internal opening (enlarged). (From a drawing by Miss Balfour.)

_o.s._ External opening of segmental organ. _p.f._ Internal opening of nephridium into the body-cavity (lateral compartment). _s._ Vesicle of segmental organ. _s_{1}. Portion of segmental organ of 4th and 5th legs, corresponding to vesicle of the other nephridia. _s.c._1. First or external portion of coiled tube of nephridium, lined by columnar epithelium with small nuclei; the cells project for very different distances, giving the inner boundary of this region a ragged appearance. _s.c._2. Region No. 2 of coiled tube of nephridium, lined by small closely-packed columnar cells. _s.c._3. Region No. 3 of coiled tube of segmental organ, lined by large flat cells with large disc-shaped nuclei. _s.c._4. Region No. 4 of coiled tube of nephridium; this region is very short and lined by small columnar cells. _s.o.t._ Terminal portion of nephridium.

Fig. 29. "Portion of nephridium of the hindermost leg of _Peripatus capensis_, seen in longitudinal and vertical section. The figure is given to shew the peritoneal funnel of the nephridium. Portions of the collecting sack (_s._) and other parts are also represented. (Zeiss B, Hartnack's camera.)"

_p.f._ Peritoneal funnel. _s._ Vesicle. _s.c.1_, _s.c.2_, _s.c.3._ Portions of coiled tube.

Fig. 30. "Section through a tracheal pit and diverging bundles of tracheal tubes" taken transversely to the long axis of the body. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)

_tr._ _Tracheæ_, shewing rudimentary spiral fibre. _tr.c._ Cells resembling those lining the tracheal pits, which occur at intervals along the course of the tracheæ. _tr.s._ Tracheal stigma. _tr.p._ Tracheal pit.

Fig. 31. "Sense organs and nerves attached from antenna of _Peripatus capensis_ (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof. Balfour.) The figure shews the arrangement of the epidermis cells round the base of the spine. The spine is seen to be continuous with the inner layer of the cuticle.

Fig. 32. Section through the skin of _Peripatus capensis_; it shews the secondary papillæ covered with minute spinous tubercles and the relation of the epidermis to them. (The cuticle in the process of cutting has been torn away from the subjacent cells.) The cells of the epidermis are provided with large oval nuclei, and there is a deposit of pigment in the outer ends of the cells. The granules in the protoplasm of the inner ends of the cells are arranged in lines, so as to give a streaked appearance. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)

_c._ Dermis. _cu._ Cuticle. _ep.c._ Epidermis cells. _pi._ Deposit of pigment in outer ends of epidermis cells. _s.p._ Secondary papillæ.

Fig. 33. Female generative organs of _Peripatus capensis_, × 5. (From a rough drawing by Prof. Balfour.) The following note was appended to this drawing: "Ovary rather to dorsal side, lying in a central compartment of body-cavity and attached to one of the longitudinal septa, dividing this from the lateral compartment between the penultimate pair of legs and that next in front. The oviducts cross before opening to the exterior, the right oviduct passing under the rectum and the left over it. They meet by opening into a common vestibule, which in its turn opens below the anus. On each side of it are a pair of short papillæ (aborted feet?)."

F. 16, 17. Last two pairs of legs. _od._ Oviduct. _ov._ Ovary. _ut._ Uterus. _v.c._ Nerve-cord.

PLATE 53.

Figs. 34-39. Five young embryos of _Peripatus capensis_; ventral view. All, excepting Fig. 37, from drawings by Miss Balfour. In figures 34 to 38_a_ denotes what is probably the anterior extremity.

Fig. 34, Stage A. Youngest embryo found, with slightly elongated blastopore.

Fig. 35, Stage B. Embryo with three mesoblastic somites and elongated blastopore. The external boundaries of the somites are not distinct.

Fig. 36, Stage C. Embryo with five somites. The blastopore is closing in its middle portion.

Fig. 37, Stage D. The blastopore has completely closed in its middle portion, and given rise to two openings, the future mouth and anus. (From a rough drawing left by Professor Balfour.) (Zeiss A, Camera Oberhaus. on level of stage.)

The following note was appended to this drawing in his handwriting: "Young larva of _Peripatus capensis_. I could not tell for certain which was the anterior end. Length, 1.34 mm."

Fig. 38, Stage E. Embryo with about thirteen mesoblastic somites in which the flexure of the hind part of the body has commenced. The remains of the original blastopore are present as the mouth, placed between the second pair of mesoblastic somites, and the anus placed on the concavity of the commencing flexure of the hind part of the body.

Fig. 39. Side view of same embryo.

Figs. 40-42. Drawings by Professor Balfour of three transverse sections through the embryo from which fig. 36 was taken. (Zeiss c, Camera.) Figs. 40 and 42 pass through the region of the blastopore.

_bl._ Blastopore. _ep._ Epiblast. _hy._ Hypoblast. _me._ Mesenteron. _mes._ Mesoblastic somite.

Fig. 43. Male generative organs of _Peripatus capensis_, viewed from the dorsal surface. (From a drawing by Miss Balfour.)

_a.g._ Enlarged crural glands of last pair of legs. F.16, 17. Last pairs of legs. _f._ Small accessory glandular tubes. _p._ Common duct into which vasa deferentia open. _p.r._ Prostate. _te._ Testes. _v.c._ Nerve-cord. _v.d._ Vas deferens.

CAMBRIDGE: PRINTED BY C. J. CLAY, M.A., AND SON, AT THE UNIVERSITY PRESS.

TRANSCRIBER'S NOTES:

Underscores surround text in italics, _like this_.

Hyphens were spaced in ranges of small numbers to ease readability, e.g., "1/2000-1/3000 of an inch" was changed to "1/2000 - 1/3000 of an inch".

Raised dots in numbers were converted to decimals. Superscript letters are enclosed in braces, e.g. P{1}.

The Greek letter, Lambda, is spelled out.

Use of periods and commas in the abbreviations within and referring to illustrations is inconsistent. Often punctuation marks do not match the illustrations to which they refer. Periods were retained; commas were added to separate figure numbers from abbreviations within the figure. Spacing within the abbreviations was standardized.

Footnotes were renumbered sequentially, indented, and moved to follow the paragraph in which the anchor occurs. There is no anchor for footnote 496; anchor was placed at the spot the transcriber deemed it likely belonged.

Changes for consistency within the text of the book:

body cavity to body-cavity body wall to body-wall choroid-slit to choroid slit develope(s) to develop(s) dog fish to dog-fish Elasmobranchs to Elasmobranchii Entwickelung to Entwicklung head-fold to head fold inter-renal to interrenal juxta-position to juxtaposition lenslike to lens-like re-agent(s) to reagent(s) omphalo-meseraic to omphalomeseraic pleuroperitoneal to pleuro-peritoneal proto-vertebra(æ) to protovertebra(æ) re-appear to reappear semi-lunar to semilunar side-fold to side fold spongework to sponge-work subgerminal to sub-germinal sub-intestinal to subintestinal sub-kingdom to subkingdom sub-notochordal to subnotochordal suboesophageal to sub-oesophageal supraoesophageal to supra-oesophageal urino-genital to urinogenital Urogenital-system to Urogenitalsystem, except where cited as a title of a work. Verwandschaft to Verwandtschaft widespread to wide-spread wood-cut(s) to woodcut(s) zool. zoot. to zool.-zoot. italics removed from eight instances of 'vide' italics, where missing, were added to 'loc. cit', 'i.e.' and 'e.g.'

Other changes:

'reremainder' to 'remainder' ... as compared with the remainder of ... 'on' to 'or' ... one or two words ... 'splachnopleure' to 'splanchnopleure' ... where the somatopleure and splanchnopleure unite,... 'Sitzen.' to 'Sitzun.' (Footnote - abbreviation for 'Sitzungsberichte') 'diffiulty' to 'difficulty' ... as a serious difficulty.... 'it' to 'is' ... is still very difficult to observe.... 'primive' to 'primitive' ... That such a condition could be a primitive one seemed scarcely possible.... 'opthalmicus' to 'ophthalmicus' ... to form ramus ophthalmicus superficialis ... ... as the _ramus ophthalmicus superficialis of the fifth nerve_ ... 'Ureierernester' to 'Ureiernester' ... nests of ova (Ureiernester),... 'vascula' to 'vascular' ... the subjacent vascular stroma ... 'Metozoa' to 'Metazoa' ... Coelenterata and the Metazoa.... duplicate word 'of' removed ...'(2) Of a layer of protoplasm' changed to '(2) a layer of protoplasm' ... 'protodæum' to 'proctodæum' ... The proctodæum (_pr._) has also grown ... 'is it' to 'it is' ... where it is attach to the side of the body ... 'is is' to 'is in' ... but is in some respects peculiar.... 'continous' to 'continuous' ... forms a continuous whole,... 'Zussammenhang' to 'Zusammenhang' ... Ueber d. Zusammenhang d. ... 'Tranverse' to 'Transverse' ... Transverse sections of the head ... 'odontophor' to 'odontophore' ... of the odontophore of a mollusc.... 'lens' to 'legs' ... the last (17th) pair of legs are enlarged ... 'Platyelminthes' to 'Platyhelminthes' ...the nervous system of Echinodermata, Platyhelminthes,...