J. Müller (_Bau u. Entwick. d. Myxinoiden_) holds that the so-called bursa

entiana of Elasmobranchii (_i.e._, the chamber placed between the part of

the intestine with the spiral valve and the end of the pylorus) is the

homologue of the more elongated portion of the small intestine which

occupies a similar position in the Sturgeon. This portion of the small

intestine is no doubt homologous with the still more elongated and coiled

portion of the small intestine in _Lepidosteus_ placed between the chamber

into which the pyloric cæca, &c., open and the region of the spiral valve.

The fact that the vitelline duct in the embryo _Lepidosteus_ is placed

close to the pyloric end of the stomach, and that the greater portion of

the small intestine is derived from part of the alimentary canal behind

this, shews that Müller is mistaken in attempting to homologise the bursa

entiana of Elasmobranchii, which is placed in front of the vitelline duct,

with the coiled part of the small intestine of the above forms. The latter

is either derived from an elongation of the very short portion of the

intestine between the vitelline duct and the primitive spiral valve, or

more probably by the conversion of the anterior part of the intestine,

originally provided with a spiral valve into a coiled small intestine not

so provided.

We have already called attention to the peculiar mesentery present in the

adult attaching the posterior straight part of the intestine to the ventral

wall of the body. This mesentery, which together with the dorsal mesentery

divides the hinder section of the body-cavity into two lateral compartments

is, we believe, a persisting portion of the ventral mesentery which, as

pointed out by one of us[550], is primitively present for the whole length

of the body-cavity. The persistence of such a large section of it as that

found in the adult _Lepidosteus_ is, so far as we know, quite exceptional.

This mesentery is shewn in section in the embryo in Plate 38, fig. 53

(_v.mt._). The small vessel in it appears to be the remnant of the

subintestinal vein.

Footnote 550: _Comparative Embryology_, Vol. II. p. 514 [the

original edition].

THE GILL ON THE HYOID ARCH.

It is well known that _Lepidosteus_ is provided with a gill on the hyoid

arch, divided on each side into two parts. An excellent figure of this gill

is given by Müller (No. 13, plate 5, fig. 6), who holds from a

consideration of the vascular supply that the two parts of this gill

represent respectively the hyoid gill and the mandibular gill (called by

Müller pseudobranch). Müller's views on this subject have not usually been

accepted, but it is the fashion to regard the whole of the gill as the

hyoid gill divided into two parts. It appeared to us not improbable that

embryology might throw some light on the history of this gill, and

accordingly we kept a look out in our embryos for traces of gills on the

hyoid and mandibular arches. The results we have arrived at are purely

negative, but are not the less surprising for this fact. The hyomandibular

cleft as shewn above, is never fully developed, and early undergoes a

complete atrophy--a fact which is, on the whole, against Müller's view; but

what astonished us most in connection with the gill in question is that we

have been unable to find any trace of it even in the oldest larva whose

head we have had (26 millims.), and at a period when the gills on the

hinder arches have reached their full development.

We imagined the gill in question to be the remnant of a gill fully formed

in extinct Ganoid types, and therefore expected to find it better developed

in the larva than in the adult. That the contrary is the fact appears to us

fairly certain, although we cannot at present offer any explanation of it.

SYSTEMATIC POSITION OF LEPIDOSTEUS.

A. Agassiz concludes his memoir on the development of _Lepidosteus_ by

pointing out that in spite of certain affinities in other directions this

form is "not so far removed from the bony Fishes as has been supposed." Our

own observations go far to confirm Agassiz' opinion.

Apart from the complete segmentation, the general development of

_Lepidosteus_ is strikingly Teleostean. In addition to the general

Teleostean features of the embryo and larva, which can only be appreciated

by those who have had an opportunity of practically working at the subject,

we may point to the following developmental features[551] as indicative of

Teleostean affinities:--

Footnote 551: The features enumerated above are not in all

cases confined to _Lepidosteus_ and Teleostei, but are always

eminently characteristic of the latter.

(1) The formation of the nervous system as a solid keel of the epiblast.

(2) The division of the epiblast into a nervous and epidermic stratum.

(3) The mode of development of the gut (vide pp. 752-754).

(4) The mode of development of the pronephros; though, as shewn on p. 822,

the pronephros of _Lepidosteus_ has primitive characters not retained by

Teleostei.

(5) The early stages in the development of the vertebral column (vide p.

779).

In addition to these, so to speak, purely embryonic characters there are

not a few important adult characters:--

(1) The continuity of the oviducts with the genital glands.

(2) The small size of the pancreas, and the presence of numerous so-called

pancreatic cæca.

(3) The somewhat coiled small intestine.

(4) Certain characters of the brain, _e.g._, the large size of the

cerebellum; the presence of the so-called lobi inferiores on the

infundibulum; and of tori semicirculares in the mid-brain.

In spite of the undoubtedly important list of features to which we have

just called attention, a list containing not less important characters,

both embryological and adult, separating _Lepidosteus_ from the Teleostei,

can be drawn up:--

(1) The character of the truncus arteriosus.

(2) The fact of the genital ducts joining the ureters.

(3) The presence of vasa efferentia in the male carrying the semen from the

testes to the kidney, and through the tubules of the latter into the kidney

duct.

(4) The presence of a well-developed opercular gill.

(5) The presence of a spiral valve; though this character may possibly

break down with the extension of our knowledge.

(6) The typical Ganoid characters of the thalamencephalon and the cerebral

hemispheres (vide pp. 769 and 770).

(7) The chiasma of the optic nerves.

(8) The absence of a pecten, and presence of a vascular membrane between

the vitreous humour and the retina.

(9) The opisthocoelous form of the vertebræ.

(10) The articulation of the ventral parts of the hæmal arches of the tail

with processes of the vertebral column.

(11) The absence of a division of the muscles into dorso-lateral and

ventro-lateral divisions.

(12) The complete segmentation of the ovum.

The list just given appears to us sufficient to demonstrate that

_Lepidosteus_ cannot be classed with the Teleostei; and we hold that

Müller's view is correct, according to which _Lepidosteus_ is a true

Ganoid.

The existence of the Ganoids as a distinct group has, however, recently

been challenged by so distinguished an Ichthyologist as Günther, and it may

therefore be well to consider how far the group as defined by Müller is a

natural one for living forms[552], and how far recent researches enable us

to improve upon Müller's definitions. In his classical memoir (No. 13) the

characters of the Ganoids are thus shortly stated:--

"These Fishes are either provided with plate-like angular or rounded

cement-covered scales, or they bear osseous plates, or are quite

naked. The fins are often, but not always, beset with a double or

single row of spinous plates or splints. The caudal fin occasionally

embraces in its upper lobe the end of the vertebral column, which may

be prolonged to the end of the upper lobe. Their double nasal openings

resemble those of Teleostei. The gills are free, and lie in a

branchial cavity under an operculum, like those of Teleostei. Many of

them have an accessory organ of respiration, in the form of an

opercular gill, which is distinct from the pseudobranch, and can be

present together with the latter; many also have spiracles like

Elasmobranchii. They have many valves in the stem of the aorta like

the latter, also a muscular coat in the stem of the aorta. Their ova

are transported from the abdominal cavity by oviducts. Their optic

nerves do not cross each other. The intestine is often provided with a

spiral valve, like Elasmobranchii. They have a swimming-bladder with a

duct, like many Teleostei. Their pelvic fins are abdominal.

"If we include in a definition only those characters which are

invariable, the Ganoids may be shortly defined as being those Fish

with numerous valves to the stem of the aorta, which is also provided

with a muscular coat; with free gills and an operculum, and with

abdominal pelvic fins."

Footnote 552: We do not profess to be able to discuss this

question for extinct forms of Fish, though of course it is a

necessary consequence of the theory of descent that the various

groups should merge into each other as we go back in geological

time.

To these distinctive characters, he adds in an appendix to his paper, the

presence of the spiral valve, and the absence of a processus falciformis

and a choroid gland.

To the distinctive set of characters given by Müller we may probably add

the following:--

(1) Oviducts and urinary ducts always unite, and open by a common

urinogenital aperture behind the anus.

(2) Skull hyostylic.

(3) Segmentation complete in the types so far investigated, though perhaps

_Amia_ may be found to resemble the Teleostei in this particular.

(4) A pronephros of the Teleostean type present in the larva.

(5) Thalamencephalon very large and well developed.

(6) The ventricle in the posterior part of the cerebrum is not divided

behind into lateral halves, the roof of the undivided part being extremely

thin.

(7) Abdominal pores always present.

The great number of characters just given are amply sufficient to

differentiate the Ganoids as a group; but, curiously enough, the only

characters amongst the whole series which have been given, which can be

regarded as peculiar to the Ganoids, are (1) the characters of the brain,

and (2) the fact of the oviducts and kidney ducts uniting together and

opening by a common pore to the exterior.

This absence of characters peculiar to the Ganoids is an indication of how

widely separated in organization are the different members of this great

group.

At the same time, the only group with which existing Ganoids have close

affinities is the Teleostei. The points they have in common with the

Elasmobranchii are merely such as are due to the fact that both retain

numerous primitive Vertebrate characters[553], and the gulf which really

separates them is very wide.

Footnote 553: As instances of this we may cite (1) the spiral

valve; (2) the frequent presence of a spiracle; (3) the

frequent presence of a communication between the pericardium

and the body-cavity; (4) the heterocercal tail.

There is again no indication of any close affinity between the Dipnoi and,

at any rate, existing Ganoids.

Like the Ganoids, the Dipnoi are no doubt remnants of a very primitive

stock; but in the conversion of the air-bladder into a true lung, the

highly specialized character of their limbs[554], their peculiar autostylic

skulls, the fact of their ventral nasal openings leading directly into the

mouth, their multisegmented bars (interspinous bars), directly prolonged

from the neural and hæmal arches and supporting the fin-rays of the

unpaired dorsal and ventral fins, and their well-developed cerebral

hemispheres, very unlike those of Ganoids and approaching the Amphibian

type, they form a very well-defined group, and one very distinctly

separated from the Ganoids.

Footnote 554: Vide F. M. Balfour, "On the Development of the

Skeleton of the Paired Fins of Elasmobranchii," _Proc. Zool.

Soc._, 1881 [This edition, No. XX.].

No doubt the Chondrostean Ganoids are nearly as far removed from the

Teleostei as from the Dipnoi, but the links uniting these Ganoids with the

Teleostei have been so fully preserved in the existing fauna of the globe,

that the two groups almost run into each other. If, in fact, we were

anxious to make any radical change in the ordinary classification of

Fishes, it would be by uniting the Teleostei and Ganoids, or rather

constituting the Teleostei into one of the sub-groups of the Ganoids,

equivalent to the Chondrostei. We do not recommend such an arrangement,

which in view of the great preponderance of the Teleostei amongst living

Fishes would be highly inconvenient, but the step from _Amia_ to the

Teleostei is certainly not so great as that from the Chondrostei to _Amia_,

and is undoubtedly less than that from the Selachii to the Holocephali.

LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF LEPIDOSTEUS.

1. Agassiz, A. "The Development of _Lepidosteus_." Part 1., _Proc. Amer.

Acad. Arts and Sciences_, Vol. XIV. 1879.

2. Agassiz, L. _Recherches s. l. Poissons Fossiles._ Neuchatel. 1833-45.

3. Boas, J. E. "Ueber Herz u. Arterienbogen bei _Ceradotus_ u.

_Protopterus_," _Morphol. Jahrbuch_, Vol. VI. 1880.

4. Davidoff, M. von. "Beiträge z. vergleich. Anat. d. hinteren Gliedmassen

d. Fische," _Morphol. Jahrbuch_, Vol. VI. 1880.

5. Gegenbaur, C. _Untersuch. z. vergleich. Anat. d. Wirbelthiere_, Heft

II., _Schultergürtel d. Wirbelthiere. Brustflosse der Fische_. Leipzig,

1865.

6. Gegenbaur, C. "Zur Entwick. d. Wirbelsäule d. _Lepidosteus_, &c."

_Jenaische Zeitschrift_, Vol. III. 1867.

7. Hertwig, O. "Ueber d. Hautskelet d. Fische (_Lepidosteus_ u.

_Polypterus_)," _Morphol. Jahrbuch_, Vol. V. 1879.

8. Hoeven, Van der. "Ueber d. zellige Schwimmblase d. _Lepidosteus_."

Müller's _Archiv_, 1841.

9. Hyrtl, J. "Ueber d. Schwimmblase von _Lepidosteus osseus_," _Sitz. d.

Wiener Akad._ Vol. VIII. 1852.

10. Hyrtl, J. "Ueber d. Pori abdominales, d. Kiemen-Arterien, u. d.

Glandula thyroidea d. Ganoiden," _Sitz. d. Wiener Akad._ Vol. VIII. 1852.

11. Hyrtl, J. _Ueber d. Zusammenhang d. Geschlechts u. Harnwerkzeuge bei d.

Ganoiden_, Wien, 1855.

12. Kölliker, A. _Ueber d. Ende d. Wirbelsäule b. Ganoiden_, Leipzig, 1860.

13. Müller, J. "Ueber d. Bau u. d. Grenzen d. Ganoiden," _Berlin Akad._

1844.

14. Schneider, H. "Ueber d. Augenmuskelnerven d. Ganoiden," _Jenaische

Zeitschrift_, Vol. XV. 1881.

15. Wilder, Burt G. "Notes on the North American Ganoids, _Amia_,

_Lepidosteus_, _Acipenser_, and _Polyodon_." _Proc. Amer. Assoc. for the

Advancement of Science_, 1875.

LIST OF REFERENCE LETTERS.

_a._ Anus. _ab._ Air-bladder. _ab´._ Aperture of air-bladder into throat.

_ac._ Anterior commissure. _af._ Anal fin. _al._ Alimentary canal. _ao._

Aorta. _ar._ Artery. _au._ Auditory pit. _b._ Brain. _bc._ Body-cavity.

_bd._ Bile duct. _bd´._ Aperture of bile duct into duodenum. _bl._

Coalesced portion of segmental ducts, forming urinogenital bladder. _bra._

Branchial arches. _brc._ Branchial clefts. _c._ Pyloric caæca. _c´._

Apertures of caæca into duodenum. _cb._ Cerebellum. _cdv._ Cardinal vein.

_ce._ Cerebrum: in figs. 47A and B, anterior lobe of cerebrum. _ce´._

Posterior lobe of cerebrum. _cf._ Caudal fin. _cn._ Centrum. _ch._

Choroidal fissure. _crv._ Circular vein of vascular membrane of eye. _csh._

Cuticular sheath of notochord. _cv._ Caudal vein. _d._ Duodenum. _dc._

Dorsal cartilage of neural arch. _df._ Dermal fin-rays. _dl._ Dorsal lobe

of caudal fin. _dlf._ Dorsal fin. _e._ Eye. _ed._ Epidermis. _ep._

Epiblast. _fb._ Fore-brain. _fe._ Pyriform bodies surrounding the zona

radiata of the ovum, probably the remains of epithelial cells. _gb._

Gall-bladder. _gd._ Genital duct. _gl._ Glomerulus. _gr._ Genital ridge.

_h._ Heart. _ha._ Hæmal arch. _hb._ Hind-brain. _hc._ Head-cavity. _hpd._

Hepatic duct. _hm._ Hyomandibular cleft. _hop._ Operculum. _hy._ Hypoblast;

in fig. 10, hyoid arch. _hyl._ Hyaloid membrane. _ic._ Intercalated

cartilaginous elements of the neural arches. _in._ Infundibulum. _ir._

Iris. _is._ Interspinous cartilage or bones. _iv._ subintestinal vein.

_ivr._ Intervertebral ring of cartilage. _k._ Kidney. _l._ Lens. _lc._

Longitudinal canal, formed by union of the vasa efferentia. _lin._ Lobi

inferiores. _ll._ Ligamentum longitudinale superius. _lr._ Liver. _lt._

Lateral line. _ly._ Lymphatic body in front of kidney. _m._ Mouth. _mb._

Mid-brain. _mc._ Medullary cord. _mel._ Membrana elastica externa. _mes._

Mesorchium. _mn._ Mandible. _md._ and _mo._ Medulla oblongata. _ms._

Mesoblast. _na._ Neural arch. _na´._ Dorsal element of neural arch. _nc._

Notochord. _nve._ Network formed by vasa efferentia on inner face of

testis. _od._ Oviduct. _od´._ Aperture of oviduct into bladder. _ol._ Nasal

pit or aperture. _olf._ Olfactory lobe. _op._ Optic vesicle. _op ch._ Optic

chiasma. _opl._ Optic lobes. _op th._ Optic thalami. _or ep._ Oral

epithelium. _ov._ Ovary. _p._ Pancreas. _pc._ Pericardium. _pcf._ Pectoral

fin. _pch._ Pigmented layer of choroid. _pf._ Peritoneal funnel of

segmental tube of mesonephros. _pfp._ Peritoneal funnel leading into

pronephric chamber. _pg._ Pectoral girdle. _plf._ Pelvic fin. _pn._ Pineal

gland. _po._ Primitive germinal cells. _pr._ Mesoblastic somite. _prc._

Pronephric chamber. _prn._ Pronephros. _pr n´._ Opening of pronephros into

pronephric chamber. _pt._ Pituitary body. _py._ Pyloric valve. _pz._

Parietal zone of blastoderm. _r._ Rostrum. _rb._ Rib. _rc._ Rectum. _s._

Spleen. _sc._ Seminal vessels passing from the longitudinal canal into the

kidney. _sd._ Suctorial disc. _sg._ Segmental or archinephric duct. _sgt._

Segmental tubules. _sh._ Granular outer portion of the sheath of the

notochord in the vertebral regions. _smx._ Superior maxillary process.

_snc._ subnotochordal rod. _so._ Somatic mesoblast. _sp._ Splanchnic

mesoblast. _spn._ Spinal nerve. _spv._ Spiral valve. _st._ Stomach. _st._

Seminal tubes of the testis. _sup._ Suctorial papillæ. _t._ Testis. _th._

Thalamencephalon. _thl._ Lobes of the roof of the thalamencephalon. _tr._

Trabeculæ. _ug._ Urinogenital aperture. _v._ Ventricle. _ve._ Vasa

efferentia. _vh._ Vitreous humour. _vl._ Ventral lobe of the caudal fin.

_vmt._ Ventral mesentery. _vn._ Vein. _vs._ Blood-vessel. _vsh._ Vascular

sheath between the hyaloid membrane and the vitreous humour. _vth._ Vesicle

of the thalamencephalon. _x._ Groove in epiblast, probably formed in

process of hardening. _y._ Yolk. _z._ Commissure in front of pineal gland.

_zr._ Outer striated portion of investing membrane (zona radiata) of ovum.

_zr´._ Inner non-striated portion of investing membrane of ovum. I.

Olfactory nerve. II. Optic nerve. III. Oculomotor nerve. V. Trigeminal

nerve. VIII. Facial and auditory nerves.

EXPLANATION OF PLATES 34-42.

PLATE 34.

Figs. 1-4. Different stages in the segmentation of the ovum.

Fig. 1. Ovum with a single vertical furrow, from above.

Fig. 2. Ovum with two vertical furrows, from above.

Fig. 3. Side view of an ovum with a completely formed blastodermic

disc.

Fig. 4. The same ovum as fig. 3, from below, shewing four vertical

furrows nearly meeting at the vegetative pole.

Figs. 5-10. External views of embryos up to time of hatching.

Fig. 5. Embryo, 3.5 millims. long, third day after impregnation.

Fig. 6. Embryo on the fifth day after impregnation.

Fig. 7. Posterior part of same embryo as fig. 6, shewing tail

swelling.

Fig. 8. Embryo on the sixth day after impregnation.

Fig. 9. Embryo on the seventh day after impregnation.

Fig. 10. Embryo on the eleventh day after impregnation (shortly before

hatching).

Fig. 11. Head of embryo about the same age as fig. 10, ventral aspect.

Fig. 12. Side view of a larva about 11 millims. in length, shortly after

hatching.

Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect.

Fig. 14. Side view of a larva about 15 millims. long, five days after

hatching.

Fig. 15. Head of a larva 23 millims. in length.

Fig. 16. Tail of a larva 11 centims. in length.

Fig. 17. Transverse section through the egg-membranes of a just-laid ovum.

We are indebted to Professor W. K. Parker for figs. 12, 14 and 15.

PLATE 35.

Figs. 18-22. Transverse sections of embryo on the third day after

impregnation.

Fig. 18. Through head, shewing the medullary keel.

Fig. 19. Through anterior part of trunk.

Fig. 20. Through same region as fig. 19, shewing a groove (_x_) in the

epiblast, probably artificially formed in the process of hardening.

Fig. 21. Through anterior part of tail region, shewing partial fusion

of layers.

Fig. 22. Through posterior part of tail region, shewing more complete

fusion of layers than fig. 21.

Figs. 23-25. Transverse sections of an embryo on the fifth day after

impregnation.

Fig. 23. Through fore-brain and optic vesicles.

Fig. 24. Through hind-brain and auditory pits.

Fig. 25. Through anterior part of trunk.

Figs. 26-27. Transverse sections of the head of an embryo on the sixth day

after impregnation.

Fig. 26. Through fore-brain and optic vesicles.

Fig. 27. Through hind-brain and auditory pits.

PLATE 36.

Figs. 28-29. Transverse sections of the trunk of an embryo on the sixth day

after impregnation.

Fig. 28. Through anterior part of trunk (from a slightly older embryo

than the other sections of this stage).

Fig. 29. Slightly posterior to fig. 28, shewing formation of segmental

duct as a fold of the somatic mesoblast.

Fig. 30. Longitudinal horizontal section of embryo on the sixth day after

impregnation, passing through the mesoblastic somites, notochord, and

medullary canal.

Figs. 31-34. Transverse sections through an embryo on the seventh day after

impregnation.

Fig. 31. Through anterior part of trunk.

Fig. 32. Through the trunk somewhat behind fig. 31.

Fig. 33. Through tail region.

Fig. 34. Further back than fig. 33, shewing constriction of tail from

the yolk.

Figs. 35-37. Transverse sections through an embryo on the eighth day after

impregnation.

Fig. 35. Through fore-brain and optic vesicles.

Fig. 36. Through hind-brain, shewing closed auditory pits, &c.

Fig. 37. Through anterior part of trunk.

Fig. 38. Section through tail of an embryo on the ninth day after

impregnation.

PLATE 37.

Fig. 39. Section through the olfactory involution and part of fore-brain of

a larva on the ninth day after impregnation, shewing olfactory nerve.

Fig. 40. Section through the anterior part of the head of the same larva,

shewing pituitary involution.

Figs. 41-43. Transverse sections through an embryo on the eleventh day

after impregnation.

Fig. 41. Through fore-part of head, shewing the pituitary body still

connected with the oral epithelium.

Fig. 42. Slightly further back than fig. 41, shewing the pituitary

body constricted off from the oral epithelium.

Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution,

eye, and hyomandibular cleft.

Fig. 44. Longitudinal section of the head of an embryo of 15 millims. in

length, a few days after hatching, shewing the structure of the brain.

Fig. 45. Longitudinal section of the head of an embryo, about five weeks

after hatching, 26 millims. in length, shewing the structure of the brain.

In the front part of the brain the section passes slightly to one side of

the median line.

Figs. 46A to 46G. Transverse sections through the brain of an embryo 25

millims. in length, about a month after hatching.

Fig. 46A. Through anterior lobes of cerebrum.

Fig. 46B. Through posterior lobes of cerebrum.

Fig. 46C. Through thalamencephalon.

Fig. 46D. Through optic thalami and optic chiasma.

Fig. 46E. Through optic lobes and infundibulum.

Fig. 46F. Through optic lobes and cerebellum.

Fig. 46G. Through optic lobes and cerebellum, slightly in front of

fig. 46F.

PLATE 38.

Figs. 47A, B, C. Figures of adult brain.

Fig. 47A. From the side.

Fig. 47B. From above.

Fig. 47C. From below.

Fig. 48. Longitudinal vertical section through the eye of an embryo, about

a week after hatching, shewing the vascular membrane surrounding the

vitreous humour.

Fig. 49. Diagram shewing the arrangement of the vessels in the vascular

membrane of the vitreous humour of adult eye.

Fig. 50. Capillaries of the same vascular membrane.

Fig. 51. Transverse section through anterior part of trunk of an embryo on

the ninth day after impregnation, shewing the pronephros and pronephric

chamber.

Fig. 52. Transverse section through the region of the stomach of an embryo

15 millims. in length, shortly after hatching, to shew the glomerulus and

peritoneal funnel of pronephros.

Fig. 53. Transverse section through posterior part of the body of an

embryo, about a month after hatching, shewing the structure of the

mesonephros, the spiral valve, &c.

PLATE 39.

Figs. 54, 55, 56, and 57 are a series of transverse sections through the

genital ridge and mesonephros of one side from a larva of 11 centims.

Fig. 54. Section of the lymphatic organ which lies in front of the

mesonephros.

Fig. 55. Section near the anterior end of the mesonephros, where the

genital sack is completely formed.

Fig. 56. Section somewhat further back, shewing the mode of formation

of the genital sack.

Fig. 57. Section posterior to the above, the formation of the genital

sack not having commenced, and the genital ridge with primitive

germinal cells projecting freely into the body-cavity.

Fig. 58A. View of the testis, mesorchium, and duct of the kidney of the

left side of an adult male example of _Lepidosteus_, 60 centims. in length,

shewing the vasa efferentia and the longitudinal canal at the base of the

mesorchium. The kidney ducts have been cut open posteriorly to shew the

structure of the interior.

Fig. 58B. Inner aspect of the posterior lobe of the testis from the same

example, to shew the vasa efferentia forming a network on the face of the

testis.

Figs. 59A and B. Two sections shewing the structure and relations of the

efferent ducts of the testis in the same example.

Fig. 59A. Section through the inner aspect of a portion of the testis

and mesorchium, to shew the network of the vasa efferentia (_nve_)

becoming continuous with the seminal tubes (_st_). The granular matter

nearly filling the vasa efferentia and the seminal tubes represent the

spermatozoa.

Fig. 59B. Section through part of the kidney and its duct and the

longitudinal canal (_lc_) at the base of the mesorchium. Canals (_sc_)

are seen passing off from the latter, which enter the kidney and join

the uriniferous tubuli. Some of the latter (as well as the seminal

tubes) are seen to be filled with granular matter, which we believe to

be the remains of spermatozoa.

Fig. 60. Diagram of the urinogenital organs of the left side of an adult

female example of _Lepidosteus_ 100 centims. in length. This figure shews

the oviduct (_od_) continuous with the investment of the ovary, opening at

_od´_ into the dilated part of the kidney duct (segmental duct). It also

shews the segmental duct and the junction of the latter with its fellow of

the right side to form the so-called bladder, this part being represented

as cut open. The kidney (_k_) and lymphatic organ (_ly_) in front of it are

also shewn.

PLATE 40.

Fig. 61. Transverse section through the developing pancreas (_p_) of a

larva 11 millims. in length.

Fig. 62. Longitudinal section through portions of the stomach, liver, and

duodenum of an embryo about a month after hatching, to shew the relations

of the pancreas (_p_) to the surrounding parts.

Fig. 63. External view of portions of the liver, stomach, duodenum, &c., of

a young Fish, 11 centims. in length, to shew the pancreas (_p_).

Fig. 64. Transverse section through the anterior part of the trunk of an

embryo, about a month after hatching, shewing the connection of the

air-bladder with the throat (_ab´_).

Fig. 65. Transverse section through the same embryo as fig. 64 further

back, shewing the posterior part of the air-bladder (_ab_).

Fig. 66. Viscera of an adult female, 100 centims. in length, shewing the

alimentary canal with its appended glands in natural position, and the

air-bladder with its aperture into the throat (_ab´_). The proximal part of

the duodenum and the terminal part of the intestine are represented as cut

open, the former to shew the pyloric valve and the apertures of the pyloric

cæca and bile duct, and the latter to shew the spiral valve.

This figure was drawn for us by Professor A. C. Haddon.

PLATE 41.

Fig. 67. Transverse section through the tail of an advanced larva, shewing

the neural and hæmal processes, the independently developed interneural and

interhæmal elements (_is_), and the commencing dermal fin-rays (_df_).

Fig. 68. Side view of the tail of a larva, 21 minims. in length, dissected

so as to shew the structure of the skeleton.

Fig. 69. Longitudinal horizontal section through the vertebral column of a

larva, 5.5 centims. in length, on the level of the hæmal arches, shewing

the intervertebral rings of cartilage continuous with the arches, the

vertebral constriction of the notochord, &c.

Figs. 70 and 71. Transverse sections through the vertebral column of a

larva of 5.5 centims. The red represents bone, and the blue cartilage.

Fig. 70. Through the vertebral region, shewing the neural and hæmal

arches, the notochordal sheath, &c.

Fig. 71. Through the intervertebral region, shewing the intervertebral

cartilage.

Figs. 72 and 73. Transverse sections through the trunk of a larva of 5.5

centims. to shew the structure of the ribs and hæmal arches.

Fig. 72. Through the anterior part of the trunk.

Fig. 73. Through the posterior part of the trunk.

PLATE 42.

Figs. 74-76. Transverse sections through the trunk of the same larva as

figs. 72 and 73.

Fig. 74. Through the posterior part of the trunk (rather further back

than fig. 73).

Fig. 75. Through the anterior part of the tail.

Fig. 76. Rather further back than fig. 75.

Fig. 77. Longitudinal horizontal section through the vertebral column of a

larva of 11 centims., passing through the level of the hæmal arches, and

shewing the intervertebral constriction of the notochord, the ossification

of the cartilage, &c.

Fig. 78. Transverse section through a vertebral region of the vertebral

column of a larva 11 centims. in length.

Fig. 79. Transverse section through an intervertebral region of the same

larva as fig. 78.

Fig. 80. Side view of two trunk vertebræ of an adult _Lepidosteus_.

Fig. 81. Front view of a trunk vertebra of adult.

In figures 80 and 81 the red does not represent bone as in the other

figures, but simply the ligamentum longitudinale superius.

XXIII. ON THE NATURE OF THE ORGAN IN ADULT TELEOSTEANS AND

GANOIDS, WHICH IS USUALLY REGARDED AS THE HEAD-KIDNEY

OR PRONEPHROS[555].

Footnote 555: From the _Quarterly Journal of Microscopical

Science_, Vol. XXII., 1882.

While working at the anatomy of _Lepidosteus_ I was led to doubt the

accuracy of the accepted accounts of the anterior part of the kidneys in

this[556] and in allied species of Fishes. In order to test my doubts I

first examined the structure of the kidneys in the Sturgeon (Acipenser), of

which I fortunately had a well-preserved specimen.

Footnote 556: I am about to publish, in conjunction with Mr

Parker, a full account of the anatomy and development of

Lepidosteus [No. XXII. of this edition], and shall therefore in

this paper make no further allusion to it.

The bodies usually described as the kidneys consist of two elongated bands,

attached to the dorsal wall of the abdomen, and extending for the greater

part of the length of the abdominal cavity. In front each of these bands

first becomes considerably narrowed, and then expands and terminates in a

great dilatation, which is usually called the head-kidney. Along the outer

border of the hinder part of each kidney is placed a wide ureter, which

ends suddenly in the narrow part of the body, some little way behind the

head-kidney. To the naked eye there is no distinction in structure between

the part of the so-called kidney in front of the ureter and that in the

region of the ureter. Any section through the kidney in the region of the

ureter suffices to shew that in this part the kidney is really formed of

uriniferous tubuli with numerous Malpighian bodies. Just in front, however,

of the point where the ureter ends the true kidney substance rapidly thins

out, and its place is taken by a peculiar tissue formed of a trabecular

work filled with cells, which I shall in future call lymphatic tissue.

_Thus the whole of that part of the apparent kidney in front of the ureter,

including the whole of the so-called head-kidney, is simply a great mass of

lymphatic tissue, and does not contain a single uriniferous tubule or

Malpighian body._

The difference in structure between the anterior and posterior parts of the

so-called kidney, although not alluded to in most modern works on the

kidneys, appears to have been known to Stannius, at least I so interpret a

note of his in the second edition of his _Comparative Anatomy_, p. 263,

where he describes the kidney of the Sturgeon as being composed of two

separate parts, viz. a spongy vascular substance (no doubt the so-called

head-kidney) and a true secretory substance.

After arriving at the above results with reference to the Sturgeon I

proceeded to the examination of the structure of the so-called head-kidney

in Teleostei.

I have as yet only examined four forms, viz. the Pike (_Esox lucius_), the

Smelt (_Osmerus eperlanus_), the Eel (_Anguilla anguilla_), and the Angler

(_Lophius piscatorius_).

The external features of the apparent kidney of the Pike have been

accurately described by Hyrtl[557]. He says: "The kidneys extend from the

second trunk vertebra to the end of the abdominal cavity. Their anterior

extremities, which have the form of transversely placed coffee beans, are

united together, and lie on the anterior end of the swimming bladder. The

continuation of the kidney backwards forms two small bands, separated from

each other by the whole breadth of the vertebral column. They gradually,

however, increase in breadth, so that about the middle of the vertebral

column they unite together and form a single symmetrical, keel-shaped

body," &c.

Footnote 557: "Das Uropoëtische System der Knochenfische,"

_Sitz. d. Wien. Akad._, 1830.

The Pike I examined was a large specimen of about 58 centimètres in length,

and with an apparent kidney of about 25-1/2 centimètres. The relations of

lymphatic tissue and kidney tissue were much as in the Sturgeon. The whole

of the anterior swelling, forming the so-called head-kidney, together with

a considerable portion of the part immediately behind, forming not far

short of half the whole length of the apparent kidney, was entirely formed

of lymphatic tissue. The posterior part of the kidney was composed of true

kidney substance, but even at 16 centimètres from the front end of the

kidney the lymphatic tissue formed a large portion of the whole.

A rudiment of the duct of the kidney extended forwards for a short way into

the lymphatic substance beyond the front part of the functional kidney.

In the Smelt (_Osmerus eperlanus_) the kidney had the typical Teleostean

form, consisting of two linear bands stretching for the whole length of the

body-cavity, and expanding into a great swelling in front on the level of

the ductus Cuvieri, forming the so-called head-kidney. The histological

examination of these bodies shewed generally the same features as in the

case of the Sturgeon and Pike. The posterior part was formed of the usual

uriniferous tubuli and Malpighian bodies. The anterior swollen part of

these bodies, and the part immediately following, were almost wholly formed

of a highly vascular lymphatic tissue; but in a varying amount in different

examples portions of uriniferous tubules were present, mainly, however, in

the region behind the anterior swelling. In some cases I could find no

tubules in the lymphatic tissue, and in all cases the number of them beyond

the region of the well-developed part of the kidney was so slight, that

there can be little doubt that they are functionless remnants of the

anterior part of the larval kidney. Their continuation into the anterior

swelling, when present, consisted of a single tube only.

In the Eel (_Anguilla anguilla_), which, however, I have not examined with

the same care as the Smelt, the true excretory part of the kidney appears

to be confined to the posterior portion, and to the portion immediately in

front of the anus, the whole of the anterior part of each apparent kidney,

which is not swollen in front, being composed of lymphatic tissue.

_Lophius piscatorius_ is one of the forms which, according to Hyrtl[558],

is provided with a head-kidney only, _i.e._ with that part of the kidney

which corresponds with the anterior swelling of the kidney of other types.

For this reason I was particularly anxious to investigate the structure of

its kidneys.

Footnote 558: "Das Uropoëtische System der Knochenfische,"

_Sitz. d. Wien. Akad._, 1830.

Each of these bodies forms a compact oval mass, with the ureter springing

from its hinder extremity, situated in a forward position in the

body-cavity. Sections through the kidneys shewed that they were throughout

penetrated by uriniferous tubules, but owing to the bad state of

preservation of my specimens I could not come to a decision as to the

presence of Malpighian bodies. The uriniferous tubules were embedded in

lymphatic tissue, similar to that which forms the anterior part of the

apparent kidneys in other Teleostean types.

With reference to the structure of the Teleostean kidneys, the account

given by Stannius is decidedly more correct than that of most subsequent

writers. In the note already quoted he gives it as his opinion that there

is a division of the kidney into the same two parts as in the Sturgeon,

viz. into a spongy vascular part and a true secreting part; and on a

subsequent page he points out the absence or poverty of the uriniferous

tubules in the anterior part of the kidney in many of our native Fishes.

Prior to the discovery that the larvæ of Teleosteans and Ganoids were

provided with two very distinct excretory organs, viz. a pronephros or

head-kidney, and a mesonephros or Wolffian body, which are usually

separated from each other by a more or less considerable interval, it was a

matter of no very great importance to know whether the anterior part of the

so-called kidney was a true excretory organ. In the present state of our

knowledge the question is, however, one of considerable interest.

In the Cyclostomata and Amphibia the pronephros is a purely larval organ,

which either disappears or ceases to be functionally active in the adult

state.

Rosenberg, to whom the earliest satisfactory investigations on the

development of the Teleostean pronephros are due, stated that he had traced

in the Pike (_Esox lucius_) the larval organ into the adult part of the

kidney, called by Hyrtl the pronephros; and subsequent investigators have

usually assumed that the so-called head-kidney of adult Teleosteans and

Ganoids is the persisting larval pronephros.

We have already seen that Rosenberg was entirely mistaken on this point, in

that the so-called head-kidney of the adult is not part of the true kidney.

From my own studies on young Fishes I do not believe that the oldest larvæ

investigated by Rosenberg were sufficiently advanced to settle the point in

question; and, moreover, as Rosenberg had no reason for doubting that the

so-called head-kidney of the adult was part of the excretory organ, he does

not appear to have studied the histological structure of the organ which he

identified with the embryonic pronephros in his oldest larva.

The facts to which I have called attention in this paper demonstrate that

in the Sturgeon the larval pronephros undoubtedly undergoes atrophy before

the adult stage is reached. The same is true for _Lepidosteus_, and may

probably be stated for Ganoids generally.

My observations on Teleostei are clearly not sufficiently extensive to

_prove_ that the larval pronephros _never_ persists in this group. They

appear to me, however, to shew that in the normal types of Teleostei the

organ usually held to be the pronephros is actually nothing of the kind.

A different interpretation might no doubt be placed upon my observations on

_Lophius piscatorius_, but the position of the kidney in this species

appears to me to be far from affording a conclusive proof that it is

homologous with the anterior swelling of the kidney of more normal

Teleostei.

When, moreover, we consider that Lophius, and the other forms mentioned by

Hyrtl as being provided with a head-kidney only, are all of them peculiarly

modified and specialized types of Teleostei, it appears to me far more

natural to hold that their kidney is merely the ordinary Teleostean kidney,

which, like many of their other organs, has become shifted in position,

than to maintain that the ordinary excretory organ present in other

Teleostei has been lost, and that a larval organ has been retained, which

undergoes atrophy in less specialized Teleostei.

As the question at present stands, it appears to me that the probabilities

are in favour of there being no functionally active remains of the

pronephros in adult Teleostei, and that in any case the burden of proof

rests with those who maintain that such remnants are to be found.

The general result of my investigations is thus to render it probable _that

the pronephros, though found in the larvæ or embryos of almost all the

Ichthyopsida, except the Elasmobranchii, is always a purely larval organ,

which never constitutes an active part of the excretory system in the adult

state_.

This conclusion appears to me to add probability to the view of Gegenbaur

that the pronephros is the primitive excretory gland of the Chordata; and

that the mesonephros or Wolffian body, by which it is replaced in existing

Ichthyopsida, is phylogenetically a more recent organ.

In the preceding pages I have had frequent occasion to allude to the

lymphatic tissue which has been usually mistaken for part of the excretory

organ. This tissue is formed of trabecular work, like that of lymphatic

glands, in the meshes of which an immense number of cells are placed, which

may fairly be compared with the similarly placed cells of lymphatic glands.

In the Sturgeon a considerable number of cells are found with peculiar

granular nuclei, which are not found in the Teleostei. In both groups, but

especially in the Teleostei, the tissue is highly vascular, and is

penetrated throughout by a regular plexus of very large capillaries, which

appear to have distinct walls, and which pour their blood into the

posterior cardinal vein as it passes through the organ. The relation of

this tissue to the lymphatic system I have not made out.

The function of the tissue is far from clear. Its great abundance, highly

vascular character, and presence before the atrophy of the pronephros,

appear to me to shew that it cannot be merely the non-absorbed remnant of

the latter organ. From its size and vascularity it probably has an

important function; and from its structure this must either be the

formation of lymph corpuscles or of blood corpuscles.

In structure it most resembles a lymphatic gland, though, till it has been

shewn to have some relation to the lymphatic system, this can go for very

little.

On the whole, I am provisionally inclined to regard it as a form of

lymphatic gland, these bodies being not otherwise represented in fishes.

XXIV.--A RENEWED STUDY OF THE GERMINAL LAYERS OF THE CHICK.

BY F. M. BALFOUR AND F. DEIGHTON[559].

Footnote 559: From the _Quarterly Journal of Microscopical

Science_, Vol. XXII. N. S. 1882.

(With Plates 43, 44, 45.)

The formation of the germinal layers in the chick has been so often and so

fully dealt with in recent years, that we consider some explanation to be

required of the reasons which have induced us to add to the long list of

memoirs on this subject. Our reasons are twofold. In the first place the

principal results we have to record have already been briefly put forward

in a _Treatise on Comparative Embryology_ by one of us; and it seemed

desirable that the data on which the conclusions there stated rest should

be recorded with greater detail than was possible in such a treatise. In

the second place, our observations differ from those of most other

investigators, in that they were primarily made with the object of testing

a theory as to the nature of the primitive streak. As such they form a

contribution to comparative embryology; since our object has been to

investigate how far the phenomena of the formation of the germinal layers

in the chick admit of being compared with those of lower and less modified

vertebrate types.

We do not propose to weary the reader by giving a new version of the often

told history of the views of various writers on the germinal layers in the

chick, but our references to other investigators will be in the main

confined to a comparison of our results with those of two embryologists who

have published their memoirs since our observations were made. One of them

is L. Gerlach, who published a short memoir[560] in April last, and the

other is C. Koller, who has published his memoir[561] still more recently.

Both of them cover part of the ground of our investigations, and their

results are in many, though not in all points, in harmony with our own.

Both of them, moreover, lay stress on certain features in the development

which have escaped our attention. We desired to work over these points

again, but various circumstances have prevented our doing so, and we have

accordingly thought it best to publish our observations as they stand, in

spite of their incompleteness, merely indicating where the most important

gaps occur.

Footnote 560: "Ueb. d. entodermale Entstehungsweise d. Chorda

dorsalis," _Biol. Centralblatt_, Vol. 1. Nos. 1 and 2.

Footnote 561: "Untersuch. üb. d. Blätterbildung im

Hühnerkeim," _Archiv f. mikr. Anat._ Vol. XX. 1881.

Our observations commence at a stage a few hours after hatching, but before

the appearance of the primitive streak.

The area pellucida is at this stage nearly spherical. In it there is a

large oval opaque patch, which is continued to the hinder border of the

area. This opaque patch has received the name of the embryonic shield--a

somewhat inappropriate name, since the structure in question has no very

definite connection with the formation of the embryo.

Koller describes, at this stage, in addition to the so-called embryonic

shield, a sickle-shaped opaque appearance at the hinder border of the area

pellucida.

We have not made any fresh investigations for the purpose of testing

Koller's statements on this subject.

Embryologists are in the main agreed as to the structure of the blastoderm

at this stage. There is (Pl. 43, Ser. A, 1 and 2) the epiblast above,

forming a continuous layer, extending over the whole of the area opaca and

area pellucida. In the former its cells are arranged as a single row, and

are cubical or slightly flattened. In the latter the cells are more

columnar, and form, in the centre especially, more or less clearly, a

double row; many of them, however, extend through the whole thickness of

the layer.

We have obtained evidence at this stage which tends to shew that at its

outer border the epiblast grows not merely by the division of its own

cells, but also by the addition of cells derived from the yolk below. The

epiblast has been observed to extend itself over the yolk by a similar

process in many invertebrate forms.

Below the epiblast there is placed, in the peripheral part of the area

opaca, simply white yolk; while in a ring immediately outside and

concentric with the area pellucida, there is a closely-packed layer of

cells, known as the _germinal wall_. The constituent cells of this wall are

in part relatively small, of a spherical shape, with a distinct nucleus,

and a granular and not very abundant protoplasm; and in part large and

spherical, filled up with highly refracting yolk particles of variable

size, which usually render the nucleus (which is probably present)

invisible (A, 1 and 2). This mass of cell rests, on its outer side, on a

layer of white yolk.

The sickle-shaped structure, visible in surface veins, is stated by Koller

to be due to a special thickening of the germinal wall. We have not found

this to be a very distinctly marked structure in our sections.

In the region of the area pellucida there is placed below the epiblast a

more or less irregular layer of cells. This layer is continuous,

peripherally, with the germinal wall; and is composed of cells, which are

distinguished both by their flattened or oval shape and more granular

protoplasm from the epiblast-cells above, to which, moreover, they are by

no means closely attached. Amongst these cells a few larger cells are

usually present, similar to those we have already described as forming an

important constituent of the germinal wall.

We have figured two sections of a blastoderm of this age (Ser. A, 1 and 2)

mainly to shew the arrangement of these cells. A large portion of them,

considerably more flattened than the remainder, form a continuous membrane

over the whole of the area pellucida, except usually for a small area in

front, where the membrane is more or less interrupted. This layer is the

hypoblast (_hy._). The remaining cells are interposed between this layer

and the epiblast. In front of the embryonic shield there are either

comparatively few or none of these cells present (Ser. A, 1), but in the

region of the embryonic shield they are very numerous (Ser. A, 2), and are,

without doubt, the main cause of the opacity of this part of the area

pellucida. These cells may be regarded as not yet completely differentiated

segmentation spheres.

In many blastoderms, not easily distinguishable in surface views from those

which have the characters just described, the hypoblastic sheet is often

much less completely differentiated, and we have met with other

blastoderms, again, in which the hypoblastic sheet was completely

established, except at the hinder part of the embryonic shield; where, in

place of it and of the cells between it and the epiblast, there was only to

be found a thickish layer of rounded cells, continuous behind with the

germinal wall.

In the next stage, of which we have examined surface views and sections,

there is already a well-formed primitive streak.

The area pellucida is still nearly spherical, the embryonic shield has

either disappeared or become much less obvious, but there is present a dark

linear streak, extending from the posterior border of the area pellucida

towards the centre, its total length being about one third, or even less,

of the diameter of the area. This streak is the _primitive streak_. It

enlarges considerably behind, where it joins the germinal wall. By Koller

and Gerlach it is described as joining the sickle-shaped structure already

spoken of. We have in some instances found the posterior end of the

primitive streak extending laterally in the form of two wings (Pl. 45,

fig. L). These extensions are, no doubt, the sickle; but the figures given

by Koller appear to us somewhat diagrammatic. One or two of the figures of

early primitive streaks in the sparrow, given by Kupffer and Benecke[562],

correspond more closely with what we have found, except that in these

figures the primitive streak does not reach the end of the area pellucida,

which it certainly usually does at this early stage in the chick.

Footnote 562: "Photogramme d. Ontogenie d. Vogel." Nova Acta.

K. Leop. Carol, _Deutschen Akad. d. Naturfor_. Bd. X. 41,

1879.

Sections through the area pellucida (Pl. 43, Ser. B and C) give the

following results as to the structure of its constituent parts.

The epiblast cells have undergone division to a considerable extent, and in

the middle part, especially, are decidedly more columnar than at an earlier

stage, and distinctly divided into two rows, the nuclei of which form two

more or less distinct layers.

In the region in front of the primitive streak the cells of the lower part

of the blastoderm have arranged themselves as a definite layer, the cells

of which are not so flat as is the case with the hypoblast cells of the

posterior part of the blastoderm, and in the older specimens of this stage

they are very decidedly more columnar than in the younger specimens.

The primitive streak is however the most interesting structure in the area

pellucida at this stage.

The feature which most obviously strikes the observer in transverse

sections through it is the fact, proved by Kölliker, that it is mainly due

to a proliferation of the epiblast cells along an axial streak, which,

roughly speaking, corresponds with the dark line visible in surface views.

In the youngest specimens and at the front end of the primitive streak, the

proliferated cells do not extend laterally beyond the region of their

origin, but in the older specimens they have a considerable lateral

extension.

The hypoblast can, in most instances, be traced as a distinct layer

underneath the primitive streak, although it is usually less easy to follow

it in that region than elsewhere, and in some cases it can hardly be

distinctly separated from the superjacent cells.

The cells, undoubtedly formed by a proliferation of the epiblast, form a

compact mass extending downwards towards the hypoblast; but between this

mass and the hypoblast there are almost always present along the whole

length of the primitive streak a number of cells, more or less loosely

arranged, and decidedly more granular than the proliferated cells. Amongst

these loosely arranged cells there are to be found a certain number of

large spherical cells filled with yolk granules. Sometimes these cells are

entirely confined to the region of the primitive streak, at other times

they are continuous laterally with cells irregularly scattered between the

hypoblast and epiblast (Ser. C, 2), which are clearly the remnants of the

undifferentiated cells of the embryonic shield. The junction between these

cells and the cells of the primitive streak derived from the epiblast is

often obscure, the two sets of cells becoming partially intermingled. The

facility with which the cells we have just spoken of can be recognized

varies moreover greatly in different instances. In some cases they are very

obvious (Ser. C), while in other cases they can only be distinguished by a

careful examination of good sections.

The cells of the primitive streak between the epiblast and the hypoblast

are without doubt mesoblastic, and constitute the first portion of the

mesoblast which is established. The section of these cells attached to the

epiblast, in our opinion, clearly originates from the epiblast; while the

looser cells adjoining the hypoblast must, it appears to us, be admitted to

have their origin in the indifferent cells of the embryonic shield, placed

between the epiblast and the hypoblast, and also very probably in a

distinct proliferation from the hypoblast below the primitive streak.

Posteriorly the breadth of the streak of epiblast which buds off the cells

of the primitive streak widens considerably, and in the case of the

blastoderm with the earliest primitive streaks extends into the region of

the area opaca. The widening of the primitive streak behind is shewn in

Ser. B, 3; Ser. C, 2; and Ser. E, 4. Where very marked it gives rise to the

sickle-shaped appearance upon which so much stress has been laid by Koller

and Gerlach. In the case of one of the youngest of our blastoderms of this

stage in which we found in surface views (Pl. 45, fig. L) a very

well-marked sickle-shaped appearance at the hind end of the primitive

streak, the appearance was caused, as is clearly brought out by our

sections, by a thickening of the hypoblast of the germinal wall.

There is a short gap in our observations between the stage with a young

primitive streak and the first described stage in which no such structure

is present. This gap has been filled up both by Gerlach and Koller.

Gerlach states that during this period a small portion of the epiblast,

within the region of the area opaca, but close to the posterior border of

the area pellucida, becomes thickened by a proliferation of its cells. This

portion gradually grows outwards laterally, forming in this way a

sickle-shaped structure. From the middle of this sickle a process next

grows forward into the area pellucida. This process is the primitive

streak, and it is formed, like the sickle, of proliferating epiblast cells.

Koller[563] described the sickle and the growth forwards from it of the

primitive streak in surface views somewhat before Gerlach; and in his later

memoir has entered with considerable detail into the part played by the

various layers in the formation of this structure.

Footnote 563: "Beitr. z. Kenntuiss d. Hühmerkeims im Beginne

d. Bebrütung," _Sitz. d. k. Akad. Wiss._ IV. Abth. 1879.

He believes, as already mentioned, that the sickle-shaped structure, which

appears according to him at an earlier stage than is admitted by Gerlach,

is in the first instance due to a thickening of the hypoblast. At a later

stage he finds that the epiblast in the centre of the sickle becomes

thickened, and that a groove makes its appearance in this thickening which

he calls the "Sichel-rinne." This groove is identical with that first

described by Kupffer and Benecke[564] in the sparrow and fowl. We have

never, however, found very clear indications of it in our sections.

Footnote 564: _Die erste Entwick. an Eier d. Reptilien._

Königsberg. 1878.

In the next stage, Koller states that, in the region immediately in front

of the "Sichel-rinne," a prominence appears which he calls the Sichelknopf,

and from this a process grows forwards which constitutes the primitive

streak. This structure is in main derived from a proliferation of epiblast

cells, but Koller admits that some of the cells just above the hypoblast in

the region of the Sichelknopf are probably derived from the hypoblast.

Since these cells form part of the mesoblast it is obvious that Koller's

views on the origin of the mesoblast of the primitive streak closely

approach those which we have put forward.

The primitive streak starting, as we have seen, at the hinder border of the

area pellucida, soon elongates till it eventually occupies at least

two-thirds of the length of the area. As Koller (_loc. cit._) has stated,

this can only be supposed to happen in one of two ways, viz. either by a

progression forward of the region of epiblast budding off mesoblast, or by

an interstitial growth of the area of budding epiblast. Koller adopts the

second of these alternatives, but we cannot follow him in doing so. The

simplest method of testing the point is by measuring the distance between

the front end of the primitive streak and the front border of the area

pellucida at different stages of growth of the primitive streak. If this

distance diminishes with the elongation of the primitive streak then

clearly the second of the two alternatives is out of the question.

We have made measurements to test this point, and find that the diminution

of the space between the front end of the primitive streak and the anterior

border of the area pellucida is very marked up to the period in which the

medullary plate first becomes established. We can further point in support

of our view to the fact that the extent of the growth lateralwards of the

mesoblast from the sides of the primitive streak is always less in front

than behind; which would seem to indicate that the front part of the streak

is the part formed latest. Our view as to the elongation of the primitive

streak appears to be that adopted by Gerlach.

Our next stage includes roughly the period commencing slightly before the

first formation of a groove along the primitive streak, known as the

primitive groove, and terminating immediately before the first trace of the

notochord makes its appearance. After the close of the last stage the

primitive streak gradually elongates, till it occupies fully two-thirds of

the diameter of the area pellucida. The latter structure also soon changes

its form from a circular to an oval, and finally becomes pyriform with the

narrow end behind, while the primitive streak occupying two-thirds of its

long axis becomes in most instances marked by a light linear band along the

centre, which constitutes the primitive groove.

In surface views the primitive streak often appears to stop short of the

hinder border of the area pellucida.

During the period in which the external changes, which we have thus briefly

described, take place in the area pellucida, great modifications are

effected in the characters of the germinal layers. The most important of

these concern the region in front of the primitive streak; but they will be

better understood if we commence our description with the changes in the

primitive streak itself.

In the older embryos belonging to our last stage we pointed out that the

mesoblast of the primitive streak was commencing to extend outwards from

the median line in the form of two lateral sheets. This growth of the

mesoblast is continued rapidly during the present stage, so that during the

latter part of it any section through the primitive streak has

approximately the characters of Ser. I, 5.

The mesoblast is attached in the median line to the epiblast. Laterally it

extends outwards to the edge of the area pellucida, and in older embryos

may even form a thickening beyond the edge (fig. G). Beneath the denser

part of the mesoblast, and attached to the epiblast, a portion composed of

stellate cells may in the majority of instances be recognized, especially

in the front part of the primitive streak. We believe these stellate cells

to be in the main directly derived from the more granular cells of the

previous stage. The hypoblast forms a sheet of flattened cells, which can

be distinctly traced for the whole breadth of the area pellucida, though

closely attached to the mesoblast above.

In sections we find that the primitive streak extends back to the border of

the area pellucida, and even for some distance beyond. The attachment to

the epiblast is wider behind; but the thickness of the mesoblast is not

usually greater in the median line than it is laterally, and for this

reason probably the posterior part of the streak fails to shew up in

surface views. The thinning out of the median portion of the mesoblast of

the primitive streak is shewn in a longitudinal section of a duck's

blastoderm of this stage (fig. D). The same figure also shews that the

hypoblastic sheet becomes somewhat thicker behind, and more independent of

the parts above.

A careful study of the peripheral part of the area pellucida, in the region

of the primitive streak, in older embryos of this stage, shews that the

hypoblast is here thickened, and that its upper part, _i.e._ that adjoining

the mesoblast, is often formed of stellate cells, many of which give the

impression of being in the act of passing into the mesoblast above. At a

later stage the mesoblast of the vascular area undoubtedly receives

accessions of cells from the yolk below; so that we see no grounds for

mistrusting the appearances just spoken of, or for doubting that they are

to be interpreted in the sense suggested.

We have already stated that during the greater part of the present stage a

groove, known as the primitive groove, is to be found along the dorsal

median line of the primitive streak.

The extent to which this groove is developed appears to be subject to very

great variation. On the average it is, perhaps, slightly deeper than it is

represented in Ser. I, 5. In some cases it is very much deeper. One of the

latter is represented in fig. G. It has here the appearance of a narrow

slit, and sections of it give the impression of the mesoblast originating

from the lips of a fold; in fact, the whole structure appears like a linear

blastopore, from the sides of which the mesoblast is growing out; and this

as we conceive actually to be the true interpretation of the structure.

Other cases occur in which the primitive groove is wholly deficient, or at

the utmost represented by a shallow depression along the median axial line

of a short posterior part of the primitive streak.

We may now pass to the consideration of the part of the area pellucida in

front of the primitive streak.

We called attention to a change in the character of the hypoblast cells of

this region as taking place at the end of the last stage. During the very

early part of this stage the change in the character of these cells becomes

very pronounced.

What we consider to be our earliest stage in this change we have only so

far met with in the duck, and we have figured a longitudinal and median

section to shew it (Pl. 43, fig. D). The hypoblast (_hy_) has become a

thick layer of somewhat cubical cells several rows deep. These cells,

especially in front, are characterized by their numerous yolk spherules,

and give the impression that part of the area pellucida has been, so to

speak, reclaimed from the area opaca. _Posteriorly, at the front end of the

primitive streak, the thick layer of hypoblast, instead of being continuous

with the flattened hypoblast under the primitive streak, falls, in the

axial line, into the mesoblast of the primitive streak_ (Pl. 43, fig. D).

In a slightly later stage, of which we have specimens both of the duck and

chick, but have only figured selected sections of a chick series, still

further changes have been effected in the constitution of the hypoblast

(Pl. 44, Ser. H, 1 and 2).

Near the front border of the area pellucida (1) it has the general

characters of the hypoblast of the duck's blastoderm just described.

Slightly further back the cells of the hypoblast have become differentiated

into stellate cells several rows deep, _which can hardly be resolved in the

axial line into hypoblast and mesoblast_, though one can fancy that in

places, especially laterally, they are partially differentiated into two

layers. The axial sheet of stellate cells is continuous laterally with

cubical hypoblast cells.

As the primitive streak is approached an axial prolongation forwards of the

rounded and closely-packed mesoblastic elements of the primitive streak is

next met with; and at the front end of the primitive streak, where this

prolongation unites with the epiblast, it also becomes continuous with the

stellate cells just spoken of. In fact, close to the end of the primitive

streak it becomes difficult to say which mesoblast cells are directly

derived from the primitive layer of hypoblast in front of the primitive

streak, and which from the forward growth of the mesoblast of the primitive

streak. There is, in fact, as in the earlier stage, a fusion of the layers

at this point.

Sections of a slightly older chick blastoderm are represented in Pl. 45,

Ser. I, 1, 2, 3, 4 and 5.

Nearly the whole of the hypoblast in front of the primitive streak has now

undergone a differentiation into stellate cells. In the second section the

products of the differentiation of this layer form a distinct mesoblast and

hypoblast laterally, while in the median line they can hardly be divided

into two distinct layers.

In a section slightly further back the same is true, except that we have

here, in the axial line above the stellate cells, rounded elements derived

from a forward prolongation of the cells of the primitive streak. In the

next section figured, passing through the front end of the primitive

streak, the axial cells have become continuous with the axial mesoblast of

the primitive streak, while below there is an independent sheet of

flattened hypoblast cells.

The general result of our observations on the part of the blastoderm in

front of the primitive streak during this stage is to shew that the

primitive hypoblast of this region undergoes considerable changes,

including a multiplication of its cells; and that these changes result in

its becoming differentiated on each side of the middle line, with more or

less distinctness, into (1) a hypoblastic sheet below, formed of a single

row of flattened cells, and (2) a mesoblast plate above formed of stellate

cells, while in the middle line there is a strip of stellate cells in which

there is no distinct differentiation into two layers.

Since the region in which these changes take place is that in which the

medullary plate becomes subsequently formed, the lateral parts of the

mesoblast plate are clearly the permanent lateral plates of the trunk, from

which the mesoblastic somites, &c., become subsequently formed; _so that

the main part of the mesoblast of the trunk is not directly derived from

the primitive streak_.

Before leaving this stage we would call attention to the presence, in one

of our blastoderms of this stage, of a deep pit at the junction of the

primitive streak with the region in front of it (Pl. 44, Ser. F, 1 and 2).

Such a pit is unusual, but we think it may be regarded as an exceptionally

early commencement of that most variable structure in the chick, the

neurenteric canal.

The next and last stage we have to deal with is that during which the first

trace of the notochord and of the medullary plate make their appearance.

In surface views this stage is marked by the appearance of a faint dark

line, extending forwards, from the front end of the primitive streak, to a

fold, which has in the mean time made its appearance near the front end of

the area pellucida, and constitutes the head fold.

Pl. 45, Ser. K, represents a series of sections through a blastoderm of

this stage, which have been selected to illustrate the mode of formation of

the notochord.

In a section immediately behind the head fold the median part of the

epiblast is thicker than the lateral parts, forming the first indication of

a medullary plate (Ser. K, 1). Below the median line of the epiblast is a

small cord of cells, not divided into two layers, but continuous laterally,

both with the hypoblast and mesoblast, which are still more distinctly

separated than in the previous stage.

A section or so further back (Ser. K, 2) the axial cord, which we need

scarcely say is the rudiment of the notochord, is thicker, and causes a

slight projection in the epiblast above. It is, as before, continuous

laterally, both with the mesoblast and with the hypoblast. The medullary

plate is more distinct, and a shallow but unmistakable medullary groove has

made its appearance.

As we approach the front end of the primitive streak the notochord becomes

(Ser. K, 3) very much more prominent, though retaining the same relation to

the germinal layers as in front.

In the section immediately behind (Ser. K, 4) the convex upper surface of

the notochord has become continuous with the epiblast for a very small

region. The section, in fact, traverses the front end of the primitive

streak.

In the next section the attachment between the epiblast and the cells below

becomes considerably wider. It will be noticed that this part of the

primitive streak is placed on the floor of the wide medullary groove, and

there forms a prominence known as the anterior swelling of the primitive

streak.

It will further be noticed that in the two sections passing through the

primitive streak, the hypoblast, instead of simply becoming continuous with

the axial thickening of the cells, as in front, forms a more or less

imperfect layer underneath it. This layer becomes in the sections following

still more definite, and forms part of the continuous layer of hypoblast

present in the region of the primitive streak.

A comparison of this stage with the previous one shews very clearly that

the notochord is formed out of the median plate of cells of the earlier

stage, which was not divided into mesoblast and hypoblast, together with

the short column of cells which grew forwards from the primitive streak.

The notochord, from its mode of origin, is necessarily continuous behind

with the axial cells of the primitive streak.

The sections immediately behind the last we have represented shew a

rudiment of the neurenteric canal of the same form as that first figured by

Gasser, viz. a pit perforating the epiblast with a great mass of rounded

cells projecting upwards through it.

* * * * *

The observations just recorded practically deal with two much disputed

points in the ontogeny of birds, viz. the origin of the mesoblast and the

origin of the notochord.

With reference to the first of these our results are briefly as follows:

The first part of the mesoblast to be formed is that which arises in

connection with the primitive streak. This part is in the main formed by a

proliferation from an axial strip of the epiblast along the line of the

primitive streak, but in part also from a simultaneous differentiation of

hypoblast cells also along the axial line of the primitive streak. The two

parts of the mesoblast so formed become subsequently indistinguishable. The

second part of the mesoblast to be formed is that which gives rise to the

lateral plates of mesoblast of the head and trunk of the embryo. This part

appears as two plates--one on each side of the middle line--which arise by

direct differentiation from the hypoblast in front of the primitive streak.

They are continuous behind with the lateral wings of mesoblast which grow

out from the primitive streak, and on their inner side are also at first

continuous with the cells which form the notochord.

In addition to the parts of mesoblast, formed as just described, the

mesoblast of the vascular area is in a large measure developed by a direct

formation of cells round the nuclei of the germinal wall.

The mesoblast formed in connection with the primitive streak gives rise in

part to the mesoblast of the allantois, and ventral part of the tail of the

embryo (?), and in part to the vascular structures found in the area

pellucida.

With reference to the formation of the mesoblast of the primitive streak,

our conclusions are practically in harmony with those of Koller; except

that Koller is inclined to minimise the share taken by the hypoblast in the

formation of the mesoblast of the primitive streak.

Gerlach, with reference to the formation of this part of the mesoblast,

adopts the now generally accepted view of Kölliker, according to which the

whole of the mesoblast of the primitive streak is derived from the

epiblast.

As to the derivation of the lateral plates of mesoblast of the trunk from

the hypoblast of the anterior part of the primitive streak, our general

result is in complete harmony with Gerlach's results, although in our

accounts of the details of the process we differ in some not unimportant

particulars.

As to the origin of the notochord, our main result is that this structure

is formed as an actual thickening of the primitive hypoblast of the

anterior part of the area pellucida. We find that it unites posteriorly

with a forward growth of the axial tissue of the primitive streak, while it

is laterally continuous, at first, both with the mesoblast of the lateral

plates and with the hypoblast. At a later period its connection with the

mesoblast is severed, while the hypoblast becomes differentiated as a

continuous layer below it.

As to the hypoblastic origin of the notochord, we are again in complete

accord with Gerlach; but we differ from him in admitting that the notochord

is continuous posteriorly with the axial tissue of the primitive streak,

and also at first continuous with the lateral plates of mesoblast.

The account we have given of the formation of the mesoblast may appear to

the reader somewhat fantastic, and on that account not very credible. We

believe, however, that if the view which has been elsewhere urged by one of

us, that the primitive streak is the homologue of the blastopore of the

lower vertebrates is accepted, the features we have described receive an

adequate explanation.

The growth outwards of part of the mesoblast from the axial line of the

primitive streak is a repetition of the well-known growth from the lips of

the blastopore. It might have been anticipated that all the layers would

fuse along the line of the primitive streak, and that the hypoblast as well

as part of the mesoblast would grow out from it. There is, however, clearly

a precocious formation of the hypoblast; but the formation of the mesoblast

of the primitive streak, partly from the epiblast and partly from the

hypoblast, is satisfactorily explained by regarding the whole structure as

the blastopore. The two parts of the mesoblast subsequently become

indistinguishable, and their difference in origin is, on the above view, to

be regarded as simply due to a difference of position, and not as having a

deeper significance.

The differentiation of the lateral plates of mesoblast of the trunk

directly from the hypoblast is again a fundamental feature of vertebrate

embryology, occurring in all types from Amphioxus upwards, the meaning of

which has been fully dealt with in the _Treatise on Comparative Embryology_

by one of us. Lastly, the formation of the notochord from the hypoblast is

the typical vertebrate mode of formation of this organ, while the fusion of

the layers at the front end of the primitive streak is the universal fusion

of the layers at the dorsal lip of the blastopore, which is so well known

in the lower vertebrate types.

EXPLANATION OF PLATES 43-45.

N. B. The series of sections are in all cases numbered from before

backwards.

LIST OF REFERENCE LETTERS.

_a.p._ Area pellucida. _ep._ Epiblast. _ch._ Notochord. _gr._ Germinal

wall. _hy._ Hypoblast. _m._ Mesoblast. _o.p._ Area opaca. _pr.g._ Primitive

groove. _pvs._ Primitive streak. _yk._ Yolk of germinal wall.

PLATE 43.

SERIES A, 1 and 2. Sections through the blastoderm before the appearance of

primitive streak.

1. Section through anterior part of area pellucida in front of embryonic

shield. The hypoblast here forms an imperfect layer. The figure represents

about half the section. 2. Section through same blastoderm, in the region

of the embryonic shield. Between the epiblast and hypoblast are a number of

undifferentiated cells. The figure represents considerably more than half

the section.

SERIES B, 1, 2 and 3. Sections through a blastoderm with a very young

primitive streak.

1. Section through the anterior part of the area pellucida in front of the

primitive streak. 2. Section through about the middle of the primitive

streak. 3. Section through the posterior part of the primitive streak.

SERIES C, 1 and 2. Sections through a blastoderm with a young primitive

streak.

1. Section through the front end of the primitive streak. 2. Section

through the primitive streak, somewhat behind 1. Both figures shew very

clearly the difference in character between the cells of the epiblastic

mesoblast of the primitive streak, and the more granular cells of the

mesoblast derived from the hypoblast.

FIG. D. Longitudinal section through the axial line of the primitive

streak, and the part of the blastoderm in front of it, of an embryo duck

with a well-developed primitive streak.

PLATE 44.

SERIES E, 1, 2, 3 and 4. Sections through blastoderm with a primitive

streak, towards the end of the first stage.

1. Section through the anterior part of the area pellucida. 2. Section a

little way behind 1 shewing a forward growth of mesoblast from the

primitive streak. 3. Section through primitive streak. 4. Section through

posterior part of primitive streak, shewing the great widening of primitive

streak behind.

SERIES F, 1 and 2. Sections through a blastoderm with primitive groove.

1. Section shewing a deep pit in front of primitive streak, probably an

early indication of the neurenteric canal. 2. Section immediately following

1.

FIG. G. Section through blastoderm with well-developed primitive streak,

shewing an exceptionally deep slit-like primitive groove.

SERIES H, 1 and 2. Sections through a blastoderm with a fully-developed

primitive streak.

1. Section through the anterior part of area pellucida, shewing the cubical

granular hypoblast cells in this region. 2. Section slightly behind 1,

shewing the primitive hypoblast cells differentiated into stellate cells,

which can hardly be resolved in the middle line into hypoblast and

mesoblast.

PLATE 45.

SERIES I, 1, 2, 3, 4 and 5. Sections through blastoderm somewhat older than

Series H.

1. Section through area pellucida well in front of primitive streak. 2.

Section through area pellucida just in front of primitive streak. 3.

Section through the front end of primitive streak. 4. Section slightly

behind 3. 5. Section slightly behind 4.

SERIES K, 1, 2, 3, 4 and 5. Sections through a blastoderm in which the

first traces of notochord and medullary groove have made their appearance.

Rather more than half the section is represented in each figure, but the

right half is represented in 1 and 3, and the left in 2 and 4.

1. Section through notochord immediately behind the head fold. 2. Section

shewing medullary groove a little behind 1. 3. Section just in front of the

primitive streak. 4 and 5. Sections through the front end of the primitive

streak.

FIG. L. Surface view of blastoderm with a very young primitive streak.

XXV. THE ANATOMY AND DEVELOPMENT OF PERIPATUS CAPENSIS[565].

Footnote 565: From the _Quarterly Journal of Microscopical

Science_, April, 1883.

(With Plates 46-53.)

INTRODUCTION.

The late Professor Balfour was engaged just before his death in

investigating the structure and embryology of _Peripatus capensis_, with

the view of publishing a complete monograph of the genus. He left numerous

drawings intended to serve as illustrations to the monograph, together with

a series of notes and descriptions of a large part of the anatomy of

_Peripatus capensis_. Of this manuscript some portions were ready for

publication, others were more or less imperfect; while of the figures many

were without references, and others were provided with only a few words of

explanation.

It was obviously necessary that Professor Balfour's work--embodying as it

did much important discovery--should be published without delay; and the

task of preparing his material for the press was confided to us. We have

printed all his notes and descriptions without alteration[566].

Explanations which appeared to be necessary, and additions to the text in

cases in which he had prepared figures without writing descriptions,

together with full descriptions of all the plates, have been added by us,

and are distinguished by enclosure in square brackets[567].

Footnote 566: Excepting in an unimportant matter of change of

nomenclature used with regard to the buccal cavity.

Footnote 567: The account of the external characters,

generative organs, and development, has been written by the

editors.

We have to thank Miss Balfour, Professor Balfour's sister, for the

important service which she has rendered by preparing a large part of the

beautiful drawings with which the monograph is illustrated. Many of these

had been executed by her under Professor Balfour's personal supervision;

and the knowledge of his work which she then acquired has been of the

greatest assistance to us in preparing the MSS. and drawings for

publication.

Since his death she has spared no pains in studying the structure of

_Peripatus_, so as to enable us to bring out the first part of the

monograph in as complete a state as possible. It is due to her skill that

the first really serviceable and accurate representation of the legs of any

species of _Peripatus_ available for scientific purposes are issued with

the present memoir[568].

Footnote 568: The drawings on Pl. 47, figs. 9 and 10 on Pl. 48,

and the drawings of the embryos (except fig. 37), have been

made by Miss Balfour since Professor Balfour's death.

We have purposely refrained from introducing comments on the general

bearing of the new and important results set forth in this memoir, and have

confined ourselves to what was strictly necessary for the presentation of

Mr Balfour's discoveries in a form in which they could be fully

comprehended.

Mr Balfour had at his disposal numerous specimens of _Peripatus novæ

zealandiæ_, collected for him by Professor Jeffrey Parker, of Christchurch,

New Zealand; also specimens from the Cape of Good Hope collected by Mr

Lloyd Morgan, and brought to England by Mr Roland Trimen in 1881; and

others given to him by Mr Wood Mason, together with all the material

collected by Mr Moseley during the "Challenger" voyage.

A preliminary account of the discoveries as to the embryology of

_Peripatus_ has already been communicated to the Royal Society[569]. It is

intended that the present memoir shall be followed by others, comprising a

complete account of all the species of the genus _Peripatus_.

H. M. MOSELEY.

A. SEDGWICK.

Footnote 569: _Proc. Royal Soc._ 1883.

PART I.

DESCRIPTION OF THE SPECIES.

_Peripatus capensis_ (fig. 1).

[The body is elongated, and slightly flattened dorso-ventrally. The dorsal

surface is arched, and darkly pigmented; while the ventral surface is

nearly flat, and of a lighter colour.

The mouth is placed at the anterior end of the body, on the ventral

surface.

The anus is posterior and terminal.

The generative opening is single and median, and placed in both sexes on

the ventral surface, immediately in front of the anus.

There are a pair of ringed antennæ projecting from the anterior end of the

head, and a pair of simple eyes, placed on the dorsal surface at the roots

of the antennæ.

The appendages of the body behind the antennæ are disposed in twenty pairs.

1. The single pair of jaws placed within the buccal cavity in front of the

true mouth opening, and consisting each of a papilla, armed at its

termination with two cutting blades.

2. The oral papillæ placed on each side of the mouth. At their apices the

ducts of the slime glands open.

3. The seventeen pairs of ambulatory appendages, each provided with a pair

of chitinous claws at its extremity.

4. The anal papillæ placed on each side of the generative opening.

_Colour._--The following statements on this head are derived from

observations of spirit specimens. The colour varies in different

individuals. It always consists of a groundwork of green and bluish grey,

with a greater or less admixture of brown. The chief variations in the

appearance of the animal, so far as colour is concerned, depend on the

shade of the green. In some it is dark, as in the specimen figured

(fig. 1); in others it is of a lighter shade.

There is present in most specimens a fairly broad light band on each side

of the body, immediately dorsal to the attachment of the legs. This band is

more prominent in the lighter coloured varieties than in the dark, and is

especially conspicuous in large individuals. It is due to a diminution in

the green pigment, and an increase in the brown.

There is a dark line running down the middle of the dorsal surface, in the

middle of which is a fine whitish line.

The ventral surface is almost entirely free from the green pigment, but

possesses a certain amount of light brown. This brown pigment is more

conspicuous and of a darker shade on the spinous pads of the foot.

In parts of the body where the pigment is scarce, it is seen to be confined

to the papillæ. This is especially evident round the mouth, where the

sparse green pigment is entirely confined to the papillæ.

In some specimens a number of white papillæ, or perhaps light brown, are

scattered over the dorsal surface; and sometimes there is a scattering of

green papillæ all over the ventral surface. These two peculiarities are

more especially noticeable in small specimens.

_Ridges and Papillæ of the Skin._--The skin is thrown into a number of

transverse ridges, along which the primary wart-like papillæ are placed.

The papillæ, which are found everywhere, are specially developed on the

dorsal surface, less so on the ventral. The papillæ round the lips differ

from the remaining papillæ of the ventral surface in containing a green

pigment. Each papilla bears at its extremity a well-marked spine.

The ridges of the skin are not continued across the dorsal middle line,

being interrupted by the whitish line already mentioned. Those which lie in

the same transverse line as the legs are not continued on to the latter,

but stop at the junction of the latter with the body. All the others pass

round to the ventral surface and are continued across the middle line; they

do not, however, become continuous with the ridges of the other side, but

passing between them gradually thin off and vanish.

The ridges on the legs are directed transversely to their long axes, _i.e._

are at right angles to the ridges of the rest of the body.

The antennæ are ringed and taper slightly till near their termination,

where they present a slight enlargement in spirit specimens, which in its

turn tapers to its termination.

The rings consist essentially of a number of coalesced primary papillæ, and

are, therefore, beset by a number of spines like those of the primary

papillæ (described below). They are more deeply pigmented than the rest of

the antenna.

The free end of the antenna is covered by a cap of tissue like that of the

rings. It is followed by four or more rings placed close together on the

terminal enlargement. There appears to be about thirty rings on the antennæ

of all adults of this species. But they are difficult to count, and a

number of small rings occur between them, which are not included in the

thirty.

The antennæ are prolongations of the dorso-lateral parts of the anterior

end of the body.

The eyes are paired and are situated at the roots of the antennæ on the

dorso-lateral parts of the head. Each is placed on the side of a

protuberance which is continued as the antenna, and presents the appearance

of a small circular crystalline ball inserted on the skin in this region.

The rings of papillæ on that part of the head from which the antennæ arise

lose their transverse arrangement. They are arranged concentrically to the

antennal rings, and have a straight course forwards between the antennæ.

The oral papillæ are placed at the side of the head. They are attached

ventro-laterally on each side of the lips. The duct of the slime gland

opens through their free end. They possess two main rings of projecting

tissue, which are especially pigmented on the dorsal side; and their

extremities are covered by papillæ irregularly arranged.

The buccal cavity, jaws, and lips are described below.

_The Ambulatory Appendages._--The claw-bearing legs are usually seventeen

in number; but in two cases of small females we have observed that the anal

papillæ bear claws, and present all the essential features of the

ambulatory appendages. In one small female specimen there were twenty pairs

of claw-bearing appendages, the last being like the claw-bearing anal

papillæ last mentioned, and the generative opening being placed between

them.

The ambulatory appendages, with the exception of the fourth and fifth pairs

in both sexes, and the last pair (seventeenth) in the male, all resemble

each other fairly closely. A typical appendage (figs. 2 and 3) will first

be described, and the small variations found in the appendages just

mentioned will then be pointed out. Each consists of two main divisions, a

larger proximal portion, the leg, and a narrow distal claw-bearing portion,

the foot.

The leg has the form of a truncated cone, the broad end of which is

attached to the ventro-lateral body-wall, of which it appears to be, and

is, a prolongation. It is marked by a number of rings of primary papillæ,

placed transversely to the long axis of the leg, the dorsal of which

contain a green and the ventral a brown pigment. These rings of papillæ, at

the attachment of the leg, gradually change their direction and merge into

the body rings. At the narrow end of the cone there are three ventrally

placed pads, in which the brown pigment is dark, and which are covered by a

number of spines precisely resembling the spines of the primary papillæ.

These spinous pads are continued dorsally, each into a ring of papillæ.

The papillæ of the ventral row next the proximal of these spinous pads are

intermediate in character between the primary papillæ and the spinous pads.

Each of these papillæ is larger than a normal papilla, and bears several

spines (fig. 2). This character of the papilla of this row is even more

marked in some of the anterior legs than in the one figured; it seems

probable that the pads have been formed by the coalescence of several rows

of papillæ on the ventral surface of the legs. On the outer and inner sides

of these pads the spines are absent, and secondary papillæ only are

present.

In the centre of the basal part of the ventral surface of the foot there

are present a group of larger papillæ, which are of a slightly paler colour

than the others. They are arranged so as to form a groove, directed

transversely to the long axis of the body, and separated at its internal

extremity by a median papilla from a deep pit which is placed at the point

of junction of the body and leg. The whole structure has the appearance,

when viewed with the naked eye, of a transverse slit placed at the base of

the leg. The segmental organs open by the deep pit placed at the internal

end of this structure. The exact arrangement of the papillæ round the outer

part of the slit does not appear to be constant.

The foot is attached to the distal end of the leg. It is slightly narrower

at its attached extremity than at its free end, which bears the two claws.

The integument of the foot is covered with secondary papillæ, but spines

and primary papillæ are absent, except at the points now to be described.

On each side of the middle ventral line of the proximal end of the foot is

placed an elliptical elevation of the integument covered with spines.

Attached to the proximal and lateral end of this is a primary papilla. At

the distal end of the ventral side of the foot on each side of the middle

line is a group of inconspicuous pale elevations, bearing spines.

On the front side of the distal end of the foot, close to the socket in

which the claws are placed, are two primary papillæ, one dorsal and the

other ventral.

On the posterior side of the foot the dorsal of these only is present. The

claws are sickle-shaped, and placed on papillæ on the terminal portion of

the foot. The part of the foot on which they are placed is especially

retractile, and is generally found more or less telescoped into the

proximal part (as in the figure).

The fourth and fifth pairs of legs exactly resemble the others, except in

the fact that the proximal pad is broken up into three, a small central and

two larger lateral. The enlarged segmental organs of these legs open on the

small central division.

The last (17) leg of the male (Pl. 47, fig. 4) is characterized by

possessing a well-marked white papilla on the ventral surface. This

papilla, which presents a slit-like opening at its apex, is placed on the

second row of papillæ counting from the innermost pad, and slightly

posterior to the axial line of the leg.

The anal papillæ, or as they should be called, generative papillæ, are

placed one on each side of the generative aperture. They are most marked in

small and least so in large specimens. That they are rudimentary ambulatory

appendages is shewn by the fact that they are sometimes provided with

claws, and resemble closely the anterior appendages.]

PART II.

ALIMENTARY CANAL.

The alimentary canal of _Peripatus capensis_ forms, in the extended

condition of the animal, a nearly straight tube, slightly longer than the

body, the general characters of which are shewn in figs. 6 and 7.

For the purposes of description, it may conveniently be divided into five

regions, viz. (1) the buccal cavity with the tongue, jaws, and salivary

glands, (2) pharynx, (3) the oesophagus, (4) the stomach, (5) the rectum.

_The Buccal Cavity._--The buccal cavity has the form of a fairly deep pit,

of a longitudinal oval form, placed on the ventral surface of the head, and

surrounded by a tumid lip.

[The buccal cavity has been shewn by Moseley to be formed in the embryo by

the fusion of a series of processes surrounding the true mouth-opening, and

enclosing in their fusion the jaws.]