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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London. Modern Notes:

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Vol. III. A Treatise on Comparative Embryology 2 (1885)

CHAPTER XVII. AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE ORGANS OF THE LATERAL LINE

Auditory Organs.

A GREAT variety of organs, very widely distributed amongst aquatic forms, and also found, though less universally, in land forms, are usually classed together as auditory organs.

In the case of all aquatic forms, or of forms which have directly inherited their auditory organs from aquatic forms, these organs are built upon a common type ; although in the majority of instances the auditory organs of the several groups have no genetic relations. All the organs have their origin in specialized portions of the epidermis. Some of the cells of a special region become provided at their free extremities with peculiar hairs, known as auditory hairs; while in other cells concretions, known as otoliths, are formed, which appear often to be sufficiently free to be acted upon by vibrations of the surrounding medium, and to be so placed as to be able in their turn to transmit their vibrations to the cells with auditory hairs 1 . The auditory regions of the epidermis are usually shut off from the surface in special sacks.

The actual function of these organs is no doubt correctly described, in the majority of instances, as being auditory; but it appears to me very possible that in some cases their function may be to enable the animals provided with them to detect the presence of other animals in their neighbourhood, through the

1 The function of the otoliths is not always clear. There is evidence to shew that they sometimes act as dampers.

AUDITORY ORGANS. 513

unclulatory movements in the water, caused by the swimming of the latter.

Auditory organs with the above characters, sometimes freely open to the external medium, but more often closed, are found in various Ccelenterata, Vermes and Crustacea, and universally or all but universally in the Mollusca and Vertebrata.

In many terrestrial Insects a different type of auditory organ has been met with, consisting of a portion of the integument modified to form a tympanum or drum, and supported at its edge by a chitinous ring. The vibrations set up in the membranous tympanum stimulate terminal nerve organs at the ends of chitinous processes, placed in a cavity bounded externally by the tympanic membrane.

The tympanum of Amphibia and Amniota is an accessory organ added, in terrestrial Vertebrata, to an organ of hearing primitively adapted to an aquatic mode of life ; and it is interesting to notice the presence of a more or less similar membrane in the two great groups of terrestrial forms, i.e. terrestrial Vertebrata and Insecta.

Nothing is known with reference to the mode of development or evolution of the tympanic type of auditory organ found in Insects, and, except in the case of Vertebrates, but little is known with reference to the development of what may be called the vesicular type of auditory organ found in aquatic forms. Some very interesting facts with reference to the evolution of such organs have however been brought to light by the brothers Hertwig in their investigations on the Ccelenterata; and I propose to commence my account of the development of the auditory organs in the animal kingdom by a short statement of the results of their researches.

Ccelenterata. Three distinct types of auditory organ have been recognised in the Medusae ; two of them resulting from the differentiation of a tentacle-like organ, and one from ectoderm cells on the under surface of the velum. We may commence with the latter as the simplest. It is found in the Medusae known as the Vesiculata. The least differentiated form of this organ, so far discovered, is present in Mitrotrocha, Tiaropsis and other genera. It has the form of an open pit ; and a series of such organs are situated along the attached edge

B. in, 33

514 AUDITORY ORGANS OF THE CCELENTERATA.

of the velum with their apertures directed downwards. The majority of the cells lining the outer, i.e. peripheral side of the

FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH DILUTE OSMIC ACID. (From Lankester; after O. and R. Hertwig.)

d l . epithelium of the upper surface of the velum; d 2 . epithelium of the under surface of the velum ; r. circular canal at the edge of the velum ; nr l . upper nervering ; h. auditory cells ; hh. auditory hairs ; np. nervous cushion formed of a prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as black, containing an otolith.

pit, contain an otolith, while a row of the cells on the inner, i.e. central side, are modified as auditory cells. The auditory cells are somewhat strap-shaped, their inner ends being continuous with the fibres of the lower nerve-ring, and their free ends being provided with bent auditory hairs, which lie in contact with the convex surfaces of the cells containing the otoliths.

By the conversion of such open pits into closed sacks a more complicated type of auditory organ, which is present in many of the Vesiculata, viz. ^Equorea, Octorchis, Phialidium, &c., is produced. A closed vesicle of this type is shewn in fig. 297. Such organs form projections on the upper surface of the velum. They are covered by a layer of the epithelium (d 1 } of the upper surface of the velum, but the lining of the vesicle (d*} is derived from what was originally part of the epithelium of the lower surface of the velum, homologous with that lining the open pits in the type already described. The general arrangement of the cells lining such vesicles is the same as that of the cells lining the open pits.

A second type of auditory organ, found in the Trachymeclusa,", appears in its simplest condition as a modified tentacle.

AUDITORY ORGANS. 515

It is formed of a basal portion, covered by auditory cells with long stiff auditory hairs, supporting at its apex a club-shaped body, attached to it by a delicate stalk. An endodermal axis is continued through the whole structure, and in one or more of the endoderm cells of the club-shaped body otoliths are always present. The tails of the auditory cells are directly continued into the upper nerve-ring.

In more complicated forms of this organ the tentacle becomes enclosed in a kind of cup, by a wall-like upgrowth of the

FIG. 298. AUDITORY ORGAN OF RHOPALONEMA. (From Lankester; after O. and R. Hertwig.)

The organ consists of a modified tentacle (hk) with auditory cells and concretions, partially enclosed in a cup.

surrounding parts (fig. 298) ; and in some forms, e.g. Geryonia, by the closure of the cup, the whole structure takes the form of a completely closed vesicle, in the cavity of which the original tentacle forms an otolith-bearing projection.

The auditory organs found in the Acraspedote Medusae approach in many respects to the type of organ found in the Trachymedusse. They consist of tentacular organs placed in grooves on the under surface of the disc. They have a swollen extremity, and are provided with an endodermal axis for half the length of which there is a diverticulum of the gastrovascular canal system. The terminal portion of the endoderm is solid, and contains calcareous concretions. The ectodermal cells at the base of these organs have the form of auditory cells.

Mollusca. Auditory vesicles are found in almost all Mollusca on the ventral side of the body in close juxtaposition to the pedal ganglia. Except possibly in some Cephalopods, these

332

516 AUDITORY ORGANS OF THE VERTEBRATA.

vesicles are closed. They are provided with free otoliths, supported by the cilia of the walls of the sack, but in addition some of the cells of the sack are provided with stiff auditory hairs.

In many forms these sacks have been observed to originate by an invagination of the epiblast of the foot (Pahtdina, Nassa, Heteropoda, Limax, Clio, Cephalopoda and Lamellibranchiata). In other instances (some Pteropods, Lymnaeus, &c.) they appear, by a secondary modification in the development, to originate by a differentiation of a solid mass of epiblast.

According to Fol the otocysts in Gasteropods are formed by cells of the wall of the auditory sacks ; and the same appears to hold good for Cephalopoda (Grenacher) 1 shewing that free otoliths have in these instances originated from otoliths originally placed in cells.

Crustacea. In the decapodous Crustacea organs, which have been experimentally proved to be true organs of hearing, are usually present on the basal joint of the anterior antennae. They may have (Hensen, No. 384) the form either of closed or of open sacks, lined by an invagination of the epidermis. They are provided with chitinous auditory hairs and free otoliths. In the case of the open sacks the otoliths appear to be simply stones transported into the interior of the sacks, but in the closed sacks the otoliths, though free, are no doubt developed within the sacks.

The Schizopods, which, as mentioned in the last chapter, are remarkable as containing a genus (Euphausia) with abnormally situated eyes, distinguish themselves again with reference to their auditory organs, in that another genus (Mysis) is characterized by the presence of a pair of auditory sacks in the inner plates of the tail. These sacks have curved auditory hairs supporting an otolith at their extremity.

The development of the auditory organs in the Crustacea has not been investigated.

The Vertebrata. The Cephalochorda are without organs of hearing, and the auditory organ of the Urochorda is constructed on a special type of its own. The primitive auditory organs of the true Vertebrata have the same fundamental characters as those of the majority of aquatic invertebrate forms. They consist of a vesicle, formed by the invagination of a patch of epiblast, and usually shut off from the exterior, but occasionally (Elasmo 1 For the somewhat complicated details as to the development of the auditory sacks of Cephalopods I must refer the reader to Vol. II., pp. 278, 279, and to Grenacher (Vol. i., No. 280).

AUDITORY ORGANS.

517

branchii) remaining open. The walls of this vesicle are always much complicated and otoliths of various forms are present in its cavity. To this vesicle accessory structures, derived from the walls of the hyomandibular cleft, are added in the majority of terrestrial Vertebrata.

The development of the true auditory vesicle will be considered separately from that of the accessory structures derived from the hyomandibular cleft.

In all Vertebrata the development of the auditory vesicle commences with the formation of a thickened patch of epiblast, at the side of the hind-brain, on the level of the second visceral cleft.

t.v.v

This patch soon becomes invaginated in the form of a pit (fig. 299, aup), to the inner side of which the ganglion of the auditory nerve (ami), which as shewn in a previous chapter is primitively a branch of the seventh nerve, closely applies itself.

In those Vertebrata (viz. Teleostei, Lepidosteus and Amphibia) in which the epiblast is early divided into a nervous and epidermic stratum, the auditory pit arises as an invagination of the nervous stratum only, and the mouth of the auditory pit is always closed -(fig. 300) by the epidermic stratum of the skin. Since the opening of the pit is retained through life in Elasmobranchii the closed form of pit in the above forms is clearly secondary.

In Teleostei the auditory pit arises as a solid invagination of the epiblast.

T/t,

FIG. 299. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT THE LEVEL OF THE AUDITORY INVOLUTION.

aup. auditory pit; aun. ganglion of auditory nerve ; iv.v. roof of fourth ventricle; a.c.v. anterior cardinal vein; aa. aorta; I.aa. aortic trunk of mandibular arch ; pp. head cavity of mandibular arch ; Ivc. alimentary pouch which will form the first visceral cleft; 77?. rudiment of thyroid body.

The mouth of the auditory vesicle gradually narrows, and in most

forms soon becomes closed, though in Elasmobranchii it remains permanently open. In any case the vesicle is gradually removed from the surface, remaining connected with it by an elongated duct, either opening on the dorsal aspect of the head (Elasmobranchii), or ending blindly close beneath the skin.

In all Vertebrata the auditory vesicle undergoes further

5 i8

AUDITORY ORGANS OF THE VERTEBRATA.

changes of a complicated kind. In the Cyclostomata these changes are less complicated than in other forms, though whether this is due to degeneration, or to the retention of a primitive

FIG. 300. SECTION THROUGH THE HEAD OK A LEPIUOSTEUS EMBRYO ON

THE SIXTH DAY AFTER IMPREGNATION. au.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast.

state of the auditory organ, is not known. In the Lamprey the auditory vesicle is formed in the usual way by an invagination

cv

cc

AOA

FIG. 301. SECTION THROUGH THE HIND-BRAIN OK A CHICK AT THE END OF THE THIRD DAY OF INCUBATION.

IV. fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. Ch. notochord ; C V. anterior cardinal vein; CC. involuted auditory vesicle (CC points to the end which will form the cochlear canal) ; RL. recessus labyrinthi (remains of passage connecting the vesicle with the exterior) ; hy. hypoblast lining the alimentary canal; AO., AO.A. aorta, and aortic arch.

AUDITORY ORGANS. 519

of the epiblast, which soon becomes vesicular, and for a considerable period retains a simple character. As pointed out by Max Schultze, a number of otoliths appears in the vesicle during larval life, and, although such otoliths are stated by J. Miiller to be absent both in the full-grown Ammoccete and in the adult, they have since been found by Ketel (No. 387). The formation of the two semicircular canals has not been investigated.

In all the higher Vertebrates the changes of the auditory sacks are more complicated. The ventral end of the sack is produced into a short process (fig. 301, CC}\ while at the dorsal end there is the canal-like prolongation of the lumen of the sack (RL}, derived from the duct which primitively opened to the exterior, and which in most cases persists as a blind diverticulum of the auditory sack, known as the recessus labyrinthi or aqueductus vestibuli. The parts thus indicated give rise to the whole of the membranous labyrinth of the ear. The main body of the vesicle becomes the utriculus and semicircular canals, while the ventral process forms the sacculus hemisphericus and cochlear canal.

The growth of these parts has been most fully studied in Mammalia, where they reach their greatest complexity, and it will be convenient to describe their development in this group, pointing out how they present, during some of the stages in their growth, a form permanently retained in lower types.

The auditory vesicle in Mammalia is at first nearly spherical, and is imbedded in the mesoblast at the side of the hind-brain. It soon becomes triangular in section, with the apex of the triangle pointing inwards and downwards. This apex gradually elongates to form the rudiment of the cochlear canal and sacculus hemisphericus (fig. 302, CC). At the same time the recessus labyrinthi (R.L) becomes distinctly marked, and the outer wall of the main body of the vesicle grows out into two protuberances, which form the rudiments of the vertical semicircular canals ( V.B}. In the lower forms (fig. 305) the cochlear process of the vestibule hardly reaches a higher stage of development than that found at this stage in Mammalia.

The parts of the auditory labyrinth thus established soon increase in distinctness (fig. 303) ; the cochlear canal (CC} becomes longer and curved ; its inner and concave surface being

520

AUDITORY ORGANS OF THE MAMMALIA.

lined by a thick layer of columnar epiblast. The recessus labyrinthi also increases in length, and just below the point where the bulgings to form the vertical semicircular canals are situated, there is formed a fresh protuberance for the horizontal semi

V.B

FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Bottcher.)

HB. the hind -brain.

The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuli R.L., and of the commencing vertical semicircular canal V.B., and of the ductus cochlearis CC., with the cavity of the primary otic vesicle are seen : on the left side, only the extreme end of the ductus cochlearis CC, and of the semicircular canal V.B. are shewn.

Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC ; on the left side the auditory nerve G and its connection N with the hind-brain are also shewn.

Below the otic vesicle on either side lies the jugular vein.

circular canal. At the same time the central parts of the walls of the flat bulgings of the vertical canals grow together, obliterating this part of the lumen, but leaving a canal round the periphery ; and, on the absorption of their central parts, each of the original simple bulgings of the wall of the vesicle becomes converted into a true semicircular canal, opening at its two extremities into the auditory vesicle. The vertical canals are first established and then the horizontal canal.

AUDITORY ORGANS.

521

Shortly after the formation of the rudiment of the horizontal semicircular canal a slight protuberance becomes apparent on the

FIG. 303. SECTION OF THE HEAD OF A FCETAL SHEEP 20 MM. IN LENGTH.

(After Bottcher.)

R. V. recessus labyrinthi ; V.B. vertical semicircular canal ; H.B. horizontal semicircular canal; C.C. cochlear canal ; G. cochlear ganglion.

inner commencement of the cochlear canal. A constriction arises on each side of the protuberance, converting it into a prominent hemispherical projection, the sacculus hemisphericus (fig. 304, S.R\

The constrictions are so deep that the sacculus is only connected with the cochlear canal on the one hand, and with the general cavity of the auditory vesicle on the other, by, in each case, a narrow though short canal.

The former of these canals (fig. 304, b) is known as the canalis reuniens. At this stage we may call the remaining cavity of the original otic vesicle, into which all the above parts open, the utriculus.

Soon after the formation of the sacculus hemisphericus, the

522 AUDITORY ORGANS OF THE MAMMALIA.

cochlear canal and the semicircular canals become invested with cartilage. The recessus labyrinthi remains however still enclosed in undifferentiated mesoblast

Between the cartilage and the parts which it surrounds there remains a certain amount of indifferent connective tissue, which is more abundant around the cochlear canal than around the semicircular canals.

As soon as they have acquired a distinct connective-tissue coat, the semicircular canals begin to be dilated at one of their terminations to form the ampullae. At about the same time a constriction appears opposite the mouth of the recessus labyrinthi, which causes its opening to be divided into two branches one towards the utriculus and the other towards the sacculus hemisphericus ; and the relations of the parts become so altered that communication between the sacculus and utriculus can only take place through the mouth of the recessus labyrinthi (fig. 305).

When the cochlear canal has come to consist of two and a half coils, the thickened epithelium which lines the lower surface of the canal forms a double ridge from which the organ of Corti is subsequently developed. Above the ridge there appears a delicate cuticular membrane, the membrane of Corti or membrana tectoria.

The epithelial walls of the utricle, the recessus labyrinthi, the semicircular canals, and the cochlear canal constitute together the highly complicated product of the original auditory vesicle. The whole structure forms a closed cavity, the various parts of which are in free communication. In the adult the fluid present in this cavity is known as the endolymph.

In the mesoblast lying between these parts and the cartilage, which at this period envelopes them, lymphatic spaces become established, which are partially developed in the Sauropsida, but become in Mammals very important structures.

They consist in Mammals partly of a space surrounding the utricle and semicircular canals, and partly of two very definite channels, which largely embrace between them the cochlear canal. The latter channels form the scala vestibuli on the upper side of the cochlear canal and the scala tympani on the lower. The scala vestibuli is in free communication with the lymphatic cavity surrounding the vestibule, and opens at the apex of the cochlea

AUDITORY ORGANS.

523

into the scala tympani. The latter ends blindly at the fenestra rotunda.

The fluid contained in the two scalae, and in the remaining lymphatic cavities of the auditory labyrinth, is known as perilymph.

The cavities just spoken of are formed by an absorption of

Ch.

JUB

C.C

FIG. 304. SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP 28 MM. IN LENGTH. (After Bottcher.)

D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior vertical semicircular canal ; U. utriculus ; H.B. horizontal semicircular canal; b. canalis reuniens ; a. constriction by means of which the sacculus hemisphericus S.R. is formed ; f. narrowed opening between sacculus hemisphericus and utriculus ; C. C. cochlea ; C.C. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate; Ch. notochord.

524 ORGAN OF CORTI.

parts of the embryonic mucous tissue between the perichondrium and the walls of the membranous labyrinth.

The scala vestibuli is formed before the scala tympani, and both scalae begin to be developed at the basal end of the cochlea : the cavity of each is continually being carried forwards towards the apex of the cochlear canal by a progressive absorption of the mesoblast. At first both scalae are somewhat narrow, but they soon increase in size and distinctness.

The cochlear canal, which is often known as the scala media of the cochlea, becomes compressed on the formation of the scalae so as to be triangular in section, with the base of the triangle outwards. This base is only separated from the surrounding cartilage by a narrow strip of firm mesoblast, which becomes the stria vascularis, etc. At the angle opposite the base the canal is joined to the cartilage by a narrow isthmus of firm material, which contains nerves and vessels. This isthmus subsequently forms the lamina spiralis, separating the scala vestibuli from the scala tympani.

The scala vestibuli lies on the upper border of the cochlear canal, and is separated from it by a very thin layer of mesoblast, bordered on the cochlear aspect by flat epiblast cells. This membrane is called the membrane ofReissner. The scala tympani is separated from the cochlear canal by a thicker sheet of mesoblast, called the basilar membrane, which supports the organ of Corti and the epithelium adjoining it. The upper extremity of the cochlear canal ends in a blind extremity called the cupola, to which the two scalae do not for some time extend. This condition is permanent in Birds, where the cupola is represented by a structure known as the lagena (fig. 305, II. L}. Subsequently the two scalae join at the extremity of the cochlear canal ; the point of the cupola still however remains in contact with the bone, which has now replaced the cartilage, but at a still later period the scala vestibuli, growing further round, separates the cupola from the adjoining osseous tissue.

The ossification around the internal ear is at first confined to the cartilage, but afterwards extends into the thick periosteum between the cartilage and the internal ear, and thus eventually makes its way into the lamina spiralis, etc.

The organ of Corti. In Mammalia there is formed from the

AUDITORY ORGANS.

525

epithelium of the cochlear canal a very remarkable organ known as the organ of Corti, the development of which is of sufficient importance to merit a brief description. A short account of this organ in the adult state may facilitate the understanding of its development.

The cochlear canal is bounded by three walls, the outer one being the osseous wall of the cochlea. The membrane of Reissner bounds it towards

U

FIG. 305. DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegenbaur.)

I. Fish. II. Bird. III. Mammal.

U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. canalis reuniens ; R. recessus labyrinthi ; UC. commencement of cochlea ; C. cochlear canal ; L. lagena ; PC. cupola at apex of cochlear canal; V. csecal sack of the vestibulum of the cochlear canal.

the scala vestibuli, and the basilar membrane towards the scala tympani. This membrane stretches from the margin of the lamina spiralis to the ligamentum spirale ; the latter being merely an expanded portion of the connective tissue lining the osseous cochlea.

The lamina spiralis is produced into two lips, called respectively the labium tympanicum and labium vestibulare ; it is to the former and longer of these that the basilar membrane is attached. At the margin of the junction of the labium tympanicum with the basilar membrane the former is perforated for the passage of the nervous fibres, and this region is called the habenula perforata.

The labium vestibulare, so called from its position, is shorter than the labium tympanicum and is raised above into numerous blunt teeth. Partly springing out from the labium vestibulare, and passing from near the inner attachment of the membrane of Reissner towards the outer wall of the cochlea, is an elastic membrane, the membrana tectoria. Resting on the basilar membrane is the organ of Corti.

Considering for the moment that a transverse section of the cochlear

$26 ORGAN OF CORTI.

canal only one cell deep is being dealt with, the organ of Corti will be found to consist of a central part composed of two peculiarly shaped rods widely separated below, but in contact above. These are the rods or fibres of Corti. On their outer side, i.e. on the side towards the osseous wall of the canal, is a reticulate membrane which passes from the inner rod of Corti towards the osseous wall of the canal. With their upper extremities fixed in that membrane, and their lower resting on the basilar membrane are three (four in man) cells with auditory hairs known as the outer 'hair cells,' which alternate with three other cells known as Deiters' cells. Between these and the outer attachment of the basilar membrane is a series of cells gradually diminishing in height in passing outwards. On the inner side of the rods of Corti is one hair cell, and then a number of peculiarly modified cells which fill up the space between the two lips of the lamina spiralis.

It will not be necessary to say much in reference to the development of the labium tympanicum and the labium vestibulare.

The labium vestibulare is formed by a growth of the connective tissue which fuses with and passes up between the epithelial cells. The epithelial cells which line its upper (vestibular) border become modified, and remain as its teeth.

The labium tympanicum is formed by the coalescence of the connective tissue layer separating the scala tympani from the cochlear canal with part of the connective tissue of the lamina spiralis. At first these two layers are separate, and the nerve fibres to the organ of Corti pass between them. Subsequently however they coalesce, and the region where they are penetrated by the nervous fibres becomes the habenula perforata.

The organ of Corti itself is derived from the epiblast cells lining the cochlear canal, and consists in the first instance of two epithelial ridges or projections. The larger of them forms the cells on the inner side of the organ of Corti, and the smaller the rods of Corti together with the inner and outer hair cells and Deiters' cells.

At first both these ridges are composed of simple elongated epithelial cells one row deep. The smaller ridge is the first to shew any change. The cells adjoining the larger ridge acquire auditory hairs at their free extremities, and form the row of inner hair cells ; the next row of cells acquires a broad attachment to the basilar membrane, and gives origin to the inner and outer rods of Corti.

Outside the latter come several rows of cells adhering together so as to form a compact mass which is quadrilateral in section. This mass is composed of three upper cells with nuclei at the same level, which form the outer hair cells, each of them ending above in auditory hairs, and three lower cells which form the cells of Deiters. Beyond this the cells gradually pass into ordinary cubical epithelial cells.

As just mentioned, the cells of the second row, resting with their broad bases on the basilar membrane, give rise to the rods of Corti. The breadth of the bases of these cells rapidly increases, and important changes take place in the structure of the cells themselves.

AUDITORY ORGANS. 527

The nucleus of each cell divides ; so that there come to be two nuclei or sometimes three which lie close together near the base of the cell. Outside the nuclei on each side a fibrous cuticular band appears. The two bands pass from the base of the cell to its apex, and there meet though widely separated below. The remaining contents of the cell, between the two fibrous bands, become granular, and are soon to a great extent absorbed ; leaving at first a round, and then a triangular space between the two fibres. The two nuclei, surrounded by a small amount of granular matter, come to lie, each at one of the angles between the fibrous bands and the basilar membrane.

The two fibrous bands become, by changes which need not be described in detail, converted into the rods of Corti each of their upper ends growing outwards into the processes which the adult rods possess.

Each pair of rods of Corti is thus (Bottcher) to be considered as the product of one cell ; and the nuclei embedded in the granular mass between them are merely the remains of the two nuclei formed by the division of the original nucleus of that cell 1 . The larger ridge is for the most part not permanent, and from being the most conspicuous part of the organ of Corti comes to be far less important than the smaller ridge. Its cells undergo a partial degeneration ; so that the epithelium in the hollow between the two lips of the lamina spiralis, which is derived from the larger ridge, comes to be composed of a single row of short and broad cells. In the immediate neighbourhood however of the inner hair cell, one or two of the cells derived from the larger ridge are very much elongated.

The membrana reticularis is a cuticular structure derived from the parts to which it is attached. .

Accessory structures connected with the organ of hearing- in Terrestrial Vertebrata.

In all the Amphibia, Sauropsida and Mammalia, except the Urodela and a few Anura and Reptilia, the first visceral or hyomandibular cleft enters into intimate relations with the organs of hearing, and from it and the adjoining parts are formed the tympanic cavity, the Eustachian tube, the tympanic membrane and the meatus auditorius externus. The tympanic membrane serves to receive from the air the sound vibrations, which are communicated to fluids contained in the true auditory labyrinth by one ossicle or by a chain of auditory ossicles.

The addition to the organ of hearing of a tympanic membrane to receive aerial sound vibrations is an interesting case of the

1 It is not clear from Bottcher's description how it comes about that the inner rods of Corti are more numerous than the outer.

528 THE TYMPANIC CAVITY.

adaptation of a structure, originally required for hearing in water, to serve for hearing in air ; and as already pointed out, the similarity of this membrane to the tympanic membrane of some Insects is also striking.

There is much that is obscure with reference to' the actual development of the above parts of the ear, which has moreover only been carefully studied in Birds and Mammals.

The Eustachian tube and tympanic cavity seem to be derived from the inner part of the first visceral or hyomandibular cleft, the external opening of which becomes soon obliterated. Kolliker holds that the tympanic cavity is simply a dorsally and posteriorly directed outgrowth of the median part of the inner section of this cleft; while Moldenhauer (No. 392) holds, if I understand him rightly, that it is formed as an outgrowth of a cavity called by him the sulcus tubo-tympanicus, derived from the inner aperture of the first visceral cleft together with the groove of the pharynx into which it opens ; and Moldenhauer is of opinion that the greater part of the original cleft atrophies.

The meatus auditorius externus is formed at the region of a shallow depression where the closure of the first visceral cleft takes place. It is in part formed by the tissue surrounding this depression growing up in the form of a wall, and Moldenhauer believes that this is the whole process. Kolliker states however that the blind end of the meatus becomes actually pushed in towards the tympanic cavity.

The tympanic membrane is derived from the tissue which separates the meatus auditorius externus from the tympanic cavity. This tissue is obviously constituted of an hypoblastic epithelium on its inner aspect, an epiblastic epithelium on its outer aspect, and a layer of mesoblast between them, and these three layers give rise to the three layers of which this membrane is formed in the adult. During the greater part of fcetal life it is relatively very thick, and presents a structure bearing but little resemblance to that in the adult.

A proliferation of the connective tissue-cells in the vicinity of the tympanic cavity causes in Mammalia the complete or nearly complete obliteration of the cavity during fcetal life.

The tympanic cavity is bounded on its inner aspect by the osseous investment of the internal ear, but at one point, known

AUDITORY ORGANS. 529

as the fenestra ovalis, the bone is deficient in the Amphibia, Sauropsida and Mammalia, and its place is taken by a membrane ; while in Mammalia and Sauropsida a second opening, the fenestra rotunda, is also present.

These two fenestrae appear early, but whether they are formed by an absorption of the cartilage, or by the nonchondrification of a small area, is not certainly known. The upper of the two, or fenestra ovalis, contains the base of a bone, known in the Sauropsida and Amphibia as the columella. The main part of the columella is formed of a stalk which is held by Parker to be derived from part of the skeleton of the visceral arches, but its nature is discussed in connection with the skeleton, while the base, forming the stapes, appears to be derived from the wall of the periotic cartilage.

In all Amphibia and Sauropsida with a tympanic cavity, the stalk of the columella extends to the tympanic membrane ; its outer end becoming imbedded in this membrane, and serving to transmit the vibrations of the membrane to the fluid in the internal ear. In Mammalia there is a stapes not directly attached to the tympanic membrane by a stalk, and two additional auditory ossicles, derived from parts of the skeleton of the visceral arches, are placed between the stapes and the tympanic membrane. These ossicles are known as the malleus and incus, and the chain of the three ossicles replaces physiologically the single ossicle of the lower forms.

These ossicles are at first imbedded in the connective tissue in the neighbourhood of the tympanic cavity, but on the full development of this cavity, become apparently placed within it ; though really enveloped in the mucous membrane lining it.

The fenestra ovalis is in immediate contiguity with the walls of the utricle, while the fenestra rotunda adjoins the scala tympani.

Hunt (No. 391) holds, from his investigations on the embryology of the pig, that " the Eustachian tube is an involution of the pharyngeal mucous membrane ;" and that "the meatus is an involution of the integument " while " the drum is formed by the Eustachian tube overlapping the extremity of the meatus." Urbantschitsch also holds that the first visceral cleft has nothing to do with the formation of the tympanic cavity and Eustachian tube, and that these parts are derived from lateral outgrowths of the oral cavity.

B. III. 34

530 THE TYMPANIC CAVITY.

The evolution of the accessory parts of the ear would be very difficult to explain on Darwinian principles if the views of Hunt and Urbantschitsch were correct ; and the accepted doctrine, originally proposed by Huschke (No. 389), according to which these structures have originated by a ' change of function' of the parts of the first visceral cleft, may fairly be held till more conclusive evidence has been brought against it than has yet been done.

Tunicata. The auditory organ of the Tunicata (fig. 306) is placed on the under surface of the anterior vesicle of the brain.

FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.)

Only the anterior part of the tail is represented.

N'. anterior swelling of neural tube ; IV. anterior swelling of spinal portion of neural tube; n. hinder part of neural tube; ch. notochord ; A", branchial region of alimentary tract ; d. cesophageal and gastric region of alimentary tract ; O. eye ; a, otolith ; o. mouth ; s. papilla for attachment.

It consists of two parts (i) a prominence of the cells of the floor of the brain forming a crista acustica, and (2) an otolith projecting into the cavity of the brain, and attached to the crista by delicate hairs.

The crista acustica is formed of very delicate cylindrical cells, and in its most projecting part is placed a vesicle with clear contents. The otolith is an oval body with its dorsal half pigmented, and its ventral half clear and highly refractive. It is balanced on the highest point of the crista.

The crista acustica would seem to be developed from the cells of the lower part of the front vesicle of the brain. The otolith however is developed from a single cell on the dorsal and right side of the brain. This cell commences to project into the cavity of the brain and its free end becomes pigmented. It gradually grows inwards till it forms a spherical prominence in the cavity of the brain, to the wall of which it is attached by a

AUDITORY ORGANS. 531

stalk. At the same time it travels round the right side of the vesicle of the brain (in a way not fully explained) till it reaches the summit of the crista, which has become in the meantime established.

The auditory organ of the simple Ascidians can hardly be brought into relation with that of the other Chordata, and has most probably been evolved within the Tunicate phylum.

BIBLIOGRAPHY.

Invertebrata.

(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeit.f. wiss. ZooL, Vol. xm. 1863.

(385) O. and R. Hertwig. Das Nervensystem u. d. Sinnesorgane d. Medusen. Leipzig, 1878.

Vertebrata.

(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d. kaiserl. Leop. Carol. Akad. d. Wissenschaft., Vol. xxxv.

(387) C. H asse. Die vergleich. Morphologic u. Histologied. hiiutigen Gehororgane d. Wirbelthiere. Leipzig, 1873.

(388) V. Hensen. "Zur Morphologic d. Schnecke." Zeit. f. wiss. ZooL, Vol. XIII. 1863.

(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim bebriiteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. vi.

(390) Reissner. De Auris internes formatione. Inaug. Diss. Dorpat, 1851.

Accessory parts of Vertebrate Ear.

(391) David Hunt. "A comparative sketch of the development of the ear and eye in the Pig. " Transactions of the International Otological Congress, \ 876.

(392) W. Moldenhaueir. "Zur Entwick. d. mittleren u. ausseren Ohres." Morphol. Jahrbuch) Vol. III. 1877.

(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trommelfelles." Mittheil. a. d. embryol. Instit. Wien, Heft i. 1877.

Olfactory organ.

Amongst the Invertebrata numerous sense organs have been described under the title of olfactory organs. In aquatic animals they often have the form of ciliated pits or grooves, while in the Insects and Crustacea delicate hairs and other structures present on the antennae are usually believed to be organs of smell. Our knowledge of all these organs is however so vague that it

342

532

OLFACTORY PIT.

would not be profitable to deal with them more fully in this place. Amongst the Chordata there are usually well developed olfactory organs.

Amongst the Urochorda (Tunicata) it is still uncertain what organs (if any) deserve this appellation. The organ on the dorsal side of the opening of the respiratory pharynx may very possibly have an olfactory function, but it is certainly not homologous with the olfactory pits of the true Vertebrata, and as mentioned above (pp. 436 and 437), may perhaps be homologous with the pituitary body.

In the Cephalochorda (Amphioxus) there is a shallow ciliated pit, discovered by Kolliker, which is situated on the left side of the head, and is closely connected with a special process of the

FlG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES AS TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F.

B. Somewhat older Scyllium embryo.

///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl. glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. midbrain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; kt. heart; Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head.

OLFACTORY ORGANS. 533

front end of the brain. It is most probably the homologue of the olfactory pits of the true Vertebrata.

In the true Vertebrata the olfactory organ has usually the form of a pair of pits, though in the Cyclostomata the organ is unpaired.

In all the Vertebrata with two olfactory pits these organs are formed from a pair of thickened patches of the epiblast, on the under side of the fore-brain, immediately in front of the mouth (fig. 307, ol). Each thickened patch of epiblast soon becomes involuted as a pit (fig. 308, N), the lining cells of which become the olfactory or Schneiderian epithelium. The surface of this epithelium is usually much increased by various foldings, which in the Elasmobranchii arise very early, and are bilaterally symmetrical, diverging on each side like the barbs of a feather from the median line. They subsequently become very pronounced (fig. 309), serving greatly to increase the surface of the olfactory epithelium. At a very early stage the olfactory nerve attaches itself to the olfactory epithelium.

In Petromyzon the olfactory organ arises as an unpaired thickening of the epiblast, which in the just hatched larva forms a shallow pit, on the ventral side of the head, immediately in front of the mouth. This pit rapidly deepens, and soon extends itself backwards nearly as far as the infundibulum (fig. 310, 0!}. By the development of the upper lip the opening of the olfactory pit is gradually carried to the dorsal surface of the head, and becomes at the same time narrowed and ciliated (fig. 47, ol). The whole organ forms an elongated sack, and in later stages becomes nearly divided by a median fold into two halves.

It is probable that the unpaired condition of the olfactory organ in the Lamprey has arisen from the fusion of two pits into one ; there is however no evidence of this in the early development ; but the division of the sack into two halves by a median fold may be regarded as an indication of such a paired character in the later stages.

In Myxine the olfactory organ communicates with the mouth through the palate, but the meaning of this communication, which does not appear to be of the same nature as the communication between the olfactory pits and the mouth by the posterior nares in the higher types, is not known.

The opening of the olfactory pit does not retain its embryonic characters. In Elasmobranchii and Chimaera it becomes enclosed by a wall of integument, often deficient on the side of the mouth, so that there is formed a groove leading from the nasal pit towards the angle of the mouth. This groove is

534

EXTERNAL AND INTERNAL NARES.

MB.

u

usually constricted in the middle, and the original single opening of the nasal sack thus becomes nearly divided into two. In Teleostei and Ganoids the division of the nasal opening into two parts becomes complete, but the ventral opening is generally carried off some distance from the mouth, and placed, by the growth of the snout, on the upper surface of the head (figs. 54 and 68). In all these instances it is

/ tftM

probable that the dorsal opening of the nasal sack is homologous with the external nares, and the ventral opening with the posterior nares of higher types. Thus the posterior nares would in fact seem to be represented in all Fishes by a ventral part of the opening of the original nasal pit which either adjoins the border of the mouth (many Elasmobranchii) or is quite separate from the mouth (Teleostei and Ganoidei). In the Dipnoi, Amphibia and all the higher types the oral region becomes extended so as to enclose the posterior nares, and then each nasal pit acquires two openings ; viz. one outside the mouth, the external nares, and one within the mouth, the internal or posterior nares. In the Dipnoi the two nasal openings are very similar to those in Ganoidei and Teleostei, but both are placed on the under surface of the head, the inner one being within the mouth, and the external one is so close to the outer border of the upper lip that it also has been considered by some anatomists to lie within the mouth.

In all the higher types the nasal pits have originally only a single opening, and the ontogenetic process by which the posterior nasal opening is formed has been studied in the Amniota and Amphibia. Amongst the Amniota we may take the Chick as representing the process in a very simple form. The general history of the process was first made out by Kolliker.

FIG. 308. SIDE VIEW OF THE HEAD OF AN EMBRYO CHICK OF THE THIRD DAY AS AN OPAQUE OBJECT. (Chromic acid preparation.)

C.H. cerebral hemispheres ; F.B. vesicle of third ventricle; M.B. mid-brain; Cb. cerebellum; H.B. medulla oblongata; N. nasal pit ; ot. auditory vesicle in the stage of a pit with the opening not yet closed up; op. optic vesicle, with /. lens and ch.f. choroidal fissure.

i F. The first visceral fold ; above it is seen the superior maxillary process.

2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts between them.

OLFACTORY ORGANS.

535

The opening of the nasal pit becomes surrounded by a ridge except on its oral side. The deficiency of this ridge on the side of the mouth gives rise to a kind of shallow groove leading from

FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)

c.h. cerebral hemispheres; oLv. olfactory vesicle; olf, olfactory pit ; Seh. Schneiderian folds ; /. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain.

the nasal pit to the mouth. The ridge enveloping the opening of the nasal pit next becomes prolonged along the sides of this groove, especially on its inner one; and at the same time the superior maxillary process grows forwards so as to bound the lower

ma

FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4-8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues.

c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum ; pn. pineal gland; mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v. ao. ventral aorta ; ht. ventricle of heart ; ch. notochord.

536 EXTERNAL AND INTERNAL NARES.

part of its outer side. The inner and outer ridges, together with the superior maxillary process, enclose a deep groove, connecting the original opening of the nasal pit with the mouth. The process just described is illustrated by fig. 311 A, and it may be seen that the ridge on the inner side of the groove forms the edge of the fronto-nasal process (k).

On the sixth day (Born, 394) the sides of this groove unite together in the middle, and convert it into a canal open at both ends the ventral openings of the canals of the two sides being placed just within the border of the mouth, and forming the posterior nares ; while the external openings form the anterior nares. The upper part of the canal, together with the original

FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS OF INCUBATION. (From Huxley.)

/". cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be seen in A ; g. nasal pits ; k. fronto-nasal process ; /. superior maxillary process ; i. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first visceral cleft.

In A the cavity of the mouth is seen enclosed by the fronto-nasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over.

In B the external opening of the mouth has become much constricted, but it is still enclosed by the fronto-nasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below.

The superior maxillary processes have united with the fronto nasal process, along nearly the whole length of the latter.

nasal pit, is alone lined by olfactory epithelium ; the remaining epithelium of the nasal cavity being indifferent epiblastic epi

OLFACTORY ORGANS.

537

thelium. Further changes subsequently take place in connection with the posterior nares, but these are described in the section dealing with the mouth.

In Mammalia the general formation of the anterior and posterior nares is the same as in Birds ; but, as shewn by Dursy and Kolliker, an outgrowth from the inner side of the canal between the two openings arises at an early period ; and becoming separate from the posterior nares and provided with a special opening into the mouth, forms the organ of Jacobson. The general relations of this organ when fully formed are shewn in fig. 312.

In Lacertilia the formation of the posterior nares differs in some particulars from that in Birds (Born). A groove is formed leading from the primitive nasal pit to the mouth, bordered on its inner side by the swollen edge of the fronto-nasal process, and on its outer by an outernasal process ; while the superior maxillary process does not assist in bounding it. On the inner side of the narrowest part of this groove there is formed a large lateral diverticulum, which is lined by a continuation of the Schneiderian epithelium, and forms the rudiment of Jacobson's organ. The nasal groove continues to grow in length, but soon becomes converted into a canal by the junction of the outer-nasal process with the fronto-nasal process. This canal is open at both ends : at its dorsal end is placed the original opening of the nasal pit, and its ventral opening is situated within the cavity of the mouth. The latter forms the primitive posterior nares. The superior maxillary process soon grows inwards on the under side of the posterior part of the nasal passage, and assists in forming its under wall. This ingrowth of the superior maxillary process is the rudiment of the hard palate.

On the conversion of the nasal groove into a closed passage, the opening of Jacobson's organ into the groove becomes concealed ; and at a later period Jacobson's organ becomes completely shut off from the nasal cavity, and opens into the mouth at the front end of an elongated groove leading back to the posterior nares.

In Amphibia the posterior nares are formed in a manner very different from that of the Amniota. At an early stage a shallow groove is formed leading from the nasal pit to the mouth ; but this groove instead

J

FIG. 312. SECTION THROUGH

THE NASAL CAVITY AND JA COBSON'S ORGAN. (From Gegenbaur.)

sn. septum nasi ; en. nasal cavity ; y. Jacobson's organ ; d, edge of upper jaw.

538 ORGANS OF THE LATERAL LINE.

of forming the posterior nares soon vanishes, and by the growth of the front of the head the nasal pits are carried farther away from the mouth.

The actual posterior nares are formed by a perforation in the palate, opening into the blind end of the original nasal pit.

Considering that the various stages in the formation of the posterior nares of the Amniota are so many repetitions of the adult states of lower forms, it may probably be assumed that the mode of formation of the posterior nares in Amphibia is secondary, as compared with that in the Amniota.

A diverticulum of the front part of the nasal cavity of the Anura is probably to be regarded as a rudimentary form of Jacobson's organ.

BIBLIOGRAPHY.

(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.

(395) A. Kollicker. " Ueber die Jacobson'schen Organe des Menschen." Festschrift f. Rienecker, 1877.

(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." Quart. Journ. of Micr. Science, Vol. xix., 1879.

Sense organs of the lateral line.

Although I do not propose dealing with the general development of various sense organs of the skin, there is one set of organs, viz. that of the lateral line, which, both from its wide extension amongst the Ichthyopsida and from the similarity of some of its parts to certain organs found amongst the Chastopoda 1 , has a great morphological importance.

The organs of the lateral line consist as a rule of canals, partly situated in the head, and partly in the trunk. These canals open at intervals on the surface, and their walls contain a series of nerve-endings. The branches of the canal in the head are innervated for the most part by the fifth pair, and those of the trunk by the nervus lateralis of the vagus nerve. There is typically but a single canal in the trunk, the openings and nerve-endings of which are segmentally arranged.

Two types of development of these organs have been found. One of these is characteristic of Teleostei ; the other of Elasmobranchii.

In just hatched Teleostei, Schulze (No. 402) found that instead of the normal canals there was present a series of sense bulbs, projecting freely on the surface and partly composed of cells with stiff hairs. In most

1 The organs which resemble those of the lateral line are the remarkable sense organs found by Eisig in the Capitellidse (Mittheil. a. d. ZooL Station zu Neapel, Vol. I.) ; but I am not inclined to think that there is a true homology between these organs and the lateral line of Vertebrata. It seems to me probable that the segmentally arranged optic organs of Polyophthalmus are a special modification of the more indifferent sense organs of the Capitellidse. The close affinity of these two types of Chsetopods is favourable to this view.

SENSE ORGANS. 539

cases each bulb is enclosed in a delicate tube open at its free extremity ; while the bulbs correspond in number with the myotomes. In some Teleostei (Gobius, Esox, etc.) such sense organs persist through life ; in most forms however each organ becomes covered by a pair of lobes of the adjacent tissue, one formed above and the other below it. The two lobes of each pair then unite and form a tube open at both ends. The linear series of tubes so formed is the commencement of the adult canal ; while the primitive sense bulbs form the sensory organs of the tubes. The adjacent tubes partially unite into a continuous canal, but at their points of apposition pores are left, which place the canal in communication with the exterior.

Besides these parts, I have found that there is present in the just hatched Salmon a linear streak of modified epidermis on the level of the lateral nerve, and from the analogy of the process described below for Elasmobranchii it appears to me probable that these streaks play some part in the formation of the canal of the lateral line.

In Elasmobranchii (Scyllium) the lateral line is formed as a linear thickening of the mucous layer of the epidermis. This thickening is at first very short, but gradually grows backwards, its hinder end forming a kind of enlarged growing point. The lateral nerve is formed shortly after the lateral line, and by the time that the lateral line has reached the level of the anus the lateral nerve has grown back for about two-thirds of that distance. The lateral nerve would seem to be formed as a branch of the vagus, but is at first half enclosed in the modified cells of the lateral line (fig. 275, nl) 1 , though it soon assumes a deeper position.

A permanent stage, more or less corresponding to the stage just described in Elasmobranchii, is retained in Chimasra, and Echinorhinus spinosus, where the lateral line has the form of an open groove (Solger, No. 404).

The epidermic thickening, which forms the lateral line, is converted into a canal, not as in Teleostei by the folding over of the sides, but by the formation of a cavity between the mucous and epidermic layers of the epiblast, and the subsequent enclosure of this cavity by the modified cells of the mucous layer of the epiblast which constitute the lateral line. The cavity first appears at the hind end of the organ, and thence extends forwards.

After its conversion into a canal the lateral line gradually recedes from the surface ; remaining however connected with the epidermis at a series of points corresponding with the segments, and at these points perforations are eventually formed to constitute the segmental apertures of the system.

The manner in which the lumen of the canal is formed in Elasmobranchs bears the same relation to the ordinary process of conversion of a groove into a canal that the formation of the auditory involution

1 Gotte and Semper both hold that the lateral nerve, instead of growing in a centrifugal manner like other nerves, is directly derived from the epiblast of the lateral line. For the reasons which prevent me accepting this view I must refer the reader to my Monograph on Elasmobranch Fishes, pp. 141 146.

540 ORGANS OF THE LATERAL LINE.

in Amphibia does to the same process in Birds. In both Elasmobranchii and Amphibia the mucous layer of the epiblast behaves exactly as does the whole epiblast in the other types, but is shut off from the surface by the passive epidermic layer of the epiblast.

The mucous canals of the head and the ampullae are formed from the mucous layer of the epidermis in a manner very similar to the lateral line ; but the nerves to them arise as simple branches of the fifth and seventh nerves, which unite with them at a series of points, but do not follow their course like the lateral nerve.

It is clear that the canal of the lateral line is secondary, as compared with the open groove of Chimaera or the segmentally arranged sense bulbs of young Teleostei ; and it is also clear that the phylogenetic mode of formation of the canal consisted in the closure of a primitively open groove. The abbreviation of this process in Elasmobranchii was probably acquired after the appearance of food-yolk in the egg, and the consequent disappearance of a free larval stage.

While the above points are fairly obvious it does not seem easy to decide a priori whether a continuous sense groove or isolated sense bulbs were the primitive structures from which the canals of the lateral line took their origin. It is equally easy to picture the evolution of the canal of the lateral line either from (i) a continuous unsegmented sense line, certain points of which became segmentally differentiated into special sense bulbs, while the whole subsequently formed a groove and then a canal ; or from (2) a series of isolated sense bulbs, for each of which a protective groove was developed ; and from the linear fusion of which a continuous canal became formed.

From the presence however of a linear streak of modified epidermis in larval Teleostei, as well as in Elasmobranchii, it appears to me more probable that a linear sense streak was the primitive structure from which all the modifications of the lateral line took their origin, and that the segmentally arranged sense bulbs of Teleostei are secondary differentiations of this primitive structure.

The, at first sight remarkable, distribution of the vagus nerve to the lateral line is probably to be explained in connection with the evolution of this organ. As is indicated both by its innervation from the vagus, as also from the region where it first becomes developed, the lateral line was probably originally restricted to the anterior part of the body. As it became prolonged backwards it naturally carried with it the vagus nerve, and thus a sensory branch of this nerve has come to innervate a region which is far beyond the limits of its original distribution.

BIBLIOGRAPHY.

(397) F. M. Balfour. A Monograph onthe development of Elasnwbranch Fishes, pp. 141 146. London, 1878.

(398) H. Eisig. "Die Segmentalorgane cl. Capitelliden." Mitthcil. a. d. zool. Station zu Neapel> Vol. I. 1879.

BIBLIOGRAPHY. 541

(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(400) Fr. Leydig. Lehrbuch d. Histologie des Memchen u. d. Thiere. Hamm.

1857 (401) Fr. Leydig. Neue Beitrdge z. anat. Kenntniss d. Hautdecke u. Hautsinnesorgane d. Fische. Halle, 1879.

(402) F. E. Schulze. " Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.

(403) C. Semper. "Das Urogenitalsy stem d. Selachier." Arbeit, a. d. zoo!.zoot. Instil. Wiirzburg, Vol. II.

(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische." Archiv f. mikr. Anat., Vol. xvn. and xvm. 1879 an< * !88o.

CHAPTER XVIII. THE NOTOCHORD, THE VERTEBRAL COLUMN, THE RIBS AND THE STERNUM

INTRODUCTION.

Amongst the products of that part of the mesoblast which constitutes the connective tissue of the body special prominence must be given to the skeleton of the Vertebrata, from its importance in relation to numerous phylogenetic and morphological problems.

The development of the skeleton is however so large a subject that it cannot be satisfactorily dealt with except in a special treatise devoted to it ; and the following description must be regarded as a mere sketch, from which detail has been as far as possible excluded.

In the lowest Chordata the sole structure present, which deserves to be called a skeleton, is the notochord. Although the notochord often persists as an important organ in the true Vertebrata, yet there are always added to it various skeletal structures developed in the mesoblast. Before entering into a systematic description of these, it will be convenient to say a few words as to the general characters of the skeleton.

Two elements, distinct both in their genesis and structure, are to be recognized in the skeleton. The one, forming the true primitive internal skeleton or endoskeleton, is imbedded within the muscles and is originally formed in cartilage. In many instances it retains a cartilaginous consistency through life, but in the majority of cases it becomes gradually ossified, and

NOTOCHORD AND VERTEBRAL COLUMN. 543

converted into true bone. Bones so formed are known as cartilage bones.

The other element is originally formed by the fusion of the ossified bases of the dermal placoid scales already described in Chapter xiv., or by the fusion of the ossified bases of teeth situated in the mucous membrane of the mouth. In both instances the plates of bone so formed may lose the teeth or spines with which they were in the first instance covered, either by absorption in the individual, or phylogenetically by their gradually ceasing to be developed. The plates of bone, which originated by the above process, become in higher types directly developed in the connective tissue beneath the skin ; and gradually acquire a deeper situation, and are finally so intimately interlocked with parts of the true internal skeleton, that the two sets of elements can only be distinguished by the fact of the one set ossifying in cartilage and the other in membrane.

It seems probable that in the Reptilia, and possibly the extinct Amphibia, dermal bones have originated in the skin without the intervention of superjacent spinous structures.

In cases where a membra nebone, as the dermal ossifications are usually called, overlies a part of the cartilage, it may set up ossification in the latter, and the cartilage bone and membrane bone may become so intimately fused as to be quite inseparable. It seems probable that in cases of this kind the compound bone may in the course of further evolution entirely lose either its cartilaginous element or its membranous element ; so that cases occasionally occur where the development of a bone ceases to be an absolutely safe guide to its evolution.

As to the processes which take place in the ossification of cartilage there is still much to be made out. Two processes are often distinguished, viz. (i) a process known as ectostosis, in which the ossification takes place in the perichondrium, and either simply surrounds or gradually replaces the cartilage, and (2) a process known as endostosis, where the ossification actually takes place between the cartilage cells. It seems probable however (Gegenbaur, Vrolik) that there is no sharp line to be drawn between these two processes ; but that the ossification almost always starts from the perichondrium. In the higher types, as a rule, the vessels of the perichondrium extend into

544 MEMBRANE BONES AND CARTILAGE BONES.

the cartilage, and the ossification takes place around these vessels within the cartilage; but in the lower types (Pisces, Amphibia) ossification is often entirely confined to the perichondrium ; and the cartilage is simply absorbed.

The regions where ossification first sets in are known as centres of ossification; and from these centres the ossification spreads outwards. There may be one or more centres for a bone.

The actual causes which in the first instance gave rise to particular centres of ossification, or to the ossification of particular parts of the cartilage, are but little understood ; nor have we as yet any satisfactory criterion for determining the value to be attached to the number and position of centres of ossification. In some instances such centres appear to have an important morphological significance, and in other instances they would seem to be determined by the size of the cartilage about to be ossified.

There is no doubt that the membrane bones and cartilage bones can as a rule be easily distinguished by their mode of development ; but it is by no means certain that this is always the case. It is necessarily very difficult to establish the homology between bones, which develop in one type from membrane and in another type from cartilage ; but there are without doubt certain instances in. which the homology between two bones would be unhesitatingly admitted were it not for the difference in their development. The most difficult cases of this kind are connected with the shoulder-girdle.

The possible sources of confusion in the development of bones are obviously two. (i) A cartilage bone by origin may directly ossify in membrane, without the previous development of cartilage, and (2) a membrane bone may in the first instance be formed in cartilage.

The occurrence of the first of these is much more easy to admit than that of the second ; and there can be little doubt that it sometimes takes place. In a large number of cases it would moreover cause no serious difficulty to the morphologist.

BIBLIOGRAPHY of the origin of the Skeleton.

(405) C. Gegenbaur. " Ueb. primare u. secundare Knochenbildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium." Jcnaischc Zcitschrifl, Vol. III. 1867.

(406) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d. Ziihne d. Selachicr." Jenaische Zeitsckrift, Vol. viu. 1874.

NOTOCHORD AND VERTEBRAL COLUMN.

545

(407) O. Hertwig. " Ueb. cl. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supplementheft, 1874.

(408) O. Hertwig. " Ueber d. Hautskelet cl. Fische." Morphol. Jahrbmh, Vol. II. 1876. (Siluroiden u. Acipenseriden.)

(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbnch, Vol. v. 1879.

(410) A. Kolliker. " Allgemeine Betrachtungen iib. die Entstehung d. knochernen Schadels d. Wirbelthiere. " Berichte r. d. kijnigl. zoot. Anstalt z. Wiirzburg, 1849.

(411) Fr. Leydig. " Histologische Bemerkungen lib. d. Polypterus bichir." Zeit.f. wiss. Zool., Vol. v. 1858.

(412) H. Miiller. " Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc." Zeit. f. wiss. ZooL, Vol. ix. 1859.

(413) Williamson. "On the structure and development of the Scales and Bones of Fishes." Phil. Trans., 1851.

(414) Vrolik. " Studien lib. d. Verknocherung u. die Knochen d. Schadels d. Teleostier. " Niederliindisches Archiv f. Zoologie, Vol. I.

NotocJtord and Vertebral column.

The primitive axial skeleton of the Chordata consists of the notochord and its sheath. It persists as such in the adult in Amphioxus, and constitutes, in embryos of all Vertebrata, for a considerable period of their early embryonic life, the sole representative of the axial skeleton.

The Notochord. The early formation of the notochord has already been described in detail (pp. 292 300). It is developed, in most if not all cases, as an axial differentiation of the hypoblast, and forms at first a solid cord of cells, without a sheath, placed between the nervous system and the dorsal wall of the alimentary tract, and extending from the base of the front of the

mid-brain to the end of the tail. The section in the region of the brain will be dealt with by itself. That H. HI.

FIG. 313. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN F IN FIG. 28.

The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.

ch. notochord ; ep. epiblast ; Vr. rudiment of vertebral body; ;;//. muscle-plate; mp'. portion of muscle-plate already differentiated into longitudinal muscles.

35

546

NOTOCHORD.

in the trunk forms the basis round which the vertebral column is moulded.

The early histological changes in the cells of the notochord are approximately the same in all the Craniata. There is formed by the superficial cells of the notochord a delicate sheath, which soon thickens, and becomes a welldefined structure. Vacuoles (one or more to each cell) are formed in the cells of the notochord, which enlarge till the whole notochord becomes almost entirely formed of large vacuoles separated by membranous septa which form a complete sponge-like reticulum 'fig. 313). In the Ichthyopsida most of the protoplasm with the nuclei is carried to the periphery, where it forms a special nucleated layer sometimes divided into definite epithelial-like cells (fig. 314), while in the meshes of the reticulum a few nuclei surrounded by a little protoplasm still remain. In the Amniotic Vertebrata, probably owing to the early atrophy of the notochord, the distribution of the nuclei in the spaces of the mesh-work remains fairly uniform.

FIG. 314. SECTION THROUGH THE SPINAL COLUMN OF A YOUNG SALMON. (From Gegenbaur.)

cs. sheath of notochord ; k. neural arch ; k'. haemal arch; m. spinal cord; a. dorsal aorta ; z'. cardinal veins.

In the early stages of development the spaces in the notochordal spongework, each containing a nucleus and protoplasm, probably represent cells. In the types in which the notochord persists in the adult the mesh-work becomes highly complicated, and then forms a peculiar reticulum filled with gelatinous material, the spaces in which do not indicate the outlines of definite cells (figs. 315 and 318).

Around the sheath of the notochord there is formed in the Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an elastic membrane usually known as the membrana elastica externa.

In most Vertebrates the notochord and its sheath either atrophy completely or become a relatively unimportant part of the axial skeleton; but in the Cyclostomata (fig. 315) and in the Selachioidean Ganoids (Acipenser, etc.) they persist as the sole representative of the true vertebral axis. The sheath becomes very much thickened; and on the membrana elastica covering

NOTOCHORD AND VERTEBRAL COLUMN.

547

Ch

it the vertebral arches directly rest. In Klasmobranchii the sheath of the notochord undergoes a more complicated series of changes, which result first of all in the formation of a definite unsegmented cartilaginous tube 1 round the notochord, and subsequently (in most forms) in the formation of true vertebral bodies. Between the membrana elastica externa and the sheath of the notochord a layer of cells becomes interposed (fig. 316, n}, which lie in a matrix not sharply separated from the sheath of the notochord. The cells which form this layer appear to be derived from a special investment of the notochord, and to have penetrated through the membrana elastica externa to reach their final situation. The layer with these cells soon increases 7/> cardmal vems in thickness, and forms a continuous unsegmented tube of fibrous tissue with flattened concentrically arranged nuclei (fig. 317, Vb}. Externally is placed c f, the membrana elastica externa (met}, while within is the cuticular sheath of the notochord. This tube is the cartilaginous tube spoken of above and is known as the cartilaginous sheath of the notochord.

FIG. 315. SECTION THROUGH THE VERTEBRAL COLUMN OF AMMOCCETES. (From Gegenbaur.)

Ch. notochord ; c s. notochordal sheath ; m. spinal cord ; a. aorta ;

^ \

FIG. 316. LONGITUDINAL SECTION THROUGH A SMALL PART OF THE NOTOCHORD AND ADJOINING PARTS OF A SCYLLIUM EMBRYO, AT THE TIME OF THE FIRST FORMATION OF THE CARTILAGINOUS SHEATH.

ch. notochord; sc. sheath of notochord; n. nuclei of cartilaginous sheath; me.e. membrana elastica externa.

The exact origin of the cartilaginous tube just described is a question of fundamental importance with reference to the origin of the vertebral column and the homologies of its constituent parts ; but is by no means easy to settle. In the account of the subject in my memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from

1 This tube consists of a peculiar form of fibrous tissue rather than true cartilage, though part of it subsequently becomes hyaline cartilage.

352

548

SHEATH OF THE NOTOCHORD.

a layer of cells outside the sheath of the notochord, on the exterior of which the membrana elastica externa was subsequently formed. To this view Gotte (No. 419) also gave his adhesion. Schneider has since (No. 429) stated that this is not the case, but that, as described above, the membrana elastica externa is formed before the layer of cartilage. I have since worked over this subject again, and am on the whole inclined to adopt Schneider's correction.

It follows from the above description that the cartilaginous tube in question is an essential part of the sheath of the notochord, and that it is to some extent homologous with the notochordal sheath of the Sturgeon and the Lamprey, and not an entirely new formation.

This sheath forms the basis of the centra of the future vertebrae. In a few adult forms, i.e. Chimaera and the Dipnoi, it

FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO AT THE ROOT OF THE TAIL.

Vb. cartilaginous sheath of the notochord ; ha. hasmal arch ; vp. process to which the rib is articulated ; mcl. membrana elastica externa ; ch. notochord ; ao. aorta ; . caudal vein.

retains its primitive condition, except that in Chimaera there are present delicate ossified rings more numerous than the arches ; while in the Notidani, Laemargi and Echinorhini the

NOTOCHORD AND VERTEBRAL COLUMN. 549

indications of vertebrae are imperfectly marked out. The further history of this sheath in the forms in which true vertebrae are formed can only be dealt with in connection with the formation of the vertebral arches.

In Teleostei there is present, as in Elasmobranchii, an elastica externa, and an inner notochordal sheath. The elastica externa contains, according to Gotte, cells. These cells, if present, are however very difficult to make out, but in any case the so-called elastica externa appears to correspond with the cartilaginous sheath of Elasmobranchii together with its enveloping elastica, since ossification, when it sets in, occurs in this layer. The sheath within becomes unusually thick.

In the Amphibia and in the Amniota no membrane is present which can be identified with the membrana elastica externa of the Elasmobranchii, Teleostei, etc. In Amphibia (Gotte) there is formed round the notochord a cellular sheath, which has very much the relations of the cartilaginous tube around the notochord of Elasmobranchii, and is developed in the same way from the perichordal connective tissue cells. It is only necessary to suppose that the rnembrana elastica externa has ceased to be developed (which in view of its extreme delicacy and unimportant function in Elasmobranchii is not difficult to do) and this cellular sheath would then obviously be homologous with the cartilaginous tube in question. In the Amniota an external sheath of the notochord cannot be traced as a distinct structure, but the connective tissue surrounding the notochord and spinal cord is simply differentiated into the vertebral bodies and vertebral arches.

Vertebral arches and Vertebral bodies.

Cyclostomata. The Cyclostomata are the most primitive forms in which true vertebral arches are present. Their ontogeny in this group has not been satisfactorily worked out. It is however noticeable in connection with them that they form for the most part isolated pieces of cartilage, the segmental arrangement of which is only imperfect.

Elasmobranchii. In the Elasmobranchii the cells forming the vertebral arches are derived from the splanchnic layer of the mesoblastic somites. They have at first the same segmentation

55O NEURAL AND H^MAL ARCHES.

as the somites (fig. 313, Vr), but this segmentation is soon lost, and there is formed round the notochord a continuous sheath of embryonic connective tissue cells, which gives rise to the arches of the vertebrae, the tissue forming the dura mater, the perichondrium, and the general investing connective tissue.

The changes which next follow result in what has been known since Remak as the secondary segmentation of the vertebral column. This segmentation, which occurs in all Vertebrata with true vertebrae, is essentially the segmentation of the continuous investment of the notochord and spinal cord into vertebral bodies and vertebral arches. It does not however follow the lines of the segmentation of the muscle-plates, but is so effected that the centres of the vertebral bodies are opposite the septa between the muscle-plates.

The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebrae had not appeared. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments.

The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each myotome shall be capable of acting on two vertebrae ; and this condition can only be fulfilled when the myotomes are opposite the intervals between the vertebrae. For this reason, when the vertebrae became formed, their centres were opposite not the middle of the myotomes but the inter-muscular septa.

These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation (fig. 313) of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present ; and has no greater morphological significance than the fact that the cells of the vertebrae were derived from the segmented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis ; till finally they became in still higher forms differentiated into vertebrae and their arches.

During the stage represented in fig. 28 g, and somewhat before the cartilaginous sheath of the notochord is formed, there appear four special concentrations of the mesoblastic tissue adjoining the notochord, two of them dorsal (neural) and two of them ventral (haemal). They are not segmented, and form four ridges, seated on the sides of the notochord. They are united

NOTOCHORD AND VERTEBRAL COLUMN.

551

with each other by a delicate layer of tissue, and constitute the substance in which the neural and haemal arches subsequently become differentiated.

At about the time when the first traces of the cartilaginous sheath of the notochord arise, differentiations take place in the neural and haemal ridges. In the neural ridge two sets of arches are formed for each myotome, one resting on the cartilaginous sheath of the notochord in the region which will afterwards form the centrum of a vertebra, and constituting a true neural arch ; and a second separate from the cartilaginous sheath, forming an intercalated piece 1 . Both of them soon become hyaline cartilage.

There is a considerable portion of the original tissue of the neural ridge, especially in the immediate neighbourhood of the notochord, which is not employed in the formation of the neural arches. This tissue has a fibrous character and becomes converted into the perichondrium and other parts.

The haemal arches are formed from the haemal ridge in precisely the same way as the neural arches, but interhsemal intercalated pieces are often present. In the region of the tail the haemal arches are continued into ventral processes which meet below, enclosing the aorta and caudal veins.

1 The presence of intercalated pieces in the neural arch system of Elasmobranchii, Chimaera, etc. is probably not the indication of an highly differentiated type of neural arch, but of a transitional type between an imperfect investment of the spinal cord by isolated cartilaginous bars, and a complete system of neural arches like that in the higher Vertebrata.

FIG. 318. SECTION THROUGH THE VERTEBRAL COLUMN OF AN ADVANCED EMBRYO OF SCYLLIUM IN THE REGION OF THE TAIL.

na. neural arch ; ha. haemal arch ; ch. notochord ; sh. inner sheath of notochord ; ne. membrana elastica externa.

552 NEURAL AND H^iMAL ARCIIKS.

Since primitively the postanal gut was placed between the aorta and the caudal vein, the haemal arches potentially invest a caudal section of the body cavity. In the trunk region they do not meet ventrally, but give support to the ribs. The structures just described are shewn in section in fig. 318, in which the neural (110) and haemal (ha) arches are shewn resting upon the cartilaginous sheath of the notochord.

While these changes are being effected in the arches the cartilaginous sheath of the notochord undergoes important differentiations. In the vertebral regions opposite the origin of the neural and haemal arches (fig. 318) its outer part becomes hyaline cartilage, while the inner parts adjoining the notochord undergo a somewhat different development, the notochord in this part becomes at the same time somewhat constricted. In the intervertebral regions the cartilaginous sheath of the notochord becomes more definitely fibrous, while the notochord is in no way constricted. A diagrammatic longitudinal section through the vertebral column, while these changes are being effected, is shewn in fig. 320 B.

These processes are soon carried further. The notochord within the vertebral body becomes gradually constricted, especially in the median plane, till it is here reduced to a fibrous band, which gradually enlarges in either direction till it reaches its maximum thickness in the median plane of the intervertebral region. The hyaline cartilage of the vertebral region forms a vertebral body in which calcification may to some extent take place. The cartilage of the base of the arches gradually spreads over it, and on the absorption of the membrana elastica externa, which usually takes place long before the adult state is reached, the arch tissue becomes indistinguishably fused with that of the vertebral bodies, so that the latter are compound structures, partly formed of the primitive cartilaginous sheath, and partly of the tissue of the bases of the neural and haemal arches. Owing to the beaded structure of the notochord the vertebral bodies take of necessity a biconcave hourglass-shaped form.

The intervertebral regions of the primitive sheath of the notochord form fibrous intervertebral ligaments enclosing the unconstricted intervertebral sections of the notochord.

NOTOCHORD AND VERTKBKAL COLUMN. 553

A peculiar fact may here be noticed with reference to the formation of the vertebral bodies in the tail of Scyllium, Raja, and possibly other forms, viz. that there are double as many -vertebral bodies as there are myotomes and spinal nerves. This is not due to a secondary segmentation of the vertebras but, as I have satisfied myself by a study of the development, takes place when the vertebral bodies first become differentiated. The possibility of such a relation of parts is probably to be explained by the fact that the segmentation of the vertebral column arose subsequently to that of the nerves and myotomes.

Ganoidei. In Acipenser and other cartilaginous Ganoids the haemal and neural arches are formed as in Elasmobranchii, and rest upon the outer sheath of the notochord. Since however the sheath of the notochord is never differentiated into distinct vertebrae, this primitive condition is retained through life.

Teleostei. In Teleostei the formation of the vertebral arches and bodies takes place in a manner, which can be reduced, except in certain minor points, to the same type as that of Elasmobranchii.

There are early formed (fig. 314 k and k] neural and haemal arches resting upon the outer sheath of the notochord. The latter structure, which, as mentioned on p. 549, corresponds to the cartilaginous sheath of the notochord of Elasmobranchii, soon becomes divided into vertebral and intervertebral regions. In the former ossification directly sets in without the sheath acquiring the character of hyaline cartilage (Gotte, 419). The latter forms the fibrous intervertebral ligaments. The notochord exhibits vertebral constrictions.

The ossified outer sheath of the notochord forms but a small part of the permanent vertebrae. The remainder is derived partly from an ossification of the connective tissue surrounding the sheath, and partly from the bases of the arches, which do not spread round the primitive vertebral bodies as in Elasmobranchii. The ossifications in the tissue surrounding the sheath usually (fig. 319) take the form of a cross, while the bases of the arches (k and k'} remain as four cartilaginous radii between the limbs of the osseous cross. In some instances the bases of the arches also become ossified, and are then with difficulty distinguishable from the other parts of the secondary vertebral body. The parts of the arches outside the vertebral bodies are for the most part ossified (fig. 319). In correlation with the vertebral constrictions of the notochord the vertebral bodies are biconcave.

Amphibia. Of the forms of Amphibia so far studied embryologically the Salamandridae present the most primitive type of formation of the vertebral column.

It has already been stated that in Amphibia there is present

554

VERTEBRAL COLUMN OF AMPHIBIA.

around the notochord a cellular sheath, equivalent to the cartilaginous sheath of Elasmobranchii. In the tissue on the dorsal side of this sheath a series of cartilaginous processes becomes formed. These processes are the commencing neural arches ; and they rest on the cellular sheath of the notochord opposite the middle of the vertebral regions.

A superficial osseous layer becomes very early formed in each vertebral region of the cellular sheath ; while in each of the intervertebral regions, which are considerably shorter than the vertebral, there is developed a ring-like cartilaginous thickening of the sheath, which projects inwards so as to constrict the notochord. At a period before this thickening has attained considerable dimensions the notochord becomes sufficiently

constricted in the centre of each FlG - 319- VERTICAL SECTIONTHROUGH THE MIDDLE OF A VER vertebral region to give a biconcave TEBRA OF Esox LUCIUS (PIKE). form to the vertebrae for a very < From Gegenbaur.) short period of fcetal life.

The stage with biconcave vertebrae is retained through life in the Perennibranchiata and Gymnophiona.

ch. notochord ; cs. notochordal sheath; /. and K. cartilaginous tissue of the neural and haemal arches ; h. osseous hremal process ; n. spinal canal.

The chief peculiarity which distinguishes the later history of their vertebral column from that of fishes consists in the immense development of the intervertebral thickenings just mentioned, which increase to such an extent as to reduce the notochord, where it passes through them, to a mere band ; while the cartilage of which they are composed becomes differentiated into two regions, one belonging to the vertebra in front, the other to that behind, the hinder one being convex, and the anterior concave. The two parts are not however absolutely separated from each other.

By these changes each vertebra comes to be composed of (i) a thin osseous somewhat hourglass-shaped cylinder with a dilated portion of the notochord in its centre, and (2 and 3) of two

NOTOCHORD AND VERTEBRAL COLUMN.

555

halves of two intervertebral cartilages, viz. an anterior convex half and a posterior concave half. The vertebrae thus come to be opisthoccelous. A longitudinal section through the vertebral column at this stage is diagrammatically shewn in fig. 320 C.

To the centre of each of these vertebrae the neural arches, the origin of which was described above, become in the meantime firmly attached ; and grow obliquely upwards and A B CUE

FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OK THE

VERTEBRA IN THE DIFFERENT TYPES. (From Gegenbaur.)

A. Ideal type in which distinct vertebrae are not established.

B. Type of Pisces with vertebral constrictions of the notochord.

C. Amphibian type, with intervertebral constrictions of the notochord by the intervertebral parts of the cellular sheath.

D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.

E. Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.

c . notochord ; cs. cuticular sheath of notochord ; s. cartilaginous sheath ; v. vertebral regions ; iv. intervertebral regions ; g, intervertebral joints.

backwards, so as to meet and unite above the spinal cord. The transverse processes of the vertebrae would seem (Pick) to be developed independently of the arches, though they very soon fuse with them. According to Gotte the transverse processes are double in the trunk, there being two pairs, one vertically above the other for each vertebra. The pair on each side eventually fuse together.

In the tail haemal arches are formed, which are similar in their mode of development to the neural arches.

The unconstricted portion of the notochord, which persists in each vertebra, becomes in part converted into cartilage.

556 VKRTKIJRAL COLUMN OF THE AMNIOTA.

Anura. In the Anura the process of formation of the vertebral column is essentially the same as that in the Salamandridte. Two types may however be observed. One of these occurs in the majority of the Anura, and mainly differs from that in Salamandra in (i) the earlier fusion of the arches with the cellular sheath of the notochord ; (2) the more rapid growth of the intervertebral thickenings of the cellular sheath, which results in the early and complete obliteration of the intervertebral parts of the notochord ; (3) the complete division of these intervertebral thickenings into anterior and posterior portions, which unite with and form the articular surfaces of two contiguous vertebras. The vertebrae are moreover proccelous instead of being opisthoccelous.

The unconstricted vertebral sections of the notochord always persist till the ossification of the vertebras has taken place. In some forms they remain through life (Rana), while in other cases they eventually either wholly or partially disappear.

The second type of vertebral development is found in Bombinator, Pseudis, Pipa, and Pelobates. In these genera the formation of the vertebra takes place almost entirely on the dorsal side of the notochord ; so that the latter forms a band on the ventral side of the vertebral column. In other respects the history of the vertebral column is the same in the two cases ; the vertebral unconstricted parts of the notochord appear however to become in part converted into cartilage. The type of formation of the vertebral column in these genera has been distinguished as epichordal in contradistinction to the more normal or perichordal type.

Amniota. In the Amniota all trace of a distinction between a cellular notochord sheath and an arch tissue is lost, and the two are developed together as a continuous whole forming an unsegmented tube round the notochord, with a neural ridge which does not at first nearly invest the neural cord. This tube becomes differentiated, in the manner already described for other types, into (i) vertebral regions with true arches, and (2) intervertebral regions.

Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous tube is formed round the notochord, which is continuous with the cartilaginous neural arches. The latter are placed in the vertebral regions, and in these regions ossification very early sets in, while the notochord remains relatively unconstricted. In the intervertebral regions the cartilage becomes thickened, as in Amphibia, and gradually constricts the notochord. The cartilage in each of the intervertebral regions soon becomes divided into two parts which form the articular faces of two contiguous vertebrae.

NOTOCHORD AND VERTEBRAL COLUMN. 557

The general character of the vertebral column on the completion of these changes is shewn in fig. 320 D. The later changes are relatively unimportant. The constricted intervertebral sections of the notochord rapidly disappear, while the vertebral sections become partially converted into cartilage, and only cease to be distinguishable at a considerably later period.

The ossification extends from the bodies of the vertebrae into the arches and into the articular surfaces, so that the whole vertebrae eventually become ossified.

The Ascalabotae (Geckos) present an exceptional type of vertebral column which has many of the characters of a developmental stage in other Lizards. The body of the vertebra is formed of a slightly hourglassshaped osseous tube, united with adjoining vertebras by a short intervertebral cartilage. There is a persistent and continuous notochord which, owing to the small development of the intervertebral cartilages, is narrower in the vertebral than in the intervertebral regions.

Aves. In Birds the cellular tube formed round the notochord is far thicker than in the Reptilia. It is continuous in the regions of the future vertebrae with neural arches, which do not at first nearly enclose the spinal cord.

On about the fifth day, in the case of the chick, it becomes differentiated into vertebral regions opposite the attachments of the neural arches, and intervertebral regions between them ; the two sets of regions being only distinguished by their histological characters. Very shortly afterwards each intervertebral region becomes segmented into two parts, which respectively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immediately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamentum suspensorium.

The notochord during these changes at first remains indifferent, but subsequently, on about the seventh day in the chick, a slight constriction of each vertebral region takes place ; so that the vertebrae have temporarily, as they have also in Amphibia, a biconcave form which repeats the permanent condition of most fishes. By the ninth and tenth days, however, this condition has completely disappeared, and in all the intervertebral portions the notochord has become distinctly constricted, and at the same time in each vertebral portion there

558

VERTEBRAL COLUMN OF MAMMALIA.

have also appeared two constrictions of the notochord giving rise to a central and to two terminal enlargements.

On the twelfth day the ossification of the cartilaginous centra commences.

The first vertebra to ossify is the second or third cervical, and the ossification gradually extends to those behind. It does not commence in the arches till somewhat later than in the bodies. For each arch there are two centres of ossification, one on each side.

The notochord persists for the greater part of foetal life and even into post-fcetal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage, and so form an integral part of the permanent vertebrae. Rudiments of the intervertebral portions of the notochord may long be detected in the ligamenta suspensoria.

Schwarck (No. 420) states that in both the intervertebral and the vertebral regions, though less conspicuously in the former, the cartilage is divided into two layers, an inner and an outer. He holds that the inner layer corresponds to the cartilaginous notochordal sheath of the lower types, and the outer to the arch tissue. Ossification (Gegenbaur) of the centra appears in a special inner layer of cartilage, which is probably the same as the inner layer of the earlier stage, though this point has not been definitely established.

FIG. 321. LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN OF AN EIGHT WEEKS' HUMAN EMBRYO IN THE THORACIC REGION. (From Kolliker.)

v. cartilaginous vertebral body ; //. intervertebral ligament; ch, notochord.

Mammalia. The early development of the perichordal cartilaginous tube and rudimentary neural arches is almost the same in Mammals as in Birds. The differentiation into vertebral and intervertebral regions is the same in both groups ; but instead of becoming divided as in Reptilia and Birds into two segments attached to two adjoining vertebrae, the intervertebral regions become in Mammals wholly converted into the intervertebral ligaments (fig. 322 /*'). There are three centres of ossifications for each vertebra, two in the arch and one in the centrum.

NOTOCHORD AND VERTEBRAL COLUMN.

559

The fate of the notochord is in important respects different from that in Birds. It is first constricted in the centre of the vertebra (figs. 320 E and 321) and disappears there shortly after the ossification ; while in the intervertebral regions it remains relatively unconstricted (figs. 320 E, 321 and 322 c] and after

FlG. 3-22. LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGAMENT AND ADJACENT PARTS OF TWO VERTEBRA FROM THE THORACIC REGION OF AN ADVANCED EMBRYO OF A SHEEP. (From Kolliker.)

la. ligamentum longitudinale anterius ; lp. ligamentum long, posterius ; li. ligamentum intervertebrale ; k, k'. epiphysis of vertebra ; w. and iv' '. anterior and posterior vertebrae ; c. intervertebral dilatation of notochord ; c'. and c'. vertebral dilatation of notochord.

undergoing certain histological changes remains through life as part of the nucleus pulposus in the axis of the invertebral ligaments 1 . There is also a slight swelling of the notochord near the two extremities of each vertebra (fig. 322 c' and c"}. In the persistent vertebral constriction of the notochord Mammals retain a more primitive and piscine mode of formation of the vertebral column than the majority either of the Reptilia or Amphibia.

1 This view was first put forward by Lushka, and his surmises have been confirmed by Kolliker and other embryologists. Leboucq (No. 424) however holds that_ the cells of the notochord in the intervertebral regions fuse with those of the adjoining tissue ; and Dursy and others deny that the nucleus pulposus is derived from the notochord.

560 BIBLIOGRAPHY.

BIBLIOGRAPHY of Notochord and Vertebral column.

(415) Cartier. "Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeitschrift furwiss. ZooL, Bd. xxv. Suppl. 1875.

(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wirbelsaule der Amphibien nnd Reptilien. Leipzig, 1862.

(417) C. Gegenbaur. " Ueber die Entwickelung der Wirbelsaule des Lepidosteus mil vergleichend anatomischen Bemerkungen." Jenaische Zeitschrift, Bd. in. 1863.

(418) C. Gegenbaur. " Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische Zeitschrift, Vol. v. 1870.

(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d. Wirbelthicre. " II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol. xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879 (Teleostier).

(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der Wirbelsaule u. s. w." Hasse, Anatomische Studien, 1872.

(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.

(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical, medic in. Gesellschaft in Wiirzburg, Bd. X.

(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier insbesondere iiber die Wirbel der Lamnoidei." Abhandlungen der senkenbergischen naturforschenden Gesellschaft in Frankfurt, Bd. v.

(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.

(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische nnd Reptilien. Berlin, 1853.

(426) Aug. Miiller. " Beobachtungen zur vergleichenden Anatomic der Wirbelsaule." Miiller's Archiv. 1853.

(427) J. Miiller. " Vergleichende Anatomic der Myxinoiden u. der Cyklostomen mil durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungen der koniglichen Akademie der Wissenschaften zu Berlin. 1834.

(428) W. Miiller. "Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.

(429) A. Schneider. Beitrage c. vergleich. Anat. u. Entwick. d. Wirbelthicre. Berlin, 1879.

Ribs and Sternum.

Ribs. Embryological evidence on the development of the ribs, though somewhat inadequate, indicates that they arise as cartilaginous bars in the connective tissue of the intermuscular septa, and that they are placed, in Elasmobranchii and

RIBS. 561

Amphibia, on the level of division between the dorso-lateral and ventro-lateral divisions of the muscle-plates. This does not appear to hold true for either Ganoidei or Teleostei. In Teleostei they are entirely below the muscles along the lines of the intermuscular septa, and this is partially true for Ganoidei, though not wholly so in Lepidosteus. They may be attached either to the haemal (Pisces) or neural (Amphibia and Amniota) arches. The connective tissue from which they are formed is continuous with the processes of the vertebrae to which they are attached ; but the conversion of the tissue into cartilage takes place more or less independently of that of the arches, although in many cases the cartilage of the two becomes continuous, the separation of the ribs being then effected by a subsequent process of segmentation (Pick, No. 431). It is possible that the ribs of Pisces may not be homologous with those of Amphibia and the Amniota, but till the reverse can be proved it is more convenient to assume that the ribs are homologous structures throughout the vertebrate series.

In Elasmobranchii the ribs are relatively of less importance in the adult than in the embryo. By a careful examination of their early development, I have satisfied myself that the differentiation of the ribs is independent of that of the haemal processes to which they are attached, although the differentiation proceeds in such a manner that, when both are converted into cartilage, they are quite continuous. Subsequently the ribs become segmented off from the haemal processes. At the junction of the tail and trunk, where the haemal processes commence to be ventrally prolonged, eventually to unite in the region of the tail below the caudal vein, the ribs are attached to short processes which spring from the sides of the haemal arches (fig. 317). The ventral haemal arches of these" fishes are therefore clearly in no part formed by the ribs.

In Ganoidei and Teleostei there is very great difficulty in determining the homologies of the ribs.

In the cartilaginous Ganoidei there are well developed rib-like structures, which might be regarded as homologous with Elasmobranch ribs, and indeed probably are so ; but at the same time their relations are in some respects very different from those of Elasmobranch ribs in the caudal region. In Ganoids the ribs, in approaching the tail, become shorter and then fuse with the ends of the haemal processes, and finally in the caudal region form together with the haemal arches a closed haemal canal which superficially resembles that in Elasmobranchii.

In Lepidosteus and Amia, especially the former, the same phenomenon is still more marked ; and in Lepidosteus it is easy, in passing backwards,

B. III. 36

562 STERNUM.

to trace the ribs bending ventral-wards, and uniting ventrally in the caudal region to form, with the haemal processes, a complete haemal canal.

It might have been anticipated that the Teleostean Ganoids would resemble the Teleostei, but, from an examination of adult Teleostei, it would seem to be clear that the relations of the parts are the same as in Elasmobranchii, i.e. that the ribs have no share in forming the haemal canal in the tail. Aug. Miiller and Gotte have however brought embryological evidence (though not of a conclusive character), to shew that in the embryo the ribs really fuse with the haemal processes in the tail, and so assist, as in the Ganoids, in forming the haemal canal. Gotte moreover holds that the ribs in Elasmobranchii are not homologous with those of Teleostei and Ganoids ; but that the haemal arches in the tail are homologous in the three groups.

Without necessarily following Gotte in these views it is worth pointing out that the undoubtedly close affinity between the bony Ganoids and the Teleostei is in favour of the view on the haemal arches of Teleostei at which he has arrived on embryological grounds.

In Amphibia the formation of the ribs from the connective tissue of the intermuscular septa, their secondary attachment to the transverse processes of the neural arches, and their subsequent separation was first clearly established by Pick (No. 431), whose statements have since been confirmed by Hasse, Born, &c., and in part by Gotte, who holds however that, though converted into cartilage independently of the transverse processes, they are formed in membrane as outgrowths of these processes.

In the Amniota the ribs are also independently established (Hasse and Born), though they subsequently become united to the transverse processes and to the bodies of the vertebrae, or to the transverse processes only. This junction is however stated by the majority of authorities, never to be effected by the fusion of the cartilage of the two parts, but always by fibrous tissue ; though Hoffmann (No. 435) takes a different view on this subject, holding that the ribs are at first continuous with the intervertebral regions of the primitive cartilaginous tube surrounding the notochord.

Sternum. In dealing with the development of the sternum it will be convenient to leave out of consideration the interclavicle or episternum which is, properly speaking, only part of the shoulder-girdle and to confine my statements to the sternum proper.

This structure is found in all the Amniota except the Ophidia, Chelonia, and some of the Amphisbaenae.

From the older researches of Rathke, and from the newer ones of Gotte, etc., it appears that the sternum is always formed from the fusion of the ventral extremities of a certain number of ribs. The extremities of the ribs unite with each other from

STERNUM. 563

before backwards, and thus give rise to two cartilaginous bands. These bands become segmented off from the ribs with which they are at first continuous, and subsequently fuse in the median ventral line to form an unpaired sternum. The Mammalian presternum (manubrium sterni) and xiphosternum have the same origin as the main body of the sternum (Ruge, No. 438).

In the Amphibia there is no structure which admits from its mode of development of a complete comparison with the sternum of the Amniota ; and it must for this reason be considered doubtful whether the median structure placed behind the coracoids in the Anura, which is usually known as the sternum, is really homologous with the sternum of the Amniota 1 .

The remaining Ichthyopsida are undoubtedly not provided with a sternum.

BIBLIOGRAPHY of Ribs and Sternum.

(430) C. Glaus. " Beitrage z. vergleich. Osteol. d. Vertebraten. I. Rippen u. unteres Bogensystem. " Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(431) A. E. Fick. " Zur Entwicklungsgeschichte . d. Rippen und Querfortsatze." Archivf. Anat. und Physiol. 1879.

(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.

(433) A. Gotte. " Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergiirtel." Archivf. mikr. Anat., Vol. xiv. 1877.

(434) C. Hasse u. G. Born. " Bemerkungen lib. d. Morphologic d. Rippen." Zoologischer A nzeiger, 1879.

(4S5) C. K. Hoffmann. "Beitrage z. vergl. Anat. d. Wirbelthiere." Niederland. Archiv Zool., Vol. IV. 1878.

(436) W. K. Parker. " A monograph on the structure and development of the shoulder-girdle and sternum." Ray Soc. 1867.

(437) H. Rathke. Ueb. d. Bau n. d. Entrmcklung d. Brustbeins d. Saurier.

i853 (438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen.

Morphol. Jahrbuch., Vol. VI. 1880.

1 The so-called sternum of the Amphibia develops in proximity with certain rudimentary abdominal ribs, and Ruge has with some force urged (against Gotte) that it may be for this reason a rudimentary structure of the same nature as the sternum of the higher types.

362

CHAPTER XIX. THE SKULL.

THREE distinct sets of elements may enter into the composition of the skull. These are (i) the cranium proper, composed of true endoskeletal elements originally formed in cartilage, to which are usually added exoskeletal osseous elements, formed in the manner already described p. 542, and known in the higher types as membrane bones. (2) The visceral arches formed primitively as cartilaginous bars, but in the higher types largely supplemented or even replaced by exoskeletal elements. (3) The labial cartilages.

These parts present themselves in the most various forms, and their study constitutes one of the most important departments of vertebrate morphology, and one which has always been a favourite subject of study with anatomists. At the end of the last century and during the first half of the present century the morphology of the skull was handled from the point of view of the adult anatomy by Goethe, Oken, Cuvier, Owen, and many other anatomists, while Duges and, nearer to our own time, Rathke, laid the foundation of an embryological study of its morphology. A new era in the study of the skull was inaugurated by Huxley in his Croonian lecture in 1858, and in his lectures on Comparative Anatomy subsequently delivered before the Royal College of Surgeons. In these lectures Huxley disproved the then widely accepted view that the skull was composed of four vertebrae ; and laid the foundation of a more satisfactory method of dealing with the homologies of its constituent parts. Since then the knowledge of the development of the skull has made great progress. In this country a number

THE SKULL.

565

of very interesting memoirs have been published on the subject by Parker, which together constitute a most striking contribution to our knowledge of the ontogeny of the skull in a series of types ; and in Germany Gegenbaur's monograph on the cephalic skeleton of Elasmobranchii has greatly promoted a scientific appreciation of the nature of the skull.

In the present chapter only the most important features in the development of the skull will be touched on.

It will be convenient to describe, in the first instance, the development of the cartilaginous elements of the skull.

The Cranium. The brain is at first enveloped in a continuous layer of mesoblast known as the membranous cranium, into the base of which the anterior part of the notochord is prolonged for some distance. The primitive cartilaginous cranium is formed by a differentiation within the membranous cranium, and is always composed of the following parts

(fig- 323) :

(1) A pair of cartilaginous plates on each side of the cephalic section of the notochord, known as the parachordals (pa. ck}. These plates together with the notochord (nc) enclosed between them form a floor for the hind- and midbrain. The continuous plate, formed by them and the notochord, is known as the basil ar plate.

(2) A pair of bars forming the floor for the fore-brain,

iff

Cl?

pa.ch.

CIA

FIG. 323. HEAD OK EMBRYO DOGFISH, SECOND STAGE ; BASAL VIEW OF CRANIUM FROM ABOVE, THE CONTENTS HAVING BEEN REMOVED. (From

Parker.)

ol. olfactory sacs ; an. auditory capsule; nc. notochord; py. pituitary body ; pa.ch. parachordal cartilage ; tr. trabecula ; inf. infundibulum ; C.ir. cornua trabeculse ; pn. prenasal element ; sp. spiracular cleft ; br. external branchiae; Cl. 2, 4. visceral clefts.

known as the trabeculae (tr). These bars are continued forward from the parachordals. They meet behind and embrace the front end of the notochord ; and after separating for some distance bend in again in such a way

566

THE PARACHORDALS AND NOTOCHORD.

as to enclose a space the pituitary space. In front of this space they remain in contact and generally unite. They extend forwards into the nasal region (pn}.

(3) The cartilaginous capsules of the sense organs. Of these the auditory (ait) and olfactory capsules (ol} unite more or less intimately with the cranial walls ; while the optic capsules, forming the usually cartilaginous sclerotics, remain distinct.

The parachordals and notochord. The first of these sets of elements, viz. the parachordals and notochord, forming together the basilar plate, is always an unsegmented continuation of the axial tissue of the vertebral column. It forms the floor for that section of the brain which belongs to the primitive postoral part of the head (vide p. 314), and its extension is roughly that of the basioccipital of the adult skull. Its mode of development is almost identical with that of the vertebral column, except that the notochord, even in many forms where it persists in the vertebral column, disappears in the basilar plate ; though in a certain number of cases remnants of it are found in the adult state.

It will be convenient to say a few words notochord in the head. It always extends along the floor of the mid- and hind-brains, but ends immediately behind the infundibulum. The limits of its anterior extension are clearly shewn in fig. 43. The front end of the notochord often becomes more or less ventrally flexed in correspondence with the cranial flexure ; its anterior end being in some instances (Elasmobranchii) almost bent backwards (fig. 324).

Kolliker has shewn that in the Rabbit 1 , and I believe that a more or less similar phenomenon may also be observed in Birds, the anterior end of the notochord is united to the hypoblast of the throat in immediate contiguity with the opening of the pituitary body ; but it is not clear whether this is to be looked upon as the remnant of a primitive attachment of the notochord to the hypoblast, or as a secondary attachment.

here with reference to the nib

FIG. 324. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO.

cer. commencement of the cerebral hemisphere; pn. pineal gland ; ///.infundibulum ; //.ingrowth from mouth to form the pituitary body ; nib. mid-brain ; cb. cerebellum ; ch, notochord; al. alimentary tract; laa. artery of mandibular arch.

" Embryologische Mittheilungen." Festschrift d. Nattirfor. G^//., Halle, 1879.

THE SKULL.

567

Before the parachordals are formed the anterior end of the notochord has usually undergone a partial atrophy ; and its front end often becomes somewhat dorsally flexed. Within the basilar plate it often exhibits two or more dilatations, which have been regarded by Parker and Kolliker as indicative of a segmentation of this plate ; but they hardly appear to me to be capable of this interpretation.

In Elasmobranchs where, as shewn above, a very primitive type of development of the vertebral column is retained, we find that the basilar plate is at first formed of (i) the notochord invested by its cartilaginous sheath, and (2) of lateral masses of cartilage, the parachordals, homologous with the arch tissue of the vertebral column. This development probably indicates that the basilar plate contains in itself the same elements as those from which the neural arches and the centra of the vertebral column are formed ; but that it never passes beyond the unsegmented stage at first characteristic of the vertebral column. The hinder end of each parachordal forms a condyle articulating with the first vertebra ; so that in the cartilaginous skull there are always two occipital condyles. The basilar plate always grows up behind (fig. 326, so], and gives rise to a complete cartilaginous ring enveloping the medulla oblongata, in the same manner that the neural arches envelope the spinal cord. This ring forms an occipital cartilaginous ring ; in front of it the basilar plate becomes laterally continuous with the periotic cartilaginous capsules, and the occipital ring above usually spreads forward to form a roof for the part of the brain between these capsules. In the higher Vertebrates the periotic cartilages may be developed continuously with the basilar plate

The trabeculae. The trabeculae, so far as their mere anatomical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as the parachordals for the mid- and hind-brains. They differ however from the parachordals in one important feature, viz. that, except at their hinder end (fig. 323), they do not embrace between them the notochord.

The notochord constitutes, as we have seen, the primitive axial skeleton of the body, and its absence in the greater part of the region of the trabeculae would probably seem to indicate, as

568

THE TRABECUL^i.

pointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance.

C V 1

FlG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL/E OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After Parker.)

In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading.

cv i. cerebral vesicle (sliced oft") ; e. eye ; nc. notochord ; iv. investing mass ; 9. foramen for the exit of the ninth nerve ; d. cochlea ; hsc. horizontal semicircular canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior end of the investing mass ; pts. pituitary space ; tr. trabeculse. The reference line tr. has been accidentally made to end a little short of the cartilage.

The nature of the trabeculae has been much disputed by morphologists. The view that they cannot be regarded as the anterior section of the vertebral axis is supported by the consideration that the forward limit of the primitive skeletal axis, as marked by the notochord, coincides exactly with the distinction we have found it necessary to recognise, on entirely independent grounds, between the fore-brain, and the remainder of the nervous axis. But while this distinction between the parachordals and the trabeculas must . I think be admitted, I see no reason against supposing that the trabecuke may be plates developed to support the floor of the fore-brain, for the same physiological reasons that the parachordals have become formed at the sides of the notochord to support the floor of the hind-brain. By some anatomists the trabeculse have been held to be a pair of branchial bars ; but this view has now been generally given up. They have also been regarded as equivalent to a complete pair of neural arches enveloping the front end of the brain. The primitive extension of the base of the fore-brain through the pituitary

THE SKULL.

569

space is an argument, not without force, which has been appealed to in support of this view.

In the majority of the lower forms the trabeculys arise quite independently of the parachordals, though the two sets of elements soon unite ; while in Birds (fig. 325) and Mammals the parachordals and trabeculae are formed as a continuous whole. The junction between the trabeculae and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid.

The trabeculae are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts (figs. 323 and 325). Into this space there

so

f

bbr

cbr

FJG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THK

SEVENTH DAY OF INCUBATION. (After Parker.)

pn. prenasal cartilage ; aln. alinasal cartilage ; ale. aliethmoid ; immediately below this is the aliseptal cartilage, eth. ethmoid ; pp. pars plana ; ps. presphenoid or interorbital ; pa. palatine ; pg. pterygoid ; z. optic nerve ; as. alisphenoid ; q. quadrate ; st. stapes ; fr. fenestra rotunda ; hso. horizontal semicircular canal ; psc. posterior vertical semicircular canal : both the anterior and the posterior semicircular canals are seen shining through the cartilage, so. supraoccipital ; eo. exoccipital ; oc. occipital condyle ; nc. notochord ; mk. Meckel's cartilage ; ch. ceratohyal ; bh. basi-hyal ; cbr. and ebr. cerato-branchial ; bbr. basibranchial.

primitively projects the whole base of the fore-brain, but the space itself gradually becomes narrowed, till it usually contains only the pituitary body. The carotid arteries always pass through it in the embryo ; but in the higher forms it ceases to be perforated in the adult. The trabeculae soon unite together both in front and behind and form a complete plate underneath the fore-brain, and extending into the nasal region 1 . A special

1 In Man (Kolliker) the trabeculce form from the first a continuous plate in front of the pituitary space, and the latter very early acquires a cartilaginous floor.

5/0 THE TKAHECUL/E.

vertical growth of this plate in the region of the orbit forms the interorbital plate of Teleostei, Lacertilia and Aves (fig. 326, ps), on the upper surface of which the front part of the brain rests. The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types two regions may be distinguished, viz. an alisphenoidal region (fig. 326, as) behind, growing out from what is known as the basisphenoidal region of the primitive trabeculae, and an orbitosphenoidal region in front growing out from the presphenoidal region of the trabecula,\ These plates form at first a continuous lateral wall of the cranium. At the front end of the brain they are continued inwards, and more or less completely separate the true cranial cavity from the nasal region in front. The region of the cartilage forming the anterior boundary of the cranial cavity is known as the lateral ethmoid region, and it is always perforated for the passage of the olfactory nerves.

The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity. In the higher types, in Mammals more especially, this roof can hardly be said to be formed at all. The region of the trabeculae in front of the brain is the ethmoid region. The basal part of this region forms an internasal plate, from which an internasal septum may grow up (fig. 326). To its sides the olfactory capsules are attached, and there are usually lateral outgrowths in front forming the trabecular cornua, while from the posterior part of the ethmoidal plate, forming the anterior boundary of the cranial cavity, there often grows out a prefrontal or lateral ethmoidal process.

These and other processes growing out from the trabeculse have occasionally been regarded as rudimentary praeoral branchial arches. I have already stated it as my view that the existence of branchial arches in this region is highly improbable, and I may add that the development of these structures as outgrowths of the skull is in itself to my mind a nearly conclusive argument against their being branchial arches, in that true branchial arches hardly ever or perhaps never arise in this way.

The sense capsules. The most important of these is the auditory capsule, which, as we have seen, fuses intimately with

THE SKULL.

the lateral walls of the skull. In front there is usually a cleft separating it from the alisphenoid region of the skull, through which the third division of the fifth nerve passes out. This cleft becomes narrowed to a small foramen (fig. 327, V). The sclerotic cartilage is always free, but profoundly modifies the region of the cranium near which it is placed. The nasal investment forms in Elasmobranchs (fig. 327, No) a capsule open

FIG. 327. SKULL OF ADULT DOGFISH, SIDE VIEW. (From Parker.) O. C, occipital condyle ; Au. periotic capsule; Pt.O. pterotic ridge ; Sp. 0. sphenotic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage; 77. optic foramen ; V. trigeminal foramen ; PL TV., Qu. pterygo-quadrate arcade ; M.Pt. metapterygoid ligament (including a small cartilage) ; Pl.Tr, ethmo-palatine or palato-trabecular ligament ; Mck. lower jaw ; Sp. spiracle; H.M. hyomandibular; C.Hy, ceratohyal ; m.h.l. mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ; E.Br. epibranchial ; C.br. ceratobranchial ; H.Br. hypobranchial ; B.Br. basibranchial ; Ex.Br. extrabranchial ; l\ 2 , 3 , 4 , 5 . labial cartilages ; the dotted lines within Mck. indicate the basihyal.

below, and continuous with the ethmoid region of the trabeculse. In most types however it becomes more closely united with the ethmoid region and the accessory parts belonging to it.

The cartilaginous cranium, the development of which has been thus briefly traced, persists in the adult without even the addition of membrane bones in the Cyclostomata, Elasmobranchii (fig. 327) and Holocephali. In the Selachioid Ganoids it is also found in the adult, but is covered over by membrane bones. In all other types it is invariably present in the embryo, but becomes in the adult more or less replaced by osseous tissue.

572 THE BRANCHIAL BARS.

Branchial skeleton.

The most primitive type of branchial skeleton in any existing form would appear to be that of the Petromyzonidae, which is developed in a superficial subdermal tissue, and consists of a series of bars united by transverse pieces, so as to form a basketwork. It is known as an extra-branchial system, and an early stage of its development in the Lamprey is shewn in fig. 47. In the higher forms this system is replaced by a series of bars, known as the branchial bars, so situated as to afford support to the successive branchial pouches. Outside these bars there may be present in some primitive forms (Elasmobranchii) cartilaginous elements, which are supposed to be remnants of the extrabranchial system (fig. 327, Ex.Br] ; while a series of membrane bones is also usually added to them, which will be dealt with in a separate section. The branchial bars are developed as simple cartilaginous rods in the deeper parts of the mesoblast which constitutes the primitive branchial arches.

The position of the branchial bars in relation to the somatopleure and splanchnopleure can be determined from their relation to the so-called head cavities. These cavities atrophy before the formation of the cartilaginous branchial bars, but it will be observed (fig. 328), that the artery of each arch (aa) is placed on the inner side of the head cavity (//). The cartilaginous bar arises at a later period on the inner side of the artery, and therefore on the inner side of the section of the body cavity primitively present in the arches.

An anterior arch, known as the mandibular arch, placed in front of the hyo-mandibular cleft, and a second arch, known as the hyoid arch, placed in front of the hyo-branchial cleft, are developed in all types. The succeeding arches are known as the true branchial arches, and are only fully developed in the Ichthyopsida.

In some Sharks (Notidani) seven branchial arches may be present (not including the hyoid and mandibular). In other Ichthyopsida five are usually present, in the embryo at any rate, while in the Amniota there are usually two or three post-hyoid membranous arches, in the interior of which a cartilaginous bar is usually formed. The general form of these bars at an early

THE SKULL.

573

Fir

HORIZONTAL

stage of development is shewn in the dog-fish (Scyllium) in fig. 329.

The simple condition of these bars in the embryo renders it highly probable that forms existed at one time with a simple branchial skeleton of this kind : at the present day however

J SECTION THROUGH THE PEN such forms no longer exist. The first ULTIMATE VISCERAL ARCH arch has in all cases changed its F RUS AN EMHRYO < function and has become converted ^ epiblast; vc. pouch of

into a supporting skeleton for the hypoblast which will form the , , ,11 -1 1 ., i i i. walls of a visceral cleft ; pp.

mouth ; the hyoid arch, though retain- segme nt of body-cavity in vis ing in Some forms its branchial func- ceral arch ;aa. aortic arch.

tion, has in most acquired additional functions and has undergone in consequence various peculiar modifications. The true branchial arches retain their branchial functions in Pisces and some Amphibia, but are secondarily modified and largely aborted in the abranchiate forms. Since the changes undergone

c.a

Bnl

ffm

LrJt

Sn.f

FIG. 329. HEAD OF EMBRYO DOGFISH, n LINES LONG. (From Parker.) TV. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn. mandibular cartilage ; Hy. hyoid arch; Br. i. first branchial arch; Sp. mandilmlohyoid cleft; C/ 1 . hyo-branchial cleft; Lch. groove below the eye; Net. olfactory rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; Hm. hemispheres; f.n.p. nasofrontal process.

by the true branchial bars are far less complicated than those of the hyoid and mandibular bars it will be convenient to treat of them in the first instance.

These bars are, as already mentioned, most numerous in certain very primitive forms (seven in Notidanus), while as we ascend the series there is a gradual tendency for the posterior of them to disappear. This tendency is the result of a gradual atrophy of the posterior branchial pouches, which commenced at

574

THE BRANCHIAL BARS.

a stage in the evolution of the Chordata long prior to the appearance of cartilaginous or osseous branchial bars, and reaches its climax in the Amniota.

In a fully developed branchial bar the primitively simple rod of cartilage becomes divided into a series of segments, usually four, articulated so as to be more or less mobile : and either remaining cartilaginous or becoming partially or wholly ossified. Each bar (fig. 327) forms a somewhat curved structure, embracing the pharynx. The dorsal and somewhat horizontally placed segment is known as the pharyngobranchial (Ph.Br), the next two as the epibranchial (E.Br) and ceratobranchial (C.Br), and the ventral segment as the hypobranchial (H.Br). There is also typically present a basal unpaired segment, uniting the bars of the two sides, known as the basibranchial (B.Br). The arches often bear cartilaginous rays which support the gill lamellae.

In Teleostei dental plates are usually developed as an exoskeletal covering on parts of the branchial arches.

In the Amphibia four or three branchial arches are present in the embryo. These parts are more or less completely retained in the Perennibranchiata and Caducibranchiata, but in the Myctodera and Anura they become largely reduced, and entirely connected with the hyoid.

In the Anura they never reach any considerable development, and are soon reduced to a plate (fig. 330) the coalesced basihyal and basibranchial plate the posterior processes of which represent the remnants of the branchial arches.

According to Parker the posterior process of this plate in the adult is a remnant of the fourth branchial bar ; the next one is the third branchial bar, while the anterior lamina behind the hyoid is stated by him (though this is somewhat doubtful) to be a remnant of the first two bars.

In the Amniota, the branchial arches become still more

Pmx

FIG. 330. YOUNG FROG, WITH TAIL JUST ABSORBED ; SIDE VIEW OF SKULL. (From Parker.)

An. auditory capsule; in front of it is the cranial side wall ; A.N. external nostril ; St. stapes; Mck. Meckelian cartilage; B.Hy. basihyobranchial plate; St.Hy. stylohyal or ceratohyal; Br.i. first branchial arch.

Bones: E-0. exoccipital; Pr.O. prootic ; Pa. parietal ; Fr. frontal ; Na. nasal ; Pmx. premaxillary ; MX. maxillary; Pt. pterygoid; Sq. squamosal; Qn-J't. quadra tojugal; Art. articular; D. dentary.

THE SKULL. 575

degenerated, in correlation with the total disappearance of a branchial respiration at all periods of life. Their remnants become more or less important parts of the hyoid bone, and are solely employed in support of the tongue. Their basal portions are best preserved, forming parts of the body of the hyoid. The posterior (thyroid) cornua of the hyoid are remnants of the true arches. Of these there are two in the Chelonia and Lacertilia, and one in the Aves and Mammalia. In Aves the cornu formed from the first branchial arch (fig. 331, cbr) is always larger than that of the true hyoid arch (cJi).

Mandibular and Hyoid arches. The adaptations of both the mandibular and hyoid bars, to functions entirely distinct from

FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL

OF A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.) cv i. cerebral vesicles ; e. eye ; fn. frontonasal process; n. nasal pit; tr. trabeculre ; pts. pituitary space ; mr. superior maxillary process ; pg. pterygoid ; pa. palatine ; q. quadrate; mk. Meckel's cartilage; ch. cerato-hyal ; bh. basihyal ; cbr. ceratobranchial ; ebr. proximal portion of the cartilage in the third visceral (first branchial) arch; bbr. basibranchial ; i. first visceral cleft; 2. second visceral cleft; 3. third visceral arch.

those which they primitively served, are most remarkable ; and the adaptations of the two bars are in many cases so intimately bound together, that it is not possible to treat them separately.

The most important change of function is undoubtedly that of the mandibular arch, which becomes entirely converted into a skeleton for the jaws. It may be noted as a peculiarity of the

576 MAND1BULAR AND HYOID BARS.

mandibular arch that it is never provided with an unpaired basal element.

The simplest forms of metamorphosis are those undergone by Elasmobranchii, of which the Dog-fish (Scyllium) and Skate (Raja) have been studied (Parker, No. 456). In some of these forms, e.g. the Skate, part of the mandibular bar is still related to the hyo-mandibular cleft (the spiracle).

Elasmobranchii. In Scyllium the hyoid and mandibular arches are at first very similar to those which follow. Soon however each of them sends an anteriorly directed dorsal process (fig. 329). The regions which may be distinguished owing to the growth of these processes have received names from ossifications in them which are found in other types. The anterior process of the mandibular arch is known as the pterygo-quadrate bar (Pl.Pt) ; the dorsal end of the primitive bar from which it starts (M.Pt] is known as the metapterygoid process; while the ventral end of the bar forms the Meckelian cartilage. The upper end of the hyoid arch is known as the hyomandibular.

In a somewhat later stage changes take place which cause these parts practically to assume the adult form (fig. 327). The mandibular arch becomes segmented at its bend into (i) a pterygo-quadrate bar (Pl.Pf) which grows forwards in front of the mouth, and forms an upper jaw, and (2) a Meckelian cartilage (Mck} which is placed behind the mouth, and forms a lower jaw. The two jaws are articulated together, and the cartilages of the two sides composing them meet each other distally.

At the articulation of the Meckelian cartilage with the quadrate part of the pterygo-quadrate is situated a ligament (M.Pf), which takes the place of the metapterygoid process of the previous stage, and passes up on the anterior side of the spiracle, to be attached to the cranium in the front part of the auditory region. This ligament, which is supplemented by a second ligament, the ethmopalatine ligament, passing from the pterygo-quadrate bar to the antorbital region of the skull, is not the most important support of the jaw. The main support is, on the contrary, given by the hyoid arch ; the hyomandibular segment of which (H.M) as well as the adjoining segment (ceratohyoid C.Hy) are firmly attached by ligament to the mandibular

THE SKULL.

577

arch. The hyomandibular is articulated with the cranium beneath the pterotic ridge (Pt.O),

In the type just described, the hyoid and mandibular arches undergo less modification than in almost any other case. The hyoid arch has altered its form, but retains its respiratory function. It has however acquired the secondary function of supporting the mandibular arch. The mandibular arch is divided into two elements, which form respectively the upper and lower jaws. It is not directly articulated with the skull, and its mode of support by the hyoid arch has been called by Huxley (No. 445) hyostylic.

The development of the hyoid and mandibular arches in the Skate is characterised by a few important features (fig. 333). The anterior element of the hyoid

arch, which forms the hyo- \ ^ Sp

mandibular (H.M], becomes entirely separate from the posterior part of the arch, and only serves to support the jaws. The posterior part of the arch (Hy} carries on the respiratory functions of the hyoid, and is closely connected with the first branchial arch. The upper or metapterygoidelementof the mandibular arch (M.Pt} has a considerable development,

FIG. 333. HEAD OF EMBRYO SKATE, i\ IN. LONG. (From Parker.)

Tr. trabecula ; Pl.Pt. pterygo- quadrate bar ; Mn. mandibular bar ; M.Pt. metapterygoid cartilage ; H.M. hyomandibular ; Hy. remainder of hyoid arch ; Br. \. first branchial arch ; Sp. mandibulo-hyoid cleft or spiracle ; Pn. pineal gland ; Au. au ditory vesicle ; C. i, C. 2, and C. 3. vesicles of the brain.

and, becoming separated from the remainder of the arch, forms a mass of cartilage with one or two branchial rays, in the front wall of the spiracle, and constitutes a section of the mandibular arch still retaining traces of its primitive function in supporting the wall of a branchial pouch.

Although the development of other Elasmobranch types is not known, it is necessary to call attention to the mode of support of the mandibular arch in certain forms, notably Notidanus, Hexanchus and Cestracion, where the pterygo-quadrate region of the mandibular arch is directly articulated to the B. in. 37

578 MANDIKULAR AND HYOID BARS.

cranium between the optic and trigeminal foramina. In the two former genera the metapterygoid region of the arch is moreover continuous with the pterygo-quadrate, and articulates with the post-orbital process of the auditory region of the skull. In spite of these attachments the mandibular arch continues to be partially supported by the hyomandibular. The skulls in which the mandibular arch has this double form of support have been called by Huxley amphistylic.

Considering the in many respects primitive characters of the forms with amphistylic skulls it seems not improbable that they

SOr

l } a.ch.

Gtly

FIG. 334. CRANIAL SKELETON OF A SALMON FRY, SECOND WEEK AFTER HATCHING; MEMBRANE BONES, EYEBALLS, AND NASAL SACS REMOVED. (From Parker.)

T.Cr. tegrnen cranii; 'S. Or. supraorbital band; Fo. superior fontanelle; Au. auditory capsule ; Pa.ch. parachordal cartilage; Ch. notochord; 7>. trabecula; above the trabecula, the interorbital septum is seen, passing into the cranial wall above and reaching the supraorbital band; //. optic foramen; V. trigeminal foramen; /', I". labial cartilages ; PI. Ft. palatopterygoid bar ; M. Pt. metapterygoid tract ; Qu. quadrate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy. symplectic tract; I.Hy. interhyal; C.Hy. ceratohyal; II. fly. hypohyal; G.ffy. glossohyal; Br.\. first branchial arch.

preserve the original mode of support of the mandibular arch ; from which differentiations in two directions have taken place, viz. differentiations in the direction of a complete support of the mandibular arch by the hyoid, which is characteristic of most Elasmobranchii and, as will be shewn below, of Ganoidei and Tclcostei ; and differentiations towards a direct articulation or attachment of the mandibular arch to the cranium, without the

THE SKULL. 579

intervention of the hyoid. The latter mode of attachment is called by Huxley autostylic. It is found in Holocephala, Dipnoi, Amphibia and the Amniota.

Teleostei. In addition to that of Elasmobranchii, the skull of the Salmon is the only hyostylic skull in which, by the admirable investigation of Parker (No. 451), the ontogeny of the hyoid and mandibular bars has been satisfactorily worked out. Apart from the presence of a series of membrane bones, the development of these bars agrees on the whole with the types already described.

The hyoid arch, though largely ossified, undergoes a process of development very similar to that in Raja. It is formed as a simple cartilaginous bar, which soon becomes segmented longi ft 1.3 Sp.

FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, AKOUT 2 INCHES LONG;

SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.)

The palato-mandibular and hyoid tracts are detached from their proper situations,

a line indicating the position where the hyomandibular is articulated beneath the

pterotic ridge.

oL olfactory fossa; c.tr. trabecular cornu; /*. /. upper labial cartilages ; p.s.

presphenoid tract ; t.cr. tegmen cranii ; s.o.b. supraorbital band; fo. superior fonta nelle; n.c. notochord; b.o. basilar cartilage; //'. trabecula; p.c. condyle for palatine

cartilage; 5. trigeminal foramen ; fa. facial foramen; 8. foramen for glossopharyngeal

and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle.

Bones: e.o. exoccipital; s.o. supraoccipital; e.p. epiotic; pt.o. pterotic; sp.o.

sphenotic ; op. opisthotic; pro. prootic; I'.s. basisphenoid ; al.s. alisphenoid; o.s.

orbitosphenoid ; I.e. ectethmoid or lateral ethmoid ; pa. palatine ; pg. pterygoid ;

m.pg. mesopterygoid ; mt.pg. metapterygoid ; qu. quadrate; ar. articular; h.m.

hyomandibular; sy. symplectic ; i.h. interhyal ; ep.h. epiceratohyal ; c.h. ceratohyal ;

h.h. hypohyal; g.h. glosso- or basihyal.

372

580 MANDIBULAR AND HYOID BARS.

tudinally into an anterior and a posterior part (fig. 334). The former constitutes the hyomandibular (H.M], while the latter, becoming more and more separated from the hyomandibular, constitutes the hyoid arch proper ; owing to the disappearance of the hyobranchial cleft, it loses its primitive function, and serves on the one hand to support the operculum covering the gills, and on the other to support the tongue. It becomes segmented into a series of parts which are ossified (fig. 335) as the epiceratohyal (ep./t) above, then a large ceratohyal (c/t), followed by a hypohyal (JiJi), while the median ventral element forms the basi- or glossohyal (gJi).

The hyomandibular itself is articulated with the skull below the pterotic process (fig. 334, H.M}. Its upper element ossifies as the hyomandibular (fig. 335, fun.}, while its lower part (fig. 334, Sy), which is firmly connected with the mandibular arch, ossifies as the symplectic (fig. 335, sy). A connecting element between the two parts of the hyoid bar forms an interhyal (i/i).

There are more important differences in the development of the mandibular arch in Elasmobranchii and the Salmon than in that of the hyoid arch, in that, instead of the whole arcade of the upper jaw being formed from the mandibular arch, a fresh element, in the form of an independently developed bar of cartilage, completes the upper arcade in front ; but even with this bar the two halves of the upper branch of the arch do not meet anteriorly, but are separated by the ends of the trabeculae.

The anterior bar of the upper arcade is known as the palatine ; but it appears to me as yet uncertain how far it is to be regarded as an element, primitively belonging to the upper arcade of the mandibular arch, which has become secondarily independent in its development ; or as an entirely distinct structure which has no counterpart in the Elasmobranch upper jaw. The latter view is adopted by Parker and Bridge, and a cartilage attached to the hinder wall of the nasal capsule of many Elasmobranchii is identified by them with the palatine rod of the Teleostei.

The arch itself is at first very similar to the succeeding arches ; its dorsal extremity soon however becomes broadened, and provided with an anteriorly directed process. This part (fig. 334, M.Pt and Qii] is then segmented from the lower region,

THE SKULL. 581

and forms what may be called the pterygo-quadrate cartilage, though not completely homologous with the similarly named cartilage in Elasmobranchs ; while the lower region forms the Meckelian cartilage (Mck], which has already grown inwards, so as to meet its fellow ventrally below the mouth. The whole arch becomes at the same time widely separated from the axial parts of the skull.

Nearly simultaneously with the first differentiation of the mandibular arch, a bar of cartilage the palatine bar already spoken of is formed on each side, below the eye, in front of the mouth. The dilated anterior extremity of this bar soon comes in contact with an anterior process of the trabeculse, known as the ethmopalatine process.

In a later stage the pterygoid end of the pterygo-quadrate cartilage unites with the distal end of the palatine bar (fig. 334, Pl.Pt], and there is then formed a continuous cartilaginous arcade for the upper jaw, which is strikingly similar to the cartilaginous upper jaw of Elasmobranchii.

A large dorsal process of the primitive pterygo-quadrate now forms a large metapterygoid tract (M.Pt] ; while the whole arch becomes firmly bound to the hyomandibular (H.M}.

In the later stages the parts formed in cartilage become ossified (fig. 335). The palatine is first ossified, the pterygoid region of the pterygo-quadrate is next ossified as a dorsal mesopterygoid (m.pg] and a ventral pterygoid proper (pg). The quadrate region, articulating with the Meckelian cartilage, becomes ossified as a distinct quadrate (qu\ while the dorsal region becomes also ossified as a metapterygoid (int.pg).

In the Meckelian cartilage a superficial ossification of the ventral edge and inner surface forms an articulare (ar) ; but the greater part of the cartilage persists through life.

Some of the above ossifications, at any rate those of the palatine and pterygoid, seem to be started by dental osseous plates adjoining the cartilage. They will be spoken of further in the section dealing with the membrane bones.

Amphibia. The development of the autostylic piscine skulls has unfortunately not yet been studied ; and the most primitive autostylic types whose development we are acquainted with are

582 MANDIBULAR AND HYOID BARS.

those of the Amphibia ; on which a large amount of light has been shed by the researches of Huxley and Parker.

The modifications of the hyoid arch are comparatively simple and uniform. It forms a rod of cartilage, which soon articulates in front with the quadrate element of the mandibular arch, and is subsequently attached by ligaments both to the quadrate and to the cranium. In those Amphibia in which external gills and gill clefts are lost, it fuses with the basal element of the hyoid (fig. 330), which, together with the basal portions of the following arches, forms a continuous cartilaginous plate. On the completion of these changes the paired parts of the hyoid arch have the form of two elongated rods, known as the anterior cornua of the hyoid, which attach the basihyal plate to the cranium behind the auditory capsule.

It is still uncertain whether there is any distinct element corresponding to the hyomandibular of fishes.

Parker holds that the columella auris of the Anura is the homologue of the hyomandibular. The columella develops comparatively late and independently of the remainder of the hyoid arch, but the similarity between its relations to the nerves and those of the hyomandibular is put forward by Parker as an argument in favour of his view. The early ligamentous connection between the quadrate and the upper end of the primitive hyoid is however an argument in favour of regarding the upper end of the primitive hyoid as the hyomandibular element, not separated from the remainder of the arch.

The history of the mandibular arch is more complicated than that of the hyoid. The part of it which corresponds with the upper jaw of Elasmobranchii exhibits most striking variations in development ; so striking indeed as to suggest that the secondary modifications it has undergone are sufficiently considerable to render great caution necessary in drawing morphological conclusions from the processes which are in some instances observable. A more satisfactory judgment on this point will be . possible after the publication of a memoir with which Parker is now engaged on the skulls of the different Anura.

The membrane bones applying themselves to the sides of the mandibular arch are relatively far more important than in the lower types. This is especially the case with the upper jaw where the maxillary and premaxillary bones functionally replace the primitive cartilaginous jaw ; while membranous pterygoids

THE SKULL.

583

and palatines apply themselves to, and largely take the place of, the cartilaginous palatine and pterygoid bars.

Two types worked out by Parker, viz. the Axolotl and the common Frog, may be selected to illustrate the development of the mandibular arch.

In the Axolotl, which may be taken as the type for the Urodela, the mandibular arch is constituted at a very early stage of (i) an enlarged dorsal element, corresponding with the pterygo-quadrate of the lower types, but usually known as the quadrate ; and (2) a ventral or Meckelian element. The Meckelian bar very early acquires its investing bones, while the dorsal part of the quadrate becomes divided into two characteristic

FIG. 336. YOUNG AXOLOTL, i\ INCHES LONG ; UNDER VIEW OF SKULL,

DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED.

(From Parker.)

nc. notochord ; oc.c. occipital condyle; f.o. fenestra ovalis; si. stapes; tr. trabecular cartilage; i.n. internal nares; c.tr. cornu trabeculse; pd. pedicle of quadrate; (/. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve.

BonCS I pa.s. parasphenoid ; e.o. exoccipital ; v. vomer; px. premaxillary ; mx. maxillary; pa. palatine; pg. pterygoid.

processes, viz. an anterior dorsal process which grows towards and soon permanently fuses with the trabecular crest, and a posterior process known as the otic process, which applies itself to the outer side of the auditory region. The anterior of these processes, as pointed out by Huxley, is probably homologous with the anterior process of the pterygo-quadrate bar in Notidanus, which articulates with the trabecular region of the cranium, while the otic process is homologous with the meta

584 MANDIBULAR AND HYOID BARS.

pterygoid process. Hardly any trace is present of an anterior process to form a pterygoid bar, but dentigerous plates forming a dermal palato-pterygoid bar have already appeared.

At a somewhat later stage a fresh process, called by Huxley the pedicle, grows out from the quadrate, and articulates with the ventral side of the auditory region (fig. 336, pd). Shortly afterwards a rod of cartilage grows forward from the quadrate under the membranous pterygoid (pg), which corresponds with the cartilaginous pterygoid bar of other types (fig. 336), and an independent palatine bar, arising even before the pterygoid process, is formed immediately dorsal to the dentigerous palatine plate (pa\ and is attached to the trabecula. These two bars eventually meet, but never become firmly united to the more important membrane bones placed superficially to them.

The mandibular arch in the Frog stands, so far as development is concerned, in striking contrast to the mandibular arch of the Axolotl, in spite of the obvious similarity in the arrangement of the adult parts in the two types. FlG . 33? . EMBRYO FROG, JUST BE In the earliest stage it FORE HATCHING ; SIDE VIEW OF HEAD,

WITH SKIN REMOVED. (From Parker.) forms a simple bar in the ,, lf , , - . , .. ,

Na. olfactory sack; E. involution for

membranous mandibular arch, eyeball; Ati. auditory sack; 7>. trabe11 i , .1 cula; Mn. mandibular : Hy. hyoid ; Br.I.

parallel to and very similar to first branchial arch . ' th / gili.buds are

are

the hyoid bar behind (figf 337, seen on the first two branchial arches; /. M \ T u, * u labial cartilages.

Mn). In the next stage ob served, that is to say in Tadpoles of four, five, to six lines long, an astonishing transformation has taken place. The mandibular arch (fig. 338) is turned directly forwards parallel to the trabecula, to which it is attached in front (p.pg) and behind (pd}. The proximal part of the arch thus forms a subocular bar, and the space between it and the trabecula a subocular fenestra. In front of the anterior attachment it is continued forwards for a short distance, and to the free end of this projecting part is articulated a small Meckelian cartilage directed upwards (mk}. The Meckelian cartilage is at this stage placed in front of the nasal sacks, in the lower lip of the suctorial

THE SKULL.

585

mouth. The greater part of the arch, parallel with the trabeculae, is equivalent to what has been called in the Axolotl the

mJr

FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG ; CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE ; THE PARACHORDAL

CARTILAGES ARE NOT YET DEFINITE. (From Parker.)

nc. notochord; ms. muscular segments; au. auditory capsule; py. region of pituitary body; tr. trabecula; c.tr, cornu trabeculae ; p-pg. palatopterygoid bar ; pd. pedicle; q. quadrate condyle; mk. Meckelian piece of mandibular arch; s.o.f. subocular fenestra ; u.l. upper labial cartilage. The dotted circle within the quadrate region indicates the position of the internal nostril.

quadrate, while its anterior attachment to the trabeculae is the rudiment of the palato-pterygoid cartilage. The posterior attachment is known as the pedicle.

The condition of the mandibular arch during this and the next stage (fig. 339) is very perplexing. Its structure appears adapted in some way to support the suctorial mouth of the Tadpole.

Reasons have been offered in a previous part of this volume for supposing that the suctorial mouth of the Tadpole is probably not simply a structure secondarily acquired by this larva, but is an organ inherited from an ancestor provided through life with a suctorial mouth.

The question thus arises, is the peculiar modification of the mandibular arch of the Tadpole an inherited or an acquired feature ?

If the first alternative is accepted we should have to admit that the mandibular arch became first of all modified in connection with the suctorial mouth, before it was converted into the jaws of the Gnathostomata ; and that the peculiar history of this arch in the Tadpole is a more or less true record of its phylogenetic development. In favour of this

586

MANDIBULAR AND HYOID BARS.

view is the striking similarity which Huxley has pointed out between the oral skeleton of the Lamprey and that of the Tadpole ; and certain peculiarities of the mandibular arch of Chimaera and the Dipnoi can perhaps best be explained on the supposition that the oral skeleton of these forms has arisen in a manner somewhat similar to that in the Frog ; though with reference to this point further developmental data are much required.

On the other hand the above suppositions would necessitate our admitting that a great abbreviation has occurred in the development of the mandibular arch of the otherwise more primitive Urodela ; and that the simple mode of growth of the jaws in Elasmobranchii, from the primitive mandibular arch, is phylogenetically a much abbreviated and modified process, instead of being, as usually supposed, a true record of ancestral history.

If the view is accepted that the characters of the mandibular arch of the Tadpole are secondary, it will be necessary to admit that the adaptation of the mandibular arch to the suctorial mouth took place after the suctorial mouth had come to be merely a larval organ.

In view of our imperfect knowledge of the development of most Piscine skulls I would refrain from expressing a decided opinion in favour of either of these alternatives.

or.p

eth

FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL

WITHOUT THE BRANCHIAL ARCHES. (From Parker.)

n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen; ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu trabeculae; ./. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle; ot.fr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; ;;//. Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The upper outline of the head is shewn by dotted lines.

As the tail of the Tadpole gradually disappears, and the metamorphosis into the Frog becomes accomplished, the mandibular arch undergoes important changes (fig. 339): the

THE SKULL.

palato-pterygoid attachment (pa.pg) of the quadrate subocular bar becomes gradually elongated ; and, as it is so, the front end of the subocular bar (su) rotates outwards and backwards, and soon forms a very considerable angle with the trabeculae. The Meckelian cartilage (ink) at its free end becomes at the same time considerably elongated. These processes of growth continue till (fig. 330) the palato-pterygoid bar (Pf) forms a subocular bar, and is considerably longer than the original subocular region of the quadrate ; while the Meckelian cartilage (Mck] has assumed its permanent position on the hinder border of the no longer suctorial mouth, and has grown forwards so as nearly to meet its fellow in the median line.

The metapterygoid region of the quadrate gives rise to a posterior and dorsal process (fig. 339, ot.pr), the end of which is constricted off as the tympanic annulus (fig. 340, a.f) ; while

pmx

FIG. 340. YOUNG FROG, NEAR END OF FIRST SUMMER ; UPPER VIEW OF SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUTWARDS. (From Parker.)

b.o. basioccipital tract; s.o. supraoccipital tract; fo. frontal fontanelle; e.n, external nostril; internal to it, internasal plate; a.t. tympanic annulus.

Bones : e.o. exoccipital; pr.o. prootic, partly overlapped by/, parietal; f. frontal ; eth. rudiment of sphenethmoid ; na. nasal ; pmx. premaxillary ; mx. maxillary; /-. pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; s<j. squamosal; ar. articular; d. dentary; m.mk. mento-Meckelian.

the proximal part of the process remains as the otic (metapterygoid) process, articulating with the auditory cartilage.

The pedicle (pd} retains its original attachment to the skull.

588 MAND1BULAR AND IIYOID BARS.

The palato-pterygoid soon becomes segmented into a transversely placed palatine, and a longitudinally placed pterygoid (fig. 340). With the exception of a few ossifications, which present no features of special interest, the parts of the mandibular arch have now reached their final condition, which is not very different from that in the Axolotl.

Sauropsida. In the Sauropsida the modifications of the hyoid and mandibular arches are fairly uniform.

The lower part of the hyoid arch, including the basihyoid, unites with the remnants of the arches behind to form the hyoid bone, to which it contributes the anterior cornu and anterior part of the body.

The columella is believed by Huxley and Parker to represent, as in the Anura, the independently developed dorsal (hyomandibular) element of the hyoid, together with the stapes with which it has become united 1 .

The membranous mandibular arch gives off in the embryos of all the Sauropsida an obvious bud to form the superior maxillary process, and the formation of this bud appears to represent the growth forwards of the pterygoid process in Elasmobranchii, which is indeed accompanied by the formation of a similar bud ; but the skeletal rod, which appears in the axis of this bud, is as a rule independent of that in the true arch (fig- SS 1 ./^. PS}- The former is the pterygo-palatine bar; the latter the Meckelian and quadrate cartilages.

The pterygo-palatine bar is usually if not always ossified directly, without the intervention of cartilage.

Born has recently shewn that Parker was mistaken in supposing that the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short cartilaginous pterygoid process of the quadrate would seem to be present (Parker, No. 458).

The quadrate and Meckelian cartilages are either from the first separate, or very early become so.

1 The strongest evidence in favour of Huxley's and Parker's view of the nature of the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with the columella (vide Huxley, No. 445). From an examination of a specimen in the Cambridge museum I do not feel satisfied that the fusion is not secondary, but have not been able to examine the junction of the hyoid and columella in section. For a different view to that of Huxley vide Peters, "Ueb. d. Gehorknochelchen u. ihr Verhaltniss zu. Zungenbeinbogen b. Sphenodon." Berlin MoHOtsbtnekU, 1874.

THE SKULL.

589

The quadrate cartilage ossifies as the quadrate bone, and supplies the permanent articulation for the lower jaw. Its upper end exhibits a tendency to divide into two processes, corresponding with the pedicle and otic processes of the Amphibia. The Meckelian cartilage becomes soon covered by investing bones, and its proximal end ossifies as the articulare. The remainder of the cartilage usually disappears.

Mammalia. The most extraordinary metamorphosis of the hyoid and mandibular arches occurs in the Mammalia, and has been in part known since the publication of the memoir of Reichert (No. 461).

Both the hyoid and mandibular arches develop at first more completely than in any of the other types above Fishes; and are

pn.ch nc

FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG ; ELEMENTS OF THE

SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.) pa.ch. parachordal cartilage; nc. notochorcl; au. auditory capsule; py. pituitary body; tr. trabeculse; c.lr. trabecular cornu; pn. prenasal cartilage; e.n. external nasal opening; ol. nasal capsule; p-pg- palatopterygoid tract enclosed in the maxillopalatine process; mn. mandibular arch ; hy. hyoid arch; th.h. first branchial arch; ja. facial nerve; 8a. glossopharyngeal ; 86. vagus; 9. hypoglossal.

articulated to each other above, while the pterygo-palatine bar is quite distinct. The main features of the subsequent development are undisputed, with the exception of that of the upper end of the hyoid, which is still controverted. The following is Parker's (No. 452) account for the Pig, which confirms in the main the view originally put forward by Huxley (No. 445).

The mandibular and hyoid arches are at first very similar

5QO MANDIBULAR AND HYOID BARS.

(fig. 341 mn and hy), their dorsal ends being somewhat incurved, and articulating together.

In a somewhat later stage (fig. 342) the upper end of the mandibular bar (mb\ without becoming segmented from the ventral part, becomes distinctly swollen, and clearly corresponds to the quadrate region of other types. The ventral part of the bar constitutes the Meckelian cartilage (mk).

The hyoid arch has in the meantime become segmented into two parts, an upper part (z), which eventually becomes one of

FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DISPLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.)

tg. tongue; ink. Meckelian cartilage; ml. body of malleus; mb. manubrium or handle of the malleus; t.ty. tegmen tympani; i. incus; st. stapes; i.hy, interhyal ligament; st.h. stylohyal cartilage; h.h. hypohyal ; ^.//.basibranchial; th.h. rudiment of first branchial arch; -ja. facial nerve.

the small bones of the ear the incus and a lower part which remains permanently as the anterior cornu of the hyoid (st./i). The two parts continue to be connected by a ligament.

The incus is articulated with the quadrate end of the mandibular arch, and its rounded head comes in contact with the stapes (fig. 342, st) which is segmented from the fenestra ovalis. The main arch of the hyoid becomes divided into a hypohyal (h.h) below and a stylohyal (st. h] above, and also becomes articulated with the basal element of the arch behind (b/i).

In the course of further development the Meckelian part of the mandibular arch becomes enveloped in a superficial ossification forming the dentary. Its upper end, adjoining the quadrate region, becomes calcified and then absorbed, and its lower, with the exception of the extreme point, is ossified and subsequently incorporated in the dentary.

The quadrate region remains relatively stationary in growth

TIIK SKULL. 591

as compared with the adjacent parts of the skull, and finally ossifies to form the malleus bone of the ear. The processus gracilis of the malleus is the primitive continuation into Meckel's cartilage.

The malleus and incus are at first embedded in the connective tissue adjoining the tympanic cavity (hyomandibular cleft, vide p. 528) ; and externally to them a bone known as the tympanic bone becomes developed so that they become placed between the tympanic bone and the periotic capsule. In late fcetal life they become transported completely within the tympanic cavity, though covered by a reflection of the tympanic mucous membrane.

The dorsal end of the part of the hyoid separated from the incus becomes ossified as the tympano-hyal, and is anchylosed with the adjacent parts of the periotic capsule. The middle part of the bar just outside the skull forms the stylo-hyal (styloid process in Man) which is attached by ligament to the anterior cornu of the hyoid (cerato-hyal).

While the account of the formation of the malleus, incus, and stapes just given is that usually accepted in this country, a somewhat different view of the development of these parts has as a rule been adopted in Germany. Reichert (No. 461) held that both the malleus and the incus were derived from the mandibular bar ; and this view has been confirmed by Giinther, Kolliker and other observers, and has recently been adopted by Salensky (No. 462) after a careful research especially directed towards this point. Reichert also held that the stapes was derived from the hyoid bar ; but, though his observations on this point have been very widely accepted, they have not met with such universal recognition as his views on the origin of the malleus and incus. Salensky has recently arrived at a view, which is in accord with that of Parker, in so far as the independence of the stapes of both the hyoid and mandibular arches is concerned. Salensky however holds that it is formed from a mass of mesoblast surrounding the artery of the mandibular arch, and that the form of the stapes is due to its perforation by the mandibular artery. A product of this artery permanently perforates the stapes in a few Mammalia, though in the majority it atrophies.

In view of the different accounts of the origin of the incus the exact nature of this bone must still be considered as an open question, but should Reichert's view be confirmed the identification of the incus with the columella of the Amphibia and Sauropsida must be abandoned.

592 MEMBRANE BONES.

Membrane bones and ossifications of the cranium.

The membrane bones of the skull may be divided into two classes, viz. (i) those derived from dermal osseous plates, which as explained above (p. 542) are primitively formed by the coalescence of the osseous plates of scales ; and (2) those formed by the coalescence of the osseous plates of teeth lining the oral cavity. Some of the bones sheathing the edge of the mouth have been formed partly by the one process and partly by the other.

In the Fishes there are found all grades of transition between simple dermal scutes, and true subdermal osseous plates forming an integral part of the internal skeleton. Dermal scutes are best represented in Acipenser and some Siluroid Fishes.

Where the membrane bones still retain the character of dermal plates, those on the dorsal surface of the cranium are usually arranged in a series of longitudinal rows, continuing in the region of the head the rows of dermal scutes of the trunk ; while the remaining cranial scutes are connected with the visceral arches. The dermal bones on the dorsal surface of the head are very different in number, size, and arrangement in different types of Fishes ; but owing to their linear disposition it is usually possible to find a certain number both of the paired and unpaired bones which have a similar situation in the different forms. These usually receive the same names, but both from general considerations as to their origin, as well as from a comparison of different species, it appears to me probable that there is no real homology between these bones in different species, but only a kind of general correspondence 1 .

It is not in fact till we get to the types above the Fishes that we can find a series of homologous dorsal membrane bones covering the roof of the skull. In these types three paired sets of such bones are usually present, viz, from behind forwards the parietals, frontals and nasals, the latter bounding the posterior surface of the external nasal opening. Even in the higher

1 For some interesting remarks on the arrangement of these bones in Fishes, vide Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878.

THE SKULL. 593

types these bones are liable to vary very greatly from the usual arrangement.

Besides these bones there is usually present in the higher forms a lacrymal bone on the anterior margin of the orbit derived from one of a series of periorbital membrane bones frequently found in Fishes. Various supraorbital and postorbital bones, etc. are also frequently found in Lacertilia, etc. which are not impossibly phylogenetically independent of the membrane bones inherited from Fishes; and may have been evolved as bony scutes in the subdermal tissue of the papillae of the sauropsidan scales.

The visceral arches of Fishes, especially of the Teleostei, are usually provided with a series of membrane bones. In the true branchial arches these take the form of dentigerous plates ; but no such plates are found in the Amphibia or Amniota.

The opercular flap attached to the hyoid arch is usually supported by a series of membrane bones, which attain their highest development in the Teleostei. One of these bones, the praeopercular, is very constant and is primitively attached along the outer edge of the hyomandibular. It seems to be retained in Amphibia as a membrane bone, overlapping the attachment of the quadrate and known as the squamosal ; though it is not impossible that this bone may be derived from a superficial membrane bone, widely distributed in Teleostei and Ganoids, which is known as the supra-temporal. In Dipnoi the bone which appears to be clearly homologous with the squamosal would seem from its position to belong to the series of dorsal plates, and therefore to be the supra-temporal ; but it is regarded by Huxley (No. 446) as the praeopercular 1 .

In the Amniota the squamosal forms an integral part, of the osseous roof of the skull ; but in the Sauropsida it continues, as in Amphibia, to be closely related to the quadrate.

A larger series of persistent membrane bones are related to the mandibular, and its palato-quadrate process.

Overlying the palato-quadrate process are two rows of bones,

1 It is not impossible that the solution of the difficulty about the praeopercular is to be found by supposing that the praeopercular as it exists in Teleostei is derived from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than in Teleostei its primitive position.

B. III. 3 8

594 MEMBRANE BONES.

one row lying at the edge of the mouth, on the outer side of the pterygo-palatine process, and the other set on the roof of the mouth superficial to the pterygo-palatine process.

The outer row is formed of the praemaxilla, maxilla, jugal, and very often quadrato-jugal. Of these bones the maxilla and prsemaxilla, as is more especially demonstrated by their ontogeny in the Urodela, are partly derived from dentigerous plates and partly from membrane plates outside the mouth; while the jugal, and quadrato-jugal when present, are entirely extra-oral. In the Amphibia and Amniota the praemaxillae and maxillae are the most important bones in the facial region, and are quite independent of any cartilaginous substratum.

The second row of bones is clearly constituted in the Dipnoi and Amphibia by the vomer in front, then the palatine, and finally the pterygoid behind. Of these bones the vomer is never related to a cartilaginous tract below, while the palatines and pterygoids usually are so. The position and growth of the three bones in many Urodela (Axolotl) are especially striking (Hertwig. No. 442). In the Axolotl they form a continuous series, the vomer and palatine being covered by teeth, but the pterygoid being without teeth. The vomer and palatine originate from the united osseous plates of the bases of the teeth, while the pterygoid is in the first instance continuous with the palatine.

In Teleostei, Amia, etc., there are dentigerous plates forming a palatine and pterygoid, which in position, at any rate, closely correspond with the similarly named bones in Amphibia ; and there is also a dentigerous vomer which may fairly be considered as equivalent to that in Amphibia.

In the Amniota the three bones found in Amphibia are always present, but with a few exceptions amongst the Lacertilia and Ophidia, are no longer dentigerous. The cartilaginous bars, which in the lower types are placed below the palatine and pterygoid membrane bones, are usually imperfectly or not at all developed.

On Meckel's cartilage important membrane bones are almost always grafted. On the outside and distal part of the cartilage a dentary is usually developed, which may envelope and replace the cartilage to a larger or smaller extent. Its oral edge

THE SKULL. 595

is usually dentigerous. The splenial membrane bone is the most important bone on the inner side of Meckel's cartilage, but other elements known as the coronoid and angular may also be added. In Mammalia the dentary is the only element present (vide p. 590).

On the roof of the mouth a median bone, the parasphenoid, is very widely present in the Amphibia and Fishes, except the Elasmobranchii and Cyclostomata, and has no doubt the same phylogenetic origin as the vomer and membranous palatines and pterygoids.

It is less important in the Sauropsida, and becomes indistinguishably fused with the sphenoid in the adult, while in Mammalia it is no longer found.

Ossification of the Cartilaginous Cranium. In certain Fishes the cartilaginous cranium remains quite unossified, while completely enveloped in dermal bones. Such for instance is its condition in the Selachioid Ganoids. In most instances, however, the investment of the cartilaginous cranium by membrane bones is accompanied by a more or less complete ossification of the cartilage itself.

In the Dipnoi this occurs to the smallest extent, the only ossifications occurring in the lateral parts of the occipital region, and forming the exoccipitals.

In Teleostei and bony Ganoids, a considerably greater number of ossifications occur in the cartilage.

In the region of the occipital cartilaginous ring there appears a basioccipital and supraoccipital and two exoccipitals. The basioccipital is the only bone on the floor of the skull ossifying that part into which the notochord is primitively continued 1 .

In the region of the periotic cartilage a large number of bones may appear. In front there is the prootic, which often meets the exoccipital behind ; behind there is above and in close connection with the supraoccipital the epiotic, and below in close connection with the exoccipital the opisthotic. On the dorsal side of the cartilage there is a projecting ridge composed mainly of a bone known as the pterotic, sometimes erroneously

1 The notochord appears also to enter into the posterior part of the region which ossifies as the basisphenoid.

383

59 6 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM.

called the squamosal, and continued in front by the sphenotic. The pterotic, or the cartilaginous region corresponding to it, always supplies the articular surface for the hyomandibular.

In the floor of the skull, in the region of the pituitary body, there is formed a basisphenoid; while in the lateral parts of the wall of this part of the cranium, there is a bone known as the alisphenoid.

In front, parts of the lateral walls of the cranium ossify as the orbitosphenoids.

In view of the very imperfect ossification of the cartilaginous cranium of the Dipnoi, and of the fact that there is certainly no direct genetic connection between the Teleostei on the one hand, and the Amphibia and Amniota on the other, it is very difficult to believe that most of the ossifications of the cranium in the Amphibia and Amniota have more than a general correspondence with those in the Teleostei.

In the Amphibia the ossifications in the cartilage are comparatively few. In the occipital region there is a lateral ossification on each side of the exoccipital. the basioccipital region being unossified, and the supraoccipital at the utmost indurated by a calcareous deposit.

The periotic capsule is ossified by a prootic centre, which meets the exoccipital behind.

The front part of the cartilaginous cranium is ossified by a complete ring of bone the sphenethmoid bone which embraces part of the ethmoid region, and of the orbitosphenoid and presphenoid regions.

In the Amphibia the cartilaginous cranium, with its centres of ossification, is easily separable from the membranous investing bones.

In the Amniota the cartilaginous cranium, whose development in the embryo has already been described, becomes in the adult much more largely ossified, and the bones which replace the primitive cartilage unite with the membrane bones to form a continuous bony cranium.

The centres of ossification become again much more numerous. In the occipital segment analogous centres to those of Teleostei are again found ; and it is probable that the exoccipitals are homologous throughout the series, the supraoccipital and basioc

THE SKULL. 597

cipital bones of the higher types being merely identical in position with the similarly named bones in Fishes.

In the periotic there are usually three centres of ossification, first recognised by Huxley. These are the prootic, the epiotic and opisthotic, the situations of which have already been defined. Of these the prootic is the most constant.

In Reptiles, the prootic and opisthotic frequently remain distinct even in the adult.

In Birds, the epiotic and opisthotic are early united with the supra- and exoccipital ; and at a later period the prootic is also indistinguishably fused with the adjacent parts.

In Mammals the three ossifications fuse into a continuous whole the periotic bone which may be partially united with the adjacent parts.

In the pituitary region of the base of the cranium a pair of osseous centres or in the higher types a single centre (Parker 1 ) gives rise to the basisphenoid bone, and in front of this another basal or pair of basal ossifications forms the presphenoid, while laterally to these two centres there are formed centres of ossification in the alisphenoid and orbitosphenoid regions, which may be extremely reduced in various Sauropsida, leaving the side walls of the skull almost entirely formed of membrane or cartilage.

In the ethmoid region there may arise a median ossification forming the mesethmoid and lateral ossifications forming the lateral ethmoids or prefrontals ; which may assist in forming the front wall of the brain-case, or be situated quite externally to the brain-case and be only related to the olfactory capsules.

The labial cartilages. In most Fishes a series of skeletal structures, known as the labial cartilages, are developed at the front and sides of the mouth, and in connection with the olfactory capsules ; and these cartilages still persist in connection with the olfactory capsules, though in a reduced form, in the higher types. They are more developed in the Cyclostomata than in any other Vertebrate type.

The meaning of these cartilages is very obscure ; but, from their being in part employed to support the lips and horny teeth of the Cyclostomata and the Tadpole, I should be inclined to regard them as remnants of a primitive skeleton supporting the suctorial mouth, with which, on the grounds already stated (p. 317), I believe the ancestors of the present Vertebrata to have been provided.

1 According to Kblliker there are two centres in Man in both the basisphenoid and presphenoid.

598 BIBLIOGRAPHY.

BIBLIOGRAPHY.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a leur differents ages." Paris, Mem. savans etrang. 1835, and An. Set. A 7 af. Vol. I. 1834.

(440) C. Gegenbaur. Untersuchwigen z. vergleich. Anat. d. Wirbelthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehororgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr. Anat., Vol. xi. 1874, suppl.

(443) T.H.Huxley. " On the theory of the vertebrate skull." Proc. Royal Soc., Vol. ix. 1858.

(444) T. H. Huxley. The Elements of Comparative Anatomy. London, 1869.

(445) T.H.Huxley. "On the Malleus and Incus." Proc. Zool. Soc., 1869.

(446) T.H.Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

(447) T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon." Journ. of Anat. and Phys., Vol. X. 1876.

(448) T.H.Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the Common Fowl (Callus Domesticus)." Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the Pig (Sus scrofa). " Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the Batrachia." Part n. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the Common Snake (Tropidonotus natrix)." Phil. Trans., 1878.

(456) W. K. Parker. " On the structure and development of the skull in Sharks and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(457) W. K. Parker. "On the structure and development of the skull in the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879.

(458) W. K. Parker. "The development of the Green Turtle." The Zoology of the Voyage of H. M.S. Challenger. Vol. I. pt. V.

(459) W. K. Parker. "The structure and development of the skull in the Batrachia." Pt. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Belt any. The Morphology of the Skull. London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Miiller's Archiv, 1837.

(462) W. Saleusky. "Beitragez. Entwick. d. knorpeligen Gehorknochelchen." Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull; Gotte (No. 296).

CHAPTER XX.

THE PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS.

TJie Pectoral girdle.

Pisces. Amongst Fishes the pectoral girdle presents itself in its simplest form in Elasmobranchii, where it consists of a bent band of cartilage on each side of the body, of somewhat variable form, meeting and generally uniting with its fellow ventrally. Its anterior border is in close proximity with the last visceral arch, and a transverse ridge on its outer and posterior border, forming the articular surface for the skeleton of the limb, divides it into a dorsal part, which may be called the scapula, and a ventral part which may be called the coracoid.

In all the remaining groups of Fishes there is added to the cartilaginous band, which may wholly or partially ossify, an osseous support composed of a series of membrane bones.

In the types with such membrane bones the cartilaginous parts do not continue to meet ventrally, except in the Dipnoi where there is a ventral piece of cartilage, distinct from that bearing the articulation of the limb. The cartilage is moreover produced into two ventral processes, an anterior and a posterior, below the articulation of the limb ; which may be called, in accordance with Gegenbaur's nomenclature, the praecoracoid and coracoid. Of these the praecoracoid is far the most

600 THE PECTORAL GIRDLE.

prominent, and in the majority of cases the coracoid can hardly be recognised. The coracoid process is however well developed in the Selachioid Ganoids, and the Siluroid Teleostei. In Teleostei the scapular region often ossifies in two parts, the smaller of which is named by Parker praecoracoid, though it is quite distinct from Gegenbaur's praecoracoid. The membrane bones, as they present themselves in their most primitive state in Acipenser and the Siluroids, are dermal scutes embracing the anterior edge of the cartilaginous girdle. In Acipenser there are three scutes on each side. A dorsal scute known as the supra-clavicle, connected above with the skull by the posttemporal ; a middle piece or clavicle, and a ventral or infraclavicle (inter-clavicle), which meets its fellow below.

In most Fishes the primitive dermal scutes have become subdermal membrane bones, and the infra-clavicle is usually not distinct, but the two clavicles form the most important part of the membranous elements of the girdle. Additional membrane bones (post-clavicles) are often present behind the main row.

The development of these parts in Fishes has been but little studied.

In Scyllium, amongst the Elasmobranchii, I find that each half of the pectoral girdle develops as a vertical bar of cartilage at the front border of the rudimentary fin, and externally to the muscle-plates.

Before the tissue forming the pectoral girdle has acquired the character of true cartilage, the bars of the two sides meet ventrally by a differentiation in situ of the mesoblastic cells, so that, when the girdle is converted into cartilage, it forms an undivided arc, girthing the ventral side of the body. There is developed in continuity with the posterior border of this arc on the level of the fin a horizontal bar of cartilage, which is continued backwards along the insertion of the fin, and, as will be shewn in the sequel, becomes the metapterygium of the adult (figs. 344, bp and 348, mp). With this bar the remaining skeletal elements of the fin are also continuous.

The foramina of the pectoral girdle are not in the first instance formed by absorption, but by the non-development of the cartilage in the region of pre-existing nerves and vessels.

THE PECTORAL GIRDLE. 6oi

The development of these parts in Teleostei has been recently investigated by 'Swirski (No. 472) who finds in the Pike (Esox) that the cartilaginous pectoral girdle is at first continuous with the skeleton of the fin. It forms a rod with a dorsal scapular and ventral coracoid process. An independent mass of cartilage gives rise to a prascoracoid, which unites with the main mass, forming a triradiate bar like that of Acipenser or the Siluroids. The coracoid process becomes in the course of development gradually reduced.

'Swirski concludes that the so-called praecoracoid bar is to some extent a secondary element, and that the coracoid bar corresponds to the whole of the ventral part of the girdle of Elasmobranchii, but his investigations do not appear to me to be as complete as is desirable.

Amphibia and Amniota. The pectoral girdle contains a more or less constant series of elements throughout the Amphibia and Amniota ; and the differences in structure between the shoulder girdle of these groups and that of Fishes are so great that it is only possible to make certain general statements respecting the homologies of the parts in the two sets of types.

The generally accepted view, founded on the researches of Parker, Huxley, and Gegenbaur, is to the effect that there is a primitively cartilaginous coraco-scapular plate, homologous with that in Fishes, and that the membrane bones in Fishes are represented by the clavicle and inter-clavicle in the Sauropsida and Mammalia, which are however usually admitted to be absent in Amphibia. These views have recently been challenged by Gotte (No. 466) and Hoffmann (No. 467), on the ground of a series of careful embryological observations ; and until the whole subject has been worked over by other observers it does not seem possible to decide satisfactorily between the conflicting views. It is on all hands admitted that the scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate becomes the scapula, which may subsequently become divided into a supra-scapula and scapula proper ; while the ventral half forms the coracoid, which is not always separated from the scapula, and is usually divided into a coracoid proper, a praecoracoid, and an epicoracoid. By the conversion of parts of the primitive cartilaginous plates into membranous tissue various fenestrae may be formed in the cartilage, and the bars

602 THE NATURE OF THE CLAVICLE.

bounding these fenestrae both in the scapula and coracoid regions have received special names ; the anterior bar of the coracoid region, forming the praecoracoid, being especially important. At the boundary between the scapula and the coracoid, on the hinder border of the plate, is placed the glenoid articular cavity to carry the head of the humerus.

The grounds of difference between Gotte and Hoffmann and other anatomists concern especially the clavicle and inter-clavicle. The clavicle is usually regarded as a membrane bone which may become to some extent cartilaginous. By. the above anatomists, and by Rathke also, it is held to be at first united with the coraco-scapular plate, of which it forms the anterior limb, free ventrally, but united dorsally with the main part of the plate ; and Gotte and Hoffmann hold that it is essentially a cartilage bone, which however in the majority of the Reptilia ossifies directly without passing through the condition of cartilage.

The interclavicle (episternum) is held by Gotte to be developed from a paired formation at the free ventral ends of the clavicles, but he holds views which are in many respects original as to its homologies in Mammalia and Amphibia. Even if Gotte's facts are admitted, it does not appear to me necessarily to follow that his deductions are correct. The most important of these is to the effect that the dermal clavicle of Pisces has no homologue in the higher types. Granting that the clavicle in these groups is in its first stage continuous with the coracoscapular plate, and that it may become in some forms cartilaginous before ossifying, yet it seems to me all the same quite possible that it is genetically derived from the clavicle of Pisces, but that it has to a great extent lost even in development its primitive characters, though these characters are still partially indicated in the fact that it usually ossifies very early and partially at least as a membrane bone 1 .

In treating the development of the pectoral girdle systematically it will be convenient to begin with the Amniota, which may be considered to fix the nomenclature of the elements of the shoulder girdle.

1 The fact of the clavicle going out of its way, so to speak, to become cartilaginous before being ossified, may perhaps be explained by supposing that its close connection with the other parts of the shoulder girdle has caused, by a kind of infection, a change in its histological characters.

II IK PECTORAL GIRDLE.

603

Lacertilia. The shoulder girdle is formed as two membranous plates, from the dorsal part of the anterior border of each of which a bar projects (Rathke, Gotte), which is free at its ventral end. This bar, which is usually (Gegenbaur, Parker) held to be independent of the remaining part of the shoulder girdle, gives rise to the clavicle and interclavicle. The scapulocoracoid plate soon becomes cartilaginous, while at the same time the clavicular bar ossifies directly from the membranous state. The ventral ends of the two clavicular bars enlarge to form two longitudinally placed plates, which unite together and ossify as the interclavicle.

Parker gives a very different account of the interclavicle in Anguis. He states that it is formed of two pairs of bones 'strapped on to the antero-inferior part of the prassternum,' which subsequently unite into one.

Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a triradiate cartilage on each side, with one dorsal and two ventral limbs. It is admitted on all hands that the dorsal limb is the scapular element, and the posterior ventral limb the coracoid ; but, while the anterior ventral limb is usually held to be the praecoracoid, Gotte and Hoffmann maintain that, in spite of its being formed of cartilage, it is homologous with the anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the homologue of the clavicle.

Parker and Huxley (doubtfully) hold that the three anterior elements of the ventral plastron (entoplastron and epiplastra) are homologous with the interclavicle and clavicles, but considering that these plates appear to belong to a secondary system of dermal ossifications peculiar to the Chelonia, this homology does not appear to me probable.

Aves. There are very great differences of view as to the development of the pectoral arch of Aves.

About the presence in typical forms of the coraco-scapular plate and two independent clavicular bars all authors are agreed. With reference to the clavicle and interclavicle Parker (No. 468) finds that the scapular end of the clavicle attaches itself to and ossifies a mass of cartilage, which he regards as the mesoscapula, while the interclavicle is formed of a mass of tissue between the ends of the clavicles where they meet ventrally, which becomes the dilated plate at their junction.

Gegenbaur holds that the two primitive clavicular bars are simply clavicles, without any element of the scapula ; and states that the clavicles are not entirely ossified from membrane, but that a delicate band of cartilage precedes the osseous bars. He finds no interclavicle.

Gotte and Rathke both state that the clavicle is at first continuous with the coraco-scapular plate, but becomes early separated, and ossifies entirely as a membrane bone. Gotte further states that the interclavicles are formed as outgrowths of the median ends of the clavicles, which extend themselves at an early period of development along the inner edges of the two halves of the sternum. They soon separate from the clavicles, which subsequently meet to form the furculum ; while the interclavicular rudiments give rise, on the junction of the two halves of the sternum, to its keel, and to the ligament

604 THK PECTORAL GIRDLE.

connecting the furculum with the sternum. The observations of Gotte, which tend to shew the keel of the sternum is really an interclavicle, appear to me of great importance.

A prascoracoid, partially separated from the coracoid by a space, is present in Struthio. It is formed by a fenestration of a primitively continuous cartilaginous coracoid plate (Hoffmann). In Dromaeus and Casuarius clavicles are present (fused with the scapula in the adult Dromaeus), though absent in other Ratitae (Parker, etc.).

Mammalia. The coracoid element of the coraco-scapular plate is much reduced in Mammalia, forming at most a simple process (except in the Ornithodelphia) which ossifies however separately 1 .

With reference to the clavicles the same divergencies of opinion met with in other types are found here also.

The clavicle is stated by Rathke to be at first continuous with the coracoscapular plate. It is however soon separated, and ossifies very early, in the human embryo before any other bone. Gegenbaur however shewed that the human clavicle is provided with a central axis of cartilage, and this observation has been confirmed by Kolliker, and extended to other Mammalia by Gotte. The mode of ossification is nevertheless in many respects intermediate between that of a true cartilage bone and a membrane bone. The ends of the clavicles remain for some time, or even permanently, cartilaginous, and have been interpreted by Parker, it appears to me on hardly sufficient grounds, as parts of the mesoscapula and praecoracoid. Parker's so-called mesoscapula may ossify separately. The homologies of the episternum are much disputed. Gotte, who has worked out the development of the parts more fully than any other anatomist, finds that paired interclavicular elements grow out backwards from the ventral ends of the clavicles, and uniting together form a somewhat T-shaped interclavicle overlying the front end of the sternum. This condition is permanent in the Ornithodelphia, except that the anterior part of the sternum undergoes atrophy. But in the higher forms the interclavicle becomes almost at once divided into three parts, of which the two lateral remain distinct, while the median element fuses with the subjacent part of the sternum and constitutes with it the presternum (manubrium sterni). If Gotte' s facts are to be trusted, and they have been to a large extent confirmed by Hoffmann, his homologies appear to be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds that Gotte is mistaken as to the origin of the presternum.

Gegenbaur admits the lateral elements as parts of the interclavicle, while Parker holds that they are not parts of an interclavicle but are homologous with the omosternum of the Frog, which is however held by Gotte to be a true interclavicle.

1 This process, known as the coracoid process, is held by Sabatier to be the pnecoracoid ; while this author also holds that the upper third of the glenoid cavity, which ossifies by a special nucleus, is the true coracoid. The absence of a praecoracoid in the Ornithodelphia is to my mind a serious difficulty in the way of Sabatier's view.

THE PECTORAL GIRDLE. 605

Amphibia. In Amphibia the two halves of the shoulder girdle are each formed as a continuous plate, the ventral or coracoid part of which is forked, and is composed of a larger posterior and a smaller anterior bar-like process, united dorsally. In the Urodela the two remain permanently free at their ventral ends, but in the Anura they become united, and the space between them then forms a fenestra. The anterior process is usually (Gegenbaur, Parker) regarded as the praecoracoid, but Gotte has pointed out that in its mode of development it strongly resembles the clavicle of the higher forms, and behaves quite differently to the so-called praecoracoid of Lizards. It is however to be noticed that it differs from the clavicle in the fact that it is never segmented off from the coraco-scapular plate, a condition which has its only parallel in the equally doubtful case of the Chelonia. Parker holds that there is no clavicle present in the Amphibia, while Gegenbaur maintains that an ossification which appears in many of the Anura (though not in the Urodela) in the perichondrium on the anterior border of the cartilaginous bar above mentioned is the representative of the clavicle. Gotte's observations on the ossification of this bone throw doubt upon this view of Gegenbaur ; while the fact that the cartilaginous bar may be completely enclosed by the bone in question renders Gegenbaur's view, that there is present both a clavicle and prsecoracoid, highly improbable.

No interclavicle is present in Urodela, but in this group and in a number of the Anura, a process grows out from the end of each of the bars (praecoracoids) which Gotte holds to be the clavicles. The two processes unite in the median line, and give rise in front to the anterior unpaired element of the shoulder girdle (omosternum of Parker). They sometimes overlap the epicoracoids behind, and fusing with them bind them together in the median line. Parker who has described the paired origin of the so-called omosternum, holds that it is not homologous with the interclavicle, but compares it with his omosternum in Mammals.

BIBLIOGRAPHY.

(463) Bruch. " Ueber die Entwicklung der Clavicula und die Farbe des Blutes. " Zeit.f. wiss. Zool., \\. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a leurs differens ages." Memoires des savants etrang. Academic royale des sciences de Finstitut de France^ Vol. vi. 1835.

(465) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wirbelthiere, 2 Heft. Schultergiirtel der Wirbelthiere. Bmstflosse der Fische. Leipzig, 1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel." Archivf. mikr, Anat. Vol. xiv. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederlandisches Archivf. ZooL,Vol.v. 1879.

(468) W. K. Parker. "A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.

606 PELVIC GIRDLE.

(469) H. Rathke. Ueber die Entwicklung der Schildkrbten. Braunschweig, 1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der Saurier, 1853.

(471) A. Sabatier. Comparaison des ceinfures et des membres antMeurs et posttrtturs d. la Serie d. Vertttrh. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. iib. d. Entwick. d. Schultergiirtels n. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

Pelvic girdle.

Pisces. The pelvic girdle of Fishes is formed of a cartilaginous band, to the outer and posterior side of which the basal element of the pelvic fin is usually articulated. This articulation divides it into a dorsal iliac, and ventral pubic section. The iliac section never articulates with the vertebral column.

In Elasmobranchii the two girdles unite ventrally, but the iliac section is only slightly developed. In Chimaera there is a well developed iliac process, but the pubic parts of the girdle are only united by connective tissue.

In the cartilaginous Ganoids the pelvic girdle is hardly to be separated from the skeleton of the fin. It is not united with its fellow, and is represented by a plate with slightly developed pubic and iliac processes.

In the Dipnoi there is a simple median cartilage, articulated with the limb, but not provided with an iliac process. In bony Ganoids and Teleostei there is on each side a bone meeting its fellow in the ventral line, which is usually held to be the rudiment of the pelvic girdle ; while Davidoff attempts to shew that it is the basal element of the fin, and that, except in Polypterus, a true pelvic girdle is absent in these types.

From my own observations I find that the mode of development of the pelvic girdle in Scyllium is very similar to that of the pectoral girdle. There is a bar on each side, continuous on its posterior border with the basal element of the fin (figs. 345 and 347). This bar meets and unites with its fellow ventrally before becoming converted into true cartilage, and though the iliac process (il) is never very considerable, yet it is better developed in the embryo than in the adult, and is at first directed nearly horizontally forwards.

Amphibia and Amniota. The primitive cartilaginous pelvic

PELVIC GIRDLE. 607

girdle of the higher types exhibits the same division as that of Pisces into a dorsal and a ventral section, which meet to form the articular cavity for the femur, known as the acetabulum. The dorsal section is always single, and is attached by means of rudimentary ribs to the sacral region of the vertebral column, and sometimes to vertebrae of the adjoining lumbar or caudal regions. It always ossifies as the ilium.

The ventral section is usually formed of two more or less separated parts, an anterior which ossifies as the pubis, and a posterior which ossifies as the ischium. The space between them is known as the obturator foramen. In the Amphibia the two parts are not separated, and resemble in this respect the pelvic girdle of Fishes. They generally meet the corresponding elements of the opposite side ventrally, and form a symphysis with them. The symphysis pubis, and symphysis ischii may be continuous (Mammalia, Amphibia).

The observations on the development of the pelvic girdle in the Amphibia and Amniota are nearly as scanty as on those of Fishes.

Amphibia. In the Amphibia (Bunge, No. 473) the two halves of the pelvic girdle are formed as independent masses of cartilage, which subsequently unite in the ventral line.

In the Urodelous Amphibia (Triton) each mass is a simple plate of cartilage divided into a dorsal and ventral section by the acetabulum. The ventral parts, which are not divided into two regions, unite in a symphysis comparatively late.

The dorsal section ossifies as the ilium. The ventral usually contains a single ossification in its posterior part which forms the ischium ; while the anterior part, which may be considered as representing the pubis, usually remains cartilaginous ; though Huxley (No. 475) states that it has a separate centre of ossification in Salamander, which however does not appear to be always present (Bunge). There is a small obturator foramen between the ischium and pubis, which gives passage to the obturator nerve. It is formed by the part of the tissue where the nerve is placed not becoming converted into cartilage.

There is a peculiar cartilage in the ventral median line in front of the pubis, which is developed independently of and much later than the true parts of the pelvic girdle. It may be called the praepubic cartilage.

Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two ventral (pubic and ischiad) processes. The acetabulum is placed on the outer side at the junction of the three processes, each of which may be

6o8 PECTORAL AND PELVIC GIRDLES.

considered to have a share in forming it. The distal ends of the pubis and ischium are close together when first formed, but subsequently separate. Each of them unites at a late stage with the corresponding process of the opposite side in a ventral symphysis. A centre of ossification appears in each of the three processes of the primitive cartilage.

Aves. In Birds the parts of the pelvic girdle no longer develop as a continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the Uuck, all the elements. The ilium early exhibits a short anterior process, but the pubis and ischium are at first placed with their long axes at right angles to that of the ilium, but gradually become rotated so as to lie parallel with it, their distal ends pointing backwards, and not uniting ventrally excepting in one or two Struthious forms.

Mammalia. In Mammalia the pelvic girdle is formed in cartilage as in the lower forms, but in Man at any rate the pubic part of the cartilage is formed independently of the remainder (Rosenberg). There are the usual three centres of ossification, which unite eventually into a single bone the innominate bone. The pubis and ischium of each side unite with each other ventrally, so as completely to enclose the obturator foramen.

Huxley holds that the so-called marsupial bones of Monotremes and Marsupials, which as shewn by Gegenbaur (No. 474) are performed in cartilage, are homologous with the praepubis of the Urodela ; but considering the great gap between the Urodela and Mammalia this homology can only be regarded as tentative. He further holds that the anterior prolongations of the cartilaginous ventral ends of the pubis of Crocodilia are also structures of the same nature.

BIBLIOGRAPHY.

(473) A. Bunge. Untersuch. z, Entwick. d. Beckengiirtels d. Amphibien, Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc." Proc. of Roy. Soc., Vol. xxvm. 1879.

(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et posterieurs dans la Serie d. Vertebrcs. Montpellier, 1880.

Comparison of Pectoral and Pelvic girdles.

Throughout the Vertebrata a more or less complete serial homology may be observed between the pectoral and pelvic girdles.

In the cartilaginous Fishes each girdle consists of a continuous band, a dorsal and ventral part being indicated by the articulation of the fin ; the former being relatively undeveloped in the pelvic

LIMBS. 609

girdle, while in the pectoral it may articulate with the vertebral column. In the case of the pectoral girdle secondary membrane bones become added to the primitive cartilage in most Fishes, which are not developed in the case of the pelvic girdle.

In the Amphibia and Amniota the ventral section of each girdle becomes divided into an anterior and a posterior part, the former constituting the praecoracoid and pubis, and the latter the coracoid and ischium ; these parts are however very imperfectly differentiated in the pelvic girdle of the Urodela. The ventral portions of the pelvic girdle usually unite below in a symphysis. They also meet each other ventrally in the case of the pectoral girdle in Amphibia, but in most other types are separated by the sternum, which has no homologue in the pelvic region, unless the praepubic cartilage is to be regarded as such. The dorsal or scapular section of the pectoral girdle remains free ; but that of the pelvic girdle acquires a firm articulation with the vertebral column.

If the clavicle of the higher types is derived from the membrane bones of the pectoral girdle of Fishes, it has no homologue in the pelvic girdle ; but if, as Gotte and Hoffmann suppose, it is a part of the primitive cartilaginous girdle, the ordinary view as to the serial homologies of the ventral sections of the two girdles in the higher types will need to be reconsidered.

Limbs.

It will be convenient to describe in this place not only the development of the skeleton of the limbs but also that of the limbs themselves. The limbs of Fishes are moreover so different from those of the Amphibia and Amniota that the development of the two types of limb may advantageously be treated separately.

In Fishes the first rudiments of the limbs appear as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.

These ridges are two in number on each side, an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected, but in some Elasmobranch embryos, more especially in Torpedo, they are connected together at their first development B. in. 39

6io

PAIRED FINS OF ELASMOBRANCHII.

by a line of columnar epiblast cells 1 . This connecting line of columnar epiblast is a very transitory structure, and after its disappearance the rudimentary fins become more prominent, consisting (fig. 343, &) of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold and the fin becomes a simple ridge of mesoblast, covered by epiblast. The pectoral fins are usually considerably ahead of the pelvic fins in development.

For the remaining history it is necessary to confine ourselves to Scylliurn as the only type which has been adequately studied.

The direction of the original ridge which connects the two fins of each side is nearly though not quite longitudinal, sloping somewhat obliquely downwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line a little way behind the anus.

The elongated ridge, forming the rudiment of each fin, gradually projects more and more, and so becomes broader in proportion to its length, but at the same time its actual attachment to the side of the body becomes shortened from behind forwards, so that what was originally the attached border becomes in part converted into the posterior border. This process is much more completely carried out in the case of the pectoral fins than in that of the pelvic, and the changes of form undergone by the pectoral fin in its development may be gathered from figs. 344 and 348.

FIG. 343. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ; cav. cardinal vein ; ua. vitelline artery ; u.v, vitelline vein ; al. duodenum ; /. liver ; sd. opening of segmented duct into the body cavity ; mp. muscle plate ; ;. umbilical canal.

1 I. M. I'alfour. Monograph on Elasmobranfh l-'hhes, pp. 1012.

LIMBS. 6ll

Before proceeding to the development of the skeleton of the fin it may be pointed out that the connection of the two rudimentary fins by a continuous epithelial line suggests the hypothesis that they are the remnants of two continuous lateral fins 1 .

Shortly after the view that the paired fins were remnants of continuous lateral fins had been put forward in my memoir on Elasmobranch Fishes, two very interesting papers were published by Thacker (No. 489) and Mivart (No. 484) advocating this view on the entirely independent grounds of the adult structure of the skeleton of the paired fins in comparison with that of the unpaired fins 2 .

The development of the skeleton has unfortunately not been as yet very fully studied. I have however made some investigations on this subject on Scyllium, and 'Swirski has also made some on the Pike.

In Scyllium the development of both the pectoral and pelvic fins is very similar.

In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate which

1 Both Maclise arid Humphry {Journal of Anat. and Pkys., Vol. v.) had previously suggested that the paired fins were related to the unpaired fins.

2 Davidoff in a Memoir (No. 477) which forms an important contribution to our knowledge of the structure of the pelvic fins has attempted from his observations to deduce certain arguments against the lateral fin theory of the limbs. His main argument is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. If this is the strongest argument which can be brought against the theory advocated in the text, there is I trust a considerable chance of its being generally accepted. For even granting that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral fin theory why the pelvic fins should not have shifted, and on the other hand the longitudinal cord connecting some of the spinal nerves in front of the pelvic fin may have another explanation. It might for instance be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their character such conclusions as those of Davidoff.

392

612

PAIRED FINS OF ELASMOBRANCHII.

extends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar.

In other words, the primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin,

FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGITUDINAL AND HORIZONTAL SECTION.

The skeleton of the fin was still in the condition of embryonic cartilage. b.p. basipterygium (eventual metapterygium) ; fr. fin rays; p.g. pectoral girdle in transverse section; /. foramen in pectoral girdle; pc. wall of peritoneal cavity.

and giving off at right angles series of rays which pass into the fin. The longitudinal bar, which may be called the basipterygium, is moreover continuous in front with the pectoral or pelvic girdle as the case may be.

The primitive skeleton of the pectoral fin is shewn in longitudinal section in fig. 344, and that of the pelvic fin at a slightly later stage in fig. 345.

A transverse section shewing the basipterygium (inpi) of the pectoral fin, and the plate passing from it into the fin, is shewn in fig. 346.

Before proceeding to describe the later history of the two fins it may be well to point out that their embryonic structure completely supports the view which has been arrived at from the consideration of the soft parts of the fin.

My observations shew that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin which has the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probably

LIMBS.

613

be due to secondary development. As pointed out by Mivart, a longitudinal bar is also occasionally formed to support the cartilaginous rays of unpaired fins. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of primitively independent rays, of which they believe the fin to have been originally composed. This view is probable enough in itself, but there is no trace

FIG. 345. PELVIC FIN OF A VERY YOUNG FEMALE EMBRYO OF SCYLLIUM STELLARE.

bb. basipterygium ; pu. pubic process of pelvic girdle ; il. iliac process of pelvic girdle.

in the embryo of the bar in question being formed by the coalesceace of rays, though the fact of its being perfectly continuous with the bases of the rays is somewhat in favour of this view 1 .

A point may be noticed here which may perhaps appear to be a difficulty, viz. that to a considerable extent in the pectoral, and to some extent in the pelvic fin the embryonic cartilage from which the fin-rays are developed is at first a continuous lamina, which subsequently segments into rays. I am however inclined to regard this merely as a result of the mode of conversion of the indifferent mesoblast into cartilage ; and in any case no conclusion adverse to the above view can be drawn from it, since I find that the rays of the unpaired fin are similarly segmented from a continuous lamina. In all cases the segmentation of the rays is to a large extent completed before the tissue in question is sufficiently differentiated to be called cartilage by an histologist.

Thacker and Mivart both hold that the pectoral and pelvic girdles have been evolved by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken to this view, and the fact of the pectoral and pelvic girdles originating continuously, and long remaining united with the

1 Thacker more especially founds his view on the adult form of the pelvic fins in the cartilaginous Ganoids ; Polyodon, in which the part which constitutes the basal plate in other forms is divided into separate segments, being mainly relied on. It is possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff, is secondary, but Thacker's view that the segmentation is a primitive character seems to me, in the absence of definite evidence to the reverse, the more natural one.

614

THE PELVIC FIN.

longitudinal bars of their respective fins is in favour of rather than against this view. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.

The later development of the skeleton of the two fins is more conveniently treated separately.

The pelvic fin. The changes in the pelvic fin are comparatively slight. The fin remains through life as a nearly horizontal lateral projection of the body, and the longitudinal bar the

FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG

EMBRYO OK SCYLLIUM STELLARE. mpt. basipterygial bar (metapterygium) ; fr. fin ray; m. muscles; hf. horny fibres.

basipterygium at its base always remains as such. It is for a considerable period attached to the pelvic girdle, but eventually becomes segmented from it. Of the fin rays the anterior remains directly articulated with the pelvic girdle on the separation of the basipterygium (fig. 347), and the remaining rays finally become segmented from the basipterygium, though they remain articulated with it. They also become to some extent transversely segmented. The posterior end of the basipterygial bar also becomes segmented off as the terminal ray.

The pelvic fin thus retains in all essential points its primitive arrangement.

LIMBS.

6l 5

The pectoral fin. The earliest stage of the pectoral fin

There

FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE.

bp. basipterygium ; m.o. process of basipterygium continued into clasper; il. iliac process of pectoral girdle ; pit. pubis.

differs from that of the pelvic fin only in minor points, is the same longitudinal or basipterygial bar to which the fin-rays are attached, whose position at the base of the fin is clearly seen in the transverse section (fig. 346, mpf). In front the bar is continuous with the pectoral girdle (figs. 344 and

348).

The changes which take place in the course of the further development are however very much more considerable in the case of the pectoral than in that of the pelvic fin. "' 3+8. F^OJJL ,,, v.

By the process spoken m p t me tapterygium (basipterygium of earlier

stage); me.p. rudiment of future pro- and mesopterygium ; sc. cut surface of scapular process ; cr. coracoid process;/;', foramen;/, horny fibres.

of above, by which the attachment of the pec

6l6 THE PECTORAL FIN.

toral fin to the body wall becomes shortened from behind forwards, the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (figs. 348 and 349, mp], constituting what Gegenbaur called the metapterygium, and eventually becomes segmented off from the pectoral girdle, simply articulating with its hinder edge.

The plate of cartilage, which is continued outwards from the basipterygium, or as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as in the case of the pelvic fin, and this is especially the case with the basal part of the plate. This basal part becomes in fact at first only divided into two parts (fig. 348) a small anterior part at the front end (me.p), and a larger posterior along the base of the remainder of the fin. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the rudiment of the mesopterygium and propterygium of Gegenbaur. It bears four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are borne by the edge of the plate continuous with the metapterygium.

The further changes in the cartilages of the limb are not important, and are easily understood by reference to fig. 349 representing the limb of a nearly full-grown embryo. The front end of the anterior basal cartilage becomes segmented off as a propterygium, bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium. The remainder of the now considerably segmented fin-rays are borne by the metapterygium.

The mode of development of the pectoral fin demonstrates that, as supposed by Mivart, the metapterygium is the homologue of the basal cartilage of the pelvic fin.

From the mode of development of the fins of Scyllium conclusions may be drawn adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom consider the primitive type of fin to be most nearly retained in Ceratodus, and to consist of a central multisegmented axis with numerous rays. Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodus, with a median axis and two

LIMBS.

6I 7

rows of rays ; but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (median or inner according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium ; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.

Gegenbaur's view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium ; not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays ; but the fact is that, as may easily be seen by an inspection of figs. 344 and 346, such a second set of lateral rays could not possibly have existed in a type . of fin like that found in the embryo 1 . With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill arches also falls ; in that his method of deriving the limbs from gill arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchs, could be derived from a visceral arch with its branchial rays 2 .

Gegenbaur's older view

FIG. 349. SKELETON OF THE PECTORAL FIN AND PART OF PECTORAL GIRDLE OF A NEARLY RIPE EMBRYO OF SCYLLIUM STELLARE.

m.p. metapterygium ; me.p. mesopterygium ; //. propterygium ; cr. coracoid process.

1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found in some Elasmobranch pectoral fins as rudiments of a second set of rays on the posterior side of the metapterygium, these rays will have to be regarded as structures in the act of being evolved, and not as persisting traces of a biserial fin.

2 Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as a result of his researches on Protopterus are interesting. The attachment which he describes between the external gills and the pectoral girdle is no doubt remarkable, but I would suggest that the observations we have on the vascular supply of these gills demonstrate that this attachment is secondary.

6l8 THE CHEIKOPTERYGIUM.

that the Elasmobranch fin retains a primitive uniserial type appears to me to be nearer the truth than his more recent view on this subject ; though I hold that the fundamental point established by the development of these parts in Scyllium is that the posterior border of the adult Elasmobranch fin is the primitive base line, i.e. the line of attachment of the fin to the side of the body.

Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The secondary character of the mesopterygium, and its total absence in the embryo Scyllium, appears to me as conclusive against Huxley's view, as the character of the embryonic fin is against that of Gegenbaur ; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchii by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.

With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski (No. 488) which unfortunately do not throw very much light upon the nature of the limb.

'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage, continuous with the pectoral girdle ; which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.

These investigations might be regarded as tending to shew that the basipterygium of Elasmobranchii is not represented in Teleostei, owing to the fin rays not having united into a continuous basal bar, but the observations are not sufficiently complete to admit of this conclusion being founded upon them with any certainty.

Tlie ckeiropterygium.

Observations on the early development of the pentadactyloid limbs of the higher Vertebrata are comparatively scanty.

The limbs arise as simple outgrowths of the sides of the body, formed both of epiblast and mesoblast. In the Amniota, at all events, they are processes of a special longitudinal ridge known as the Wolffian ridge. In the Amniota they also bear at their extremity a thickened cap of epiblast, which may be compared with the epiblastic fold at the apex of the Elasmobranch fin.

Both limbs have at first a precisely similar position, both being directed backwards and being parallel to the surface of the body.

I 111: CHEIROPTERYGIUM.

619

In the Urodela (Gotte) the ulnar and fibular sides are primitively dorsal, and the radial and tibial ventral : in Mammalia however Kolliker states that the radial and tibial edges are from the first anterior.

The exact changes of position undergone by the limbs in the course of development are not fully understood. To suit a terrestrial mode of life the flexures of the two limbs become gradually more and more opposite, till in Mammalia the corresponding joints of the two limbs are turned in completely opposite directions.

Within the mesoblast of the limbs a continuous blastema becomes formed, which constitutes the first trace of the skeleton of the limb. The corresponding elements of the two limbs, viz. the humerus and femur, radius and tibia, ulna and fibula, carpal and tarsal bones, metacarpals and metatarsals, and digits, become differentiated within this, by the conversion of definite regions into cartilage, which may either be completely distinct or be at first united. These cartilaginous elements subsequently ossify.

The later development of the parts, more especially of the carpus and tarsus, has been made the subject of considerable study ; and important results have been thereby obtained as to the homology of the various carpal and tarsal bones throughout the Vertebrata ; but this subject is too special to be treated of here. The early development, including the succession of the growth of the different parts, and the extent of continuity primitively obtaining between them, has on the other hand been but little investigated ; recently however the development of the limbs in the Urodela has been worked out in this way by two anatomists, Gotte (No. 482) and Strasser (No. 487), and their results, though not on all points in complete harmony, are of considerable interest, more especially in their bearing on the derivation of the pentadactyloid limb from the piscine fin. Till however further investigations of the same nature have been made upon other types, the conclusions to be drawn from Gotte and Strasser's observations must be regarded as somewhat provisional, the actual interpretation of various ontological processes being very uncertain.

The forms investigated are Triton and Salamandra. We may remind the reader that the hand of the Urodela has four digits, and the foot five, the fifth digit being absent in the hand 1 . In Triton the proximal row of carpal bones consists (using Gegenbaur's nomenclature) of (i) a radiale, and (2 and 3) an intermedium and ulnare, partially united. The distal row is formed of four carpals, of which the first often does not support the first 1 This seems to me clearly to follow from Gotte and Strasser's observations.

620 THE GHE1ROPTERYGIUM.

metacarpal ; while the second articulates with both the first and second metacarpals. In the foot the proximal row of tarsals consists of a tibiale, an intermedium and a fibulare. The distal row is formed of four tarsals, the first, like that in the hand, often not articulating with the first metatarsal, the second supporting the first and second metatarsals ; and the fourth the fourth and fifth metatarsals.

The mode of development of the hand and foot is almost the same. The most remarkable feature of development is the order of succession of the digits. The two anterior (radial or tibial) are formed in the first instance, and then the third, fourth and fifth in succession.

As to the actual development of the skeleton Strasser, whose observations were made by means of sections, has arrived at the following results.

The humerus with the radius and ulna, and the corresponding parts in the hind limb, are the first parts to be differentiated in the continuous plate of tissue from which the skeleton of the limb is formed. Somewhat later a cartilaginous centre appears at the base of the first and second fingers (which have already appeared as prominences at the end of the limb) in the situation of the permanent second carpal of the distal row of carpals ; and the process of chondrification spreads from this centre into the fingers and into the remainder of the carpus. In this way a continuous carpal plate of cartilage is established, which is on the one hand continuous with the cartilage of the two metacarpals, and on the other with the radius and ulna.

In the cartilage of the carpus two special columns may be noticed, the one on the radial side, most advanced in development, being continuous with the radius ; the other less developed column on the side of the ulna being continuous both with the ulna and with the radius. The ulna and radius are not united with the humerus.

In the further growth the third and fourth digits, and in the foot the fifth digit also, gradually sprout out in succession from the ulnar side of the continuous carpal plate. The carpal plate itself becomes segmented from the radius and ulna, and divided up into the carpal bones.

The original radial column is divided into three elements, a proximal the radiale, a middle element the first carpal, and a distal the second carpal already spoken of. The first carpal is thus situated between the basal cartilage of the second digit and the radiale, and would therefore appear to be the representative of a primitive middle row of carpal bones, of which the centrale is also another representative.

The centrale and intermedium are the middle and proximal products of the segmentation of the ulnar column of the primitive carpus, the distal second carpal being common both to this column and to the radial column.

The ulnar or fibular side of the carpus or tarsus becomes divided into a proximal element the ulnare or fibulare the ulnare remaining partially united with the intermedium. There are also formed from this plate two carpals to articulate with digits 3 and 4 ; while in the foot the corresponding elements articulate respectively with the third digit, and with the fourth and fifth digits.

THE CIIF.IROPTERYGIUM. 621

Gotte, whose observations were made in a somewhat different method to those of Strasser, is at variance with him on several points. He finds that the primitive skeleton of the limb consists of a basal portion, the humerus, continued into a radial and an ulnar ray, which are respectively prolonged into the two first digits. The two rays next coalesce at the base of the fingers to form the carpus, and thus the division of the limb into the brachium, antebrachium and manus is effected.

The ulna, which is primitively prolonged into the second digit, is subsequently separated from it and is prolonged into the third ; from the side of the part of the carpus connecting the ulna with the third digit the fourth digit is eventually budded out, and in the foot the fourth and fifth digits arise from the corresponding region. Each of the three columns connected respectively with the first, second, and third digits becomes divided into three successive carpal bones, so that Gotte holds the skeleton of the hand or foot to be formed of a proximal, a middle, and a distal row of carpal bones each containing potentially three elements. The proximal row is formed of the radiale, intermedium and ulnare ; the middle row of carpal i, the centrale and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two coalesced elements) and carpal 3.

The derivation of the cheiropterygium from the ichthyoptcrygium. All anatomists are agreed that the limbs of the higher Vertebrata are derived from those of Fishes, but the gulf between the two types of limbs is so great that there is room for a very great diversity of opinion as to the mode of evolution of the cheiropterygium. The most important speculations on the subject are those of Gegenbaur and Huxley.

Gegenbaur holds that the cheiropterygium is derived from a uniserial piscine limb, and that it consists of a primitive stem, to which a series of lateral rays are attached on one (the radial) side ; while Huxley holds that the cheiropterygium is derived from a biserial piscine limb by the "lengthening of the axial skeleton, accompanied by the removal of its distal elements further away from the shoulder-girdle and by a diminution in the number of the rays."

Neither of these theories is founded upon ontology, and the only ontological evidence we have which bears on this question is that above recorded with reference to the development of the Urodele limb.

Without holding that this evidence can be considered as in any way conclusive, its tendency would appear to me to be in favour of regarding the cheiropterygium as derived from a uniserial type of fin. The humerus or femur would appear to be the basipterygial bars (metapterygium), which have become directed outwards instead of retaining their original position parallel to the length of the body at the base of the fin. The anterior (proximal) fin-rays and the pro- and mesopterygium must be supposed to have become aborted, while the radius or ulna, and tibia or fibula are two posterior fin-rays (probably each representing several coalesced rays like the pro- and mesopterygium) which support at their distal extremities more numerous fin-rays consisting of the rows of carpal and tarsal bones.

622 THE CHEIROPTERYGIUM.

This view of the cheiropterygium corresponds in some respects with that put forward by Gotte as a result of his investigations on the development of the Urodele limbs, though in other respects it is very different. A difficulty of this view is the fact that it involves our supposing that the radial edge of the limb corresponds with the metapterygial edge of the piscine fin. The difficulties of this position have been clearly pointed out by Huxley, but the fact that in the primitive position of the Urodele limbs the radius is ventral and the ulna dorsal shews that this difficulty is not insuperable, in that it is easy to conceive the radial border of the fin to have become rotated from its primitive Elasmobranch position into the vertical position it occupies in the embryos of the Urodela, and then to have been further rotated from this position into that which it occupies in the adult Urodela and in all higher forms.

BIBLIOGRAPHY of the Limbs.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuckungen z. vergleich. Anat. d. Wirbelthiere. Leipzig, 1864 5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d. Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeitsckrift, Vol. V. 1870.

(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol. vii. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere." Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entivick. u. Regeneration d. Gliedmaassenskelets d. Molche. Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeil.f. iviss. Zool., Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi d. Menschen. " Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u. Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Untersitch. iib. d. Entwick. d. Schultergitrtels u. d. Skelcls d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history of the Vertebrate limbs." Trans, of the Connecticut Acad., Vol. ill. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut Acad., Vol. iv. 1877.

CHAPTER XXI.

THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE VASCULAR GLANDS.

The Body cavity.

IN the Ccelenterata no body cavity as distinct from the alimentary cavity is present ; but in the remaining Invertebrata the body cavity may (i) take the form of a wide space separating the wall of the gut from the body wall, or (2) may be present in a more or less reduced form as a number of serous spaces, or (3) only be represented by irregular channels between the muscular and connective-tissue cells filling up the interior of the body. The body cavity, in whatever form it presents itself, is probably filled with fluid, and the fluid in it may contain special cellular elements. A well developed body cavity may coexist with an independent system of serous spaces, as in the Vertebrata and the Echinodermata ; the perihaemal section of the body cavity of the latter probably representing the system of serous spaces.

In several of the types with a well developed body cavity it has been established that this cavity originates in the embryo from a pair of alimentary diverticula, and the cavities resulting from the formation of these diverticula may remain distinct, the adjacent walls of the two cavities fusing to form a dorsal and a ventral mesentery.

It is fairly certain that some groups, e.g. the Tracheata, with imperfectly developed body cavities are descended from ancestors which were provided with well developed body cavities, but how far this is universally the case cannot as yet be definitely decided, and for additional information on this subject the

624 CIIORDATA.

reader is referred to pp. 355 360 and to the literature there referred to.

In the Chaetopoda and the Tracheata the body cavity arises as a series of paired compartments in the somites of mesoblast (fig. 350) which have at first a very restricted extension on the ventral side of the body, but eventually extend dorsalwards and vcntralwards till each cavity is a half circle investing the alimentary tract ; on the dorsal side the walls separating the two

FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA LABYRINTHICA.

The section is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk segments and their nuclei.

i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument.

half cavities usually remain as the dorsal mesentery, while ventrally they are in most instances absorbed. The transverse walls, separating the successive compartments of the body cavity, generally become more or less perforated.

Chordata. In the Chordata the primitive body cavity is cither directly formed from a pair of alimentary diverticula (Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic plates of the two sides of the body (fig. 20).

As already explained (pp. 294 300) the walls of the dorsal sections of the primitive body cavity soon become separated from those of the ventral, and becoming segmented constitute the muscle plates, while the cavity within them becomes

I

THE BODY CAVITY.

625

the

obliterated : they are dealt with in a separate chapter. The ventral part of the primitive cavity alone constitutes the permanent body cavity.

The primitive body cavity in the lower Vertebrata is at first continued forwards into the region of the head, but on the formation of the visceral clefts the cephalic section of the body cavity becomes divided into a series of separate compartments. Subsequently these sections of the body cavity become obliterated ; and, since their walls give rise to muscles, they may probably be looked upon as equivalent to the dorsal sections of the body cavity in the trunk, and will be treated of in connection with the muscular system.

As a result of its mode of origin the body cavity in trunk is at first divided into two lateral halves ; and part of the mesoblast lining it soon becomes distinguished as a special layer of epithelium, known as the peritoneal epithelium, of which the part bounding the outer wall forms the somatic layer, and that bounding the inner wall the splanchnic layer. Between the two splanchnic layers is placed the gut. On the ventral side, in the region of the permanent gut, the two halves of the body cavity soon coalesce, the septum between them becoming absorbed, and the splanchnic layers of epithelium of the two sides uniting at the ventral side of the gut, and the somatic layers at the median ventral line of the body wall (fig.

In the lower Vertebrata the body cavity is originally present even in the post-anal region of the trunk, but usually atrophies early, frequently before the two halves coalesce.

On the dorsal side of the gut the B. III.

FIG. 351. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white matter of spinal cord ; pr. posterior nerve-roots ; cA. notochord ; x. sub-notochordal rod ; ao. aorta ; nip. muscle-plate ; nip 1 , inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body ; si. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.

40

626 ABDOMINAL PORES.

two halves of the body cavity never coalesce, but eventually the splanchnic layers of epithelium of the two sides, together with a thin layer of interposed mesoblast, form a delicate membrane, known as the mesentery, which suspends the gut from the dorsal wall of the body (figs. 119 and 351). On the dorsal side the epithelium lining of the body cavity is usually more columnar than elsewhere (fig. 351), and its cells partly form a covering for the generative organs, and partly give rise to the primitive germinal cells. This part of the epithelium is often known as the germinal epithelium.

Over the greater part of the body cavity the lining epithelium becomes in the adult intimately united with a layer of the subjacent connective tissue, and constitutes with it a special lining membrane for the body cavity, known as the peritoneal membrane.

Abdominal pores. In the Cyclostomata, the majority of the Elasmobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida (Chelonia and Crocodilia) the body cavity is in communication with the exterior by a pair of pores, known as abdominal pores, the external openings of which are usually situated in the cloaca 1 .

The ontogeny of these pores has as yet been but very slightly investigated. In the Lamprey they are formed as apertures leading from the body cavity into the excretory section of the primitive cloaca. This section would appear from Scott's (No. 87) observations to be derived from part of the hypoblastic cloacal section of the alimentary tract.

In all other cases they are formed in a region which appears to belong to the epiblastic region of the cloaca ; and from my observations on Elasmobranchs it may be certainly concluded that they are formed there in this group. They may appear as perforations (i) at the apices of papilliform prolongations of the body cavity, or (2) at the ends of cloacal pits directed from the exterior towards the body cavity, or (3) as simple slit-like openings.

Considering the difference in development between the abdominal pores of most types, and those of the Cyclostomata, it is open to doubt whether these two types of pores are strictly homologous.

In the Cyclostomata they serve for the passage outwards of the generative products, and they also have this function in some of the few Teleostei in which they are found ; and Gegenbaur and Bridge hold that the primitive mode of exit of the generative products, prior to the development of the Miillerian ducts, was probably by means of these pores. I have elsewhere

1 For a full account of these structures the reader is referred to T. W. Bridge, "Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879.

THE BODY CAVITY.

627

suggested that the abdominal pores are perhaps remnants of the openings of segmental tubes ; there does not however appear to be any definite evidence in favour of this view, and it is more probable that they may have arisen as simple perforations of the body wall.

Pericardial cavity, pleural cavities, and diaphragm.

In all Vertebrata the heart is at first placed in the body cavity (fig. 353 A), but the part of the body cavity containing it afterwards becomes separated as a distinct cavity known as the pericardial cavity. In Elasmobranchii, Acipenser, etc. a passage is however left between the pericardial cavity and the body cavity ; and in the Lamprey a separation between the two cavities does not occur during the Ammoccete stage. In Elasmobranchii the pericardial cavity becomes established as a distinct space in front of the body cavity in the following way. When the two ductus Cuvieri, leading transversely from the sinus venosus to the cardinal veins, become developed, a horizontal septum, shewn on the right side in fig. 352, is formed to support them, stretching across from the splanchnic to the somatic side of the body cavity, and dividing the body cavity (fig. 352) in this part into (i) a dorsal section formed of a right and left division constituting the true body cavity (pp), and (2) a ventral part the pericardial cavity (pc). The septum is at first of a very small longitudinal extent, so that both in front and behind it (fig. 352 on the left side) the dorsal and ventral sections of the body cavity are in free communication. The septum soon however becomes prolonged, and ceasing to be quite horizontal, is directed obliquely upwards and forwards till it meets the dorsal wall of the body

40 2

-ht

FIG. 352. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F.

The figure shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the narrow passage which remains connecting the two cavities.

sp.c. spinal canal ; w. white matter of spinal cord ; pr. commissure connecting the posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; sv. sinus venosus ; cav. cardinal vein ; ht. heart ; pp. body cavity ; pc. pericardial cavity ; as. solid oesophagus ; /. liver ; nip. muscle-plate.

628 THE PERICARDIAL CAVITY.

Anteriorly all communication is thus early shut off between the body cavity and the pericardial cavity, but the two cavities still open freely into each other behind.

The front part of the body cavity, lying dorsal to the pericardial cavity, becomes gradually narrowed, and is wholly obliterated long before the close of embryonic life, so that in adult Elasmobranch Fishes there is no section of the body cavity dorsal to the pericardial cavity. The septum dividing the body cavity from the pericardial cavity is prolonged backwards, till it meets the ventral wall of the body at the point where the liver is attached by its ventral mesentery (falciform ligament). In this way the pericardial cavity becomes completely shut off from the body cavity, except, it would seem, for the narrow communications found in the adult. The origin of these communications has not however been satisfactorily worked out.

The septum between the pericardial cavity and the body cavity is attached on its dorsal aspect to the liver. It is at first nearly horizontal, but gradually assumes a more vertical position, and then, owing to the obliteration of the primitive anterior part of the body cavity, appears to mark the front boundary of the body cavity. The above description of the mode of formation of the pericardial cavity, and the explanation of its relations to the body cavity, probably holds true for Fishes generally.

In the higher types the earlier changes are precisely the same as those in Elasmobranch Fishes. The heart is at first placed within the body cavity attached to the ventral wall of the gut by a mesocardium (fig. 353 A). A horizontal septum is then formed, in which the ductus Cuvieri are placed, dividing the body cavity for a short distance into a dorsal (/./) and ventral (p.c) section (fig. 353 B). In Birds and Mammals, and probably also in Reptilia, the ventral and dorsal parts of the body cavity are at first in free communication both in front of and behind this septum. This is shewn for the Chick in fig- 353 A an d B, which are sections of the same chick, A being a little in front of B. The septum is soon continued forwards so as completely to separate the ventral pericardial and the dorsal body cavity in front, the pericardial cavity extending at this period considerably further forwards than the body cavity.

Since the horizontal septum, by its mode of origin, is

THE BODY CAVITY.

629

necessarily attached to the ventral side of the gut, the dorsal part of the primitive body space is divided into two halves by a median vertical septum formed of the gut and its mesentery (fig- 353 B). Posteriorly the horizontal septum grows in a slightly ventral direction along the under surface of the liver (fig- 354)j till it meets the abdominal wall of the body at the insertion of the falciform ligament, and thus completely shuts off the pericardial cavity from the body cavity. The horizontal septum forms, as is obvious from the above description, the dorsal wall of the pericardial cavity 1 .

A. B.

FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTYONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAI, CAVITY, A. BEING THE ANTERIOR SECTION.

p.p. body cavity; p.c. pericardial cavity; al. alimentary cavity ; au. auricle; v. ventricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; nip. muscle-plate; me. medullary cord.

With the complete separation of the pericardial cavity from the body cavity, the first period in the development of these parts is completed, and the relations of the body cavity to the

1 Kolliker's account of this septum, which he calls the mesocardium laterale (No. 298, p. 295), would seem to imply that in Mammals it is completed posteriorly even before the formation of the liver. I doubt whether this takes place quite so early as he implies, but have not yet determined its exact period by my own observations.

630

THE PERICARDIAL CAVITY.

pericardial cavity become precisely those found in the embryos of Elasmobranchii. The later changes are however very different. Whereas in Fishes the right and left sections of the body cavity dorsal to the pericardial cavity soon atrophy, in the higher types, in correlation with the relatively backward situation of the heart, they rapidly become larger, and receive the lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into the splanchnic mesoblast, in front of the body cavity ; but as they grow, they extend into the two anterior compartments of the body cavity, each attached by its mesentery to the mesentery of the gut (fig. 354, lg). They soon moreover extend beyond the region of the pericardium into the undivided body cavity behind. This holds not only for the embryos of the Amphibia and Sauropsida, but also for those of Mammalia.

To understand the further

rrianfrps in rhp nerirardial ravitv FlG> 354- SECTION THROUGH

pencaraiai cavity THECARDIACREGION OF AN EMBRYO

it is necessary to bear in mind its OF LACERTA MURALIS OF 9 MM. TO

, ,. ,, ,. . . SHEW THE MODE OF FORMATION OF

relations to the adjoining parts. THE PERICARDIAL CAVITY.

'-/it

It lies at this period completely ventral to the two anterior pro

ht. heart ; pc. pericardial cavity ; al. alimentary tract; lg. lung; /. liver ; pp. body cavity ; md. open longations of the body Cavity COn- end of Mullerian duct ; wd. Wolffian . . duct; vc. vena cava inferior; ao.

taming the lungs (fig. 354). Its aorta; ch. notochord; me. medullary

dorsal wall is attached to the gut, cord>

and is continuous with the mesentery of the gut passing to the dorsal abdominal wall, forming the posterior mediastinum of human anatomy.

The changes which next ensue consist essentially in the enlargement of the sections of the body cavity dorsal to the pericardial cavity. This enlargement takes place partly by the elongation of the posterior mediastinum, but still more by the two divisions of the body cavity which contain the lungs extending themselves ventrally round the outside of the peri

THE BODY CAVITY.

631

cardial cavity. This process is illustrated by fig. 355, taken from an embryo Rabbit. The two dorsal sections of the body cavity (pl.p] finally extend so as completely to envelope the pericardial cavity (pc\ remaining however separated from each other below by a lamina extending from the ventral wall of the pericardial cavity to the body wall, which forms the anterior mediastinum of human anatomy.

By these changes the pericardial cavity is converted into a closed bag, completely surrounded at its sides by the two lateral halves of the body cavity, which were primitively placed

SJ3. C.

FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL CAVITIES.

ht. heart; pc. pericardial cavity; //./ pleural cavity; Ig. lung; al. alimentary tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.

dorsally to it. These two sections of the body cavity, which in Amphibia and Sauropsida remain in free communication with the undivided peritoneal cavity behind, may, from the fact of their containing the lungs, be called the pleural cavities.

In Mammalia a further change takes place, in that, by the formation of a vertical partition across the body cavity, known as the diaphragm, the pleural cavities, containing the lungs,

632 THE VASCULAR SYSTEM.

become isolated from the remainder of the body or peritoneal cavity. As shewn by their development the so-called pleurae or pleural sacks are simply the peritoneal linings of the anterior divisions of the body cavity, shut off from the remainder of the body cavity by the diaphragm.

The exact mode of formation of the diaphragm is not fully made out ; the account of it recently given by Cadiat (No. 491) not being in my opinion completely satisfactory.

BIBLIOGRAPHY.

(491) M. Cadiat. "Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 1'cesophage." Journal de F Anatomic et de la Physiologic, Vol. xiv. 1878.

Vascular System.

The actual observations bearing on the origin of the vascular system, using the term to include the lymphatic system, are very scanty. It seems probable, mainly it must be admitted on d priori grounds, that vascular and lymphatic systems have originated from the conversion of indefinite spaces, primitively situated in the general connective tissue, into definite channels. It is quite certain that vascular systems have arisen independently in many types ; a very striking case of the kind being the development in certain parasitic Copepoda of a closed system of vessels with a red non-corpusculated blood (E. van Beneden, Heider), not found in any other Crustacea. Parts of vascular systems appear to have arisen in some cases by a canalization of cells.

The blood systems may either be closed or communicate with the body cavity. In cases where the primitive body cavity is atrophied or partially broken up into separate compartments (Insecta, Mollusca, Discophora, etc.) a free communication between the vascular system and the body cavity is usually present ; but in these cases the communication is no doubt secondary. On the whole it would seem probable that the vascular system has in most instances arisen independently of the body cavity, at least in types where the body cavity is

THE VASCULAR SYSTEM. 633

present in a well-developed condition. As pointed out by the Hertwigs, a vascular system is always absent where there is not a considerable development of connective tissue.

As to the ontogeny of the vascular channels there is still much to be made out both in Vertebrates and Invertebrates.

The smaller channels often rise by a canalization of cells. This process has been satisfactorily studied by Lankester in the Leech 1 , and may easily be observed in the blastoderm of the Chick or in the epiploon of a newlyborn Rabbit (Schafer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise to the walls, and the blood-corpuscles being derived either from nucleated masses set free within the vessels (the Chick) or from blood-corpuscles directly differentiated in the axes of the cells (Mammals).

Larger vessels would seem to be formed from solid cords of cells, the central cells becoming converted into the corpuscles, and the peripheral cells constituting the walls. This mode of formation has been observed by myself in the case of the Spider's heart, and by other observers in other Invertebrata. In the Vertebrata a more or less similar mode of formation appears to hold good for the larger vessels, but further investigations are still required on this subject. Gotte finds that in the Frog the larger vessels are formed as longitudinal spaces, and that the walls are derived from the indifferent cells bounding these spaces, which become flattened and united into a continuous layer.

The early formation of vessels in the Vertebrata takes place in the splanchnic mesoblast ; but this appears due to the fact that the circulation is at first mainly confined to the vitelline region, which is covered by splanchnic mesoblast.

The Heart.

The heart is essentially formed as a tubular cavity in the splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this cavity are formed of two layers, an outer thicker layer, which has at first only the form of a half tube, being incomplete on its dorsal side; and an inner lamina formed of delicate flattened cells. The latter is the epithelioid lining of the heart, and the cavity it contains the true cavity of the heart. The outer layer gives rise to the muscular wall and peritoneal covering of the heart. Though at first it has only the form of a half tube (fig.

1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science, Vol. XX. 1880.

634

THE HEART.

356), it soon becomes folded in on the dorsal side so as to form for the heart a complete muscular wall. Its two sides, after thus meeting to complete the tube of the heart, remain at first continuous with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery the mesocardium which attaches the heart to the ventral wall of the throat. The superficial stratum of the wall of the heart differentiates itself as the peritoneal covering. The inner epithelioid tube takes its origin at the time when the general cavity of the heart is being formed by the separation of the splanchnicmesoblastfrom the hypoblast. During this process (fig. 357) a layer of mesoblast remains close to the hypoblast, but connected with the main mass

FIG. 356. SECTION THROUGH THE DEVELOPING HEART OF AN EMBRYO OF AN ELASMOBRANCH (Pristiurus).

al. alimentary tract ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ht. heart.

FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.

hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochorcl ; x. thickening of hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ht. heart; //. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast.

THE VASCULAR SYSTEM.

635

of the mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form together the epithelioid lining of the heart ; between them is the cavity of the heart, which soon loses the protoplasmic trabeculae which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast, and resembles in its mode of origin that of other large vascular trunks.

The above description applies only to the development of the heart in those types in which it is formed at a period after the throat has become a closed tube (Elasmobranchii, Amphibia, Cyclostomata, Ganoids (?)). In a number of other cases, in which the heart is formed before the conversion of the throat into a closed tube, of which the most notable is that of Mammals (Hensen, Gotte, Kolliker), the heart arises as two independent

A.

B.

mes fir

FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE

SAME AGE AS FIG. 144 B. (From Kolliker.) B is a more highly magnified representation of part of A.

rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ; dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast; ^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of the pharynx.

636

THE HEART.

tubes (fig. 358), which eventually coalesce into an unpaired structure.

In Mammals the two tubes out of which the heart is formed appear at the sides of the cephalic plates, opposite the region of the mid- and hindbrain (fig. 358). They arise at a time when the lateral folds which form the ventral wall of the throat are only just becoming visible. Each half of the heart originates in the same way as the whole heart in Elasmobranchii, etc. ; and the layer of the splanchnic mesoblast, which forms the muscular wall for each part (ahh), has at first the form of a half tube open below to the hypoblast.

On the formation of the lateral folds of the splanchnic walls, the two halves of the heart become carried inwards and downwards, and eventually

FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE FORMATION OF THE HEART.

fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchnopleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins.

THE VASCULAR SYSTEM.

637

meet on the ventral side of the throat. For a short time they here remain distinct, but soon coalesce into a single tube.

In Birds, as in Mammals, the heart makes its appearance as two tubes, but arises at a period when the formation of the throat is very much more advanced than in the case of Mammals. The heart arises immediately behind the point up to which the ventral wall of the throat is established and thus has at first a A -shaped form. At the apex of the A , which forms the anterior end of the heart, the two halves are in contact (fig. 357), though they have not coalesced; while behind they diverge to be continued as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce from before backwards into a single structure. Fig. 359 A and B shews the heart during this process. The two halves have coalesced anteriorly (A) but are still widely separated behind (B). In Teleostei the heart is formed as in Birds and Mammals by the coalescence of two tubes, and it arises before the formation of the throat.

The fact that the heart arises in so many instances as a double tube might lead to the supposition that the ancestral Vertebrate had two tubes in the place of the present unpaired heart.

The following considerations appear to me to prove that this conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the formation of the heart, it is clear that a modification in the development of the heart would occur, in that the two halves of the heart would necessarily be formed widely apart, and only eventually united on the folding in of the wall of the throat. It is therefore possible to explain the double formation of the heart without having recourse to the above hypothesis of an ancestral Vertebrate with two hearts. If the explanation just suggested is the true one the heart should only be formed as two tubes when it arises prior to the formation of the throat, and as a single tube when formed after the formation of the throat. Since this is invariably found to be so, it may be safely concluded that the formation of the heart as two cavities is a secondary mode of development, which has been brought about by variations in the period of the closing in of the wall of the throat.

The heart arises continuously with the sinus venosus, which in the Amniotic Vertebrata is directly continued into the vitelline veins. Though at first it ends blindly in front, it is very soon connected with the foremost aortic arches.

638 THE HEART.

The simple tubular heart, connected as above described, grows more rapidly than the chamber in which it is contained, and is soon doubled upon itself, acquiring in this way an S-shaped curvature, the posterior portion being placed dorsally, and the anterior ventrally. A constriction soon appears between the dorsal and ventral portions.

The dorsal section becomes partially divided off behind from the sinus venosus, and constitutes the relatively thin-walled auricular section of the heart; while the ventral portion, after becoming distinct anteriorly from a portion continued forwards from it to the origin of the branchial arteries, which may be called the truncus arteriosus, acquires very thick spongy muscular walls, and becomes the ventricular division of the heart.

The further changes in the heart are but slight in the case of the Pisces. A pair of simple membranous valves becomes established at the auriculoventricular orifice, and further changes take place in the truncus arteriosus. This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a posterior section, called the conus arteriosus, provided with a series of transverse rows of valves, and an anterior section, called the bulb us arteriosus, not provided with valves, and leading into the branchial arteries. In most Teleostei (except Butirinus and a few other forms) the conus arteriosus is all but obliterated, and the anterior row of its valves alone preserved ; and the bulbus is very much enlarged 1 .

In the Dipnoi important changes in the heart are effected, as compared with other Fishes, by the development of true lungs. Both the auricular and ventricular chamber may be imperfectly divided into two, and in the conus a partial longitudinal septum is developed in connection with a longitudinal row of valves 2 .

In Amphibia the heart is in many respects similar to that of the Dipnoi. Its curvature is rather that of a screw than of a simple S. The truncus arteriosus lies to the left, and is continued into the ventricle which lies ventrally and more to the right, and this again into the dorsally placed auricular section.

After the heart has reached the piscine stage, the auricular section (Bombinator) becomes prolonged into a right and left auricular appendage^ A septum next grows from the roof of the auricular portion of the heart

1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit., Vol. n. 1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterienbogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.," Morphol. Jahrb., Vol. VI. 1880.

2 Boas holds that the longitudinal septum is formed by the coalescence of a row of longitudinal valves, but this is opposed to Lankester's statements, "On the hearts of Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. x. 1879.

THE VASCULAR SYSTEM. 639

obliquely backwards and towards the left, and divides it in two chambers ; the right one of which remains continuous with the sinus venosus, while the left one is completely shut off from the sinus, though it soon enters into communication with the newly established pulmonary veins. The truncus arteriosus 1 is divided into a posterior conus arteriosus (pylangium) and an anterior bulbus (synangium). The former is provided with a proximal row of valves at its ventricular end, and a distal row at its anterior end near the bulbus. It is also provided with a longitudinal septum, which is no doubt homologous with the septum in the conus arteriosus of the Dipnoi. The bulbus is well developed in many Urodela, but hardly exists in the Anura.

In the Amniota further changes take place in the heart, resulting in the abortion of the distal rows of valves of the conus arteriosus 2 , and in the splitting up of the whole truncus arteriosus into three vessels in Reptilia, and two in Birds and Mammals, each opening into the ventricular section of the heart, and provided with a special set of valves at its commencement. In Birds and Mammals the ventricle becomes moreover completely divided into two chambers, each communicating with one of the divisions of the primitive truncus, known in the higher types as the systemic and pulmonary aortae. The character of the development of the heart in the Amniota will be best understood from a description of what takes place in the Chick.

In Birds the originally straight heart (fig. 109) soon becomes doubled up upon itself. The ventricular portion becomes placed on the ventral and right side, while the auricular section is dorsal and to the left. The two parts are separated from each other by a slight constriction known as the canalis auricularis. Anteriorly the ventricular cavity is continued into the truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out from the auricle at a very early period. The general appearance of the heart, as seen from the ventral side on the fourth day, is shewn in fig. 360. Although the external divisions of the heart are well marked even before this stage, it is not till the end of the third day that the internal partitions become apparent ; and, contrary to what might have been anticipated from the evolution of these parts in the lower types, the ventricular septum is the first to be established.

1 For a good description of the adult heart vide Huxley, Article "Amphibia," in the Encyclopedia Britannic a.

2 It is just possible that the reverse may be true, vide note on p. 640. If however, as is most probable, the statement in the text is correct, the valves at the mouth of the ventricle in Teleostei are not homologous with those of the Amniota ; the former being the distal rov/ of the valves of the conus, the latter the proximal.

640 THE HEART OF AVES.

It commences on the third day as a crescentic ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, and on the fourth day grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement of the truncus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to

the right and below. These commu- A ^) ) CA

nicate with each other, above the free edge of the partition, along its whole length.

Externally the ventricular portion as yet shews no division into two parts.

By the fifth day the venous end of the heart, though still lying somewhat to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON

forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION

VIEWED FROM THE VENTRAL SURFACE.

organ appearing to be drawn together.

The ventricular septum is complete. L ?.- lef t a , uricular appendage; C.A.

, e .. , . , , canahs auricularis ; v. ventricle ; b. trun The apex of the ventricles becomes cus arteriosus.

more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the auricular septum, while in the canalis auricularis, which is now at its greatest length, there is also to be seen a commencement of the valvular structures tending to separate the cavity of the auricles from those of the ventricles.

About the io6th hour, a septum begins to make its appearance in the truncus arteriosus in the form of a longitudinal fold, which according to Tonge (No. 495) starts at the end of the truncus furthest removed from the heart. It takes origin from the wall of the truncus between the fourth and fifth pairs of arches, and grows downwards in such a manner as to divide the truncus into two channels, one of which leads from the heart to the third and fourth pairs of arches, and the other to the fifth pair. Its course downwards is not straight but spiral, and thus the two channels into which it divides the truncus arteriosus wind spirally the one round the other.

At the time when the septum is first formed, the opening of the truncus arteriosus into the ventricles is narrow or slit-like, apparently in order to prevent the flow of the blood back into the heart. Soon after the appearance of the septum, however, semilunar valves (Tonge, No. 495) are developed from the wall of that portion of the truncus which lies between the free edge of the septum and the cavity of the ventricles 1 .

1 If Tonge is correct in his statement that the semilunar valves develop at some distance from the mouth of the ventricle, it would seem possible that the portion of the truncus between them and the ventricle ought to be regarded as the embryonic conus arteriosus, and that the distal row of valves of the conus (and not the proximal as suggested above, p. 639) has been preserved in the higher types.

THE VASCULAR SYSTEM.

641

The ventral and the dorsal pairs of valves are the first to appear : the former as two small solid prominences separated from each other by a narrow groove ; the latter as a single ridge, in the centre of which is a prominence indicating the point where the ridge will subsequently become divided into two. The outer valves appear opposite each other, at a considerably later period.

As the septum grows downwards towards the heart, it finally reaches the position of these valves. One of its edges then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart, and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the truncus now

A. B.

FlG. 361. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY

OF INCUBATION.

A. from the ventral, B. from the dorsal side.

La. left auricular appendage; r.a. right auricular appendage ; r.v. right ventricle; l.v. left ventricle; b. truncus arteriosus.

fuses with the ventricular septum, and thus the division of the truncus into two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete ; the position of the valves not being very different from that in the adult heart.

That division of the truncus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle. The former becomes the pulmonary artery, the latter the commencement of the systemic aorta.

The external constriction actually dividing the truncus into two vessels does not begin to appear till the septum has extended some way back towards the heart.

The semilunar valves become pocketed at a period considerably later than their first formation (from the H7th to the,i65th hour) in the order of their appearance.

At the end of the sixth day, and even on the fifth day (figs. 361 and 362), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that of the adult. The original

B. III.

4 1

642

THE HEART OF MAMMALIA.

r.a

l.v

FIG. 362. HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FROM THE VENTRAL SURFACE.

La. left auricular appendage ; r,a. right auricular appendage ; r.v. right ventricle ; l.v. left ventricle ; b. truncus arteriosus.

protuberance to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart. The most noticeable difference (in the ventral view) is the still externally undivided condition of the truncus arteriosus.

The subsequent changes which the heart undergoes are concerned more with its internal structure than with its external shape. Indeed, during the next three days, viz. the eighth, ninth, and tenth, the external form of the heart remains nearly unaltered.

In the auricular portion, however, the septum which commenced on the fifth day becomes now more conspicuous. It is placed vertically, and arises from the ventral wall ; commencing at the canalis auricularis and proceeding towards the opening into the sinus venosus.

This latter structure gradually becomes reduced so as to become a special appendage of the right auricle. The inferior vena cava

enters the sinus obliquely from the right, so that its blood has a tendency to flow towards the left auricle of the heart, which is at this time the larger of the two.

The valves between the ventricles and auricles are now well developed, and it is about this time that the division of the truncus arteriosus into the aorta and pulmonary artery becomes visible from the exterior.

By the eleventh to the thirteenth day the right auricle has become as large as the left, and the auricular septum much more complete, though there is still a small opening, the foramen ovale, by which the two cavities communicate with each other.

The most important feature in which the development of the Reptilian heart differs from that of Birds is the division of the truncus into three vessels, instead of two. The three vessels remain bound up in a common sheath, and appear externally as a single trunk. The vessel not represented in Birds is that which is continued into the left aortic arch.

In Mammals the early stages in the development of the heart present no important points of difference from those of Aves. The septa in the truncus, in the ventricular, and in the auricular cavities are formed, so far as is known, in the same way and at the same relative periods in both groups. In the embryo Man, the Rabbit, and other Mammals the division of the ventricles is made apparent externally by a deep cleft, which, though evanescent in these forms, is permanent in the Dugong.

The attachment of the auriculo-ventricular valves to the wall of the ventricle, and the similar attachment of the left auriculo-ventricular valves in Birds, have been especially studied by Gegenbaur and Bernays (No. 492),

ARTERIAL SYSTEM. 643

and deserve to be noticed. In the primitive state the ventricular walls have throughout a spongy character ; and the auriculo-ventricular valves are simple membranous projections like the auriculo-ventricular valves of Fishes. Soon however the spongy muscular tissue of both the ventricular and auricular walls, which at first pass uninterruptedly the one into the other, grows into the bases of the valves, which thus become in the main muscular projections of the walls of the heart. As the wall of the ventricle thickens, the muscular trabeculas, connected at one end with the valves, remain at the other end united with the ventricular wall, and form special bands passing between the two. The valves on the other hand lose their muscular attachment to the auricular walls. This is the condition permanent in Ornithorhynchus. In higher Mammalia the ends of the muscular bands inserted into the valves become fibrous, from the development of intermuscular connective tissue, and the atrophy of the muscular elements. The fibrous parts now form the chordae tendinea?, and the muscular the musculi papillares.

The sinus venosus in Mammals becomes completely merged into the right auricle, and the systemic division of the truncus arteriosus is apparently not homologous with that in Birds.

In the embryos of all the Craniata the heart is situated very far forwards in the region of the head. This position is retained in Pisces. In Amphibia the heart is moved further back, while in all the Amniota it gradually shifts its position first of all into the region of the neck and finally passes completely within the thoracic cavity. The steps in the change of position may be gathered from figs. 109, in, and 118.

BIBLIOGRAPHY of the Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." Morphol. Jahrbuch,^o\. II. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f. mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX. 1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), and Balfour (292).

Arterial System.

In the embryos of Vertebrata the arterial system consists of a forward continuation of the truncus arteriosus, on the ventral

41 2

644

ARTERIES OF PISCES.

side of the throat (figs. 363, abr, and 364, a), which, with a few exceptions to be noticed below, divides into as many branches on each side as there are visceral arches. These branches, after traversing the visceral arches, unite on the dorsal side of the throat into a common trunk on each side. This trunk (figs. 363 and 364) after giving off one (or more) vessels to the head (c and c] turns backv/ards, and bends in towards the middle line, close to its fellow, immediately below the notochord (figs. 21 and 116) and runs backwards in this situation towards the end of the tail. The two parallel trunks below the notochord fuse very early into a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}.

ttbr v "a,

FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)

a. auricle ; v. ventricle ; abr. branchial artery ; c'. carotid ; ad. dorsal aorta ; s. branchial clefts; sv. sinus venosus; dc. ductus Cuvieri; n. nasal pit

There is given off from each collecting trunk from the visceral arches, or from the commencement of the dorsal aorta, a subclavian artery to each of the anterior limbs ; from near the anterior end of the dorsal aorta a vitelline artery (or before the dorsal aortae have united a pair of arteries fig. 125, R of A and L of A) to the yolk-sack, which subsequently becomes the main visceral artery 1 ; and from the dorsal aorta opposite the hind limbs one (or two) arteries on each side the iliac arteries to the hind limbs ; from these arteries the allantoic arteries are given off in the higher types, which remain as the hypogastric arteries after the disappearance of the allantois.

The primitive arrangement of the arterial trunks is with a few modifications retained in Fishes. With the development of the gills the vessels to the arches become divided into two parts connected by a capillary system in the gill folds, viz. into the

1 In Mammalia the superior inesenteric artery arises from the vitelline artery, which may probably be regarded as a primitive crclinco-mescnteric artery.

ARTERIAL SYSTEM.

branchial arteries bringing the blood to the gills from the truncus arteriosus, and the branchial veins transporting it to the dorsal aorta. The branchial vessels to those arches which do not bear gills, either wholly or partially atrophy; thus in Elasmobranchii the mandibular trunk, which is fully developed in the embryo (fig. 193, \av}, atrophies, except for a small remnant bringing blood to the rudimentary gill of the spiracle from the branchial vein of the hyoid arch. In Ganoids the mandibular artery atrophies, but the hyoid is usually preserved. In Teleostei both mandibular 1 and hyoid arteries are absent in the adult, except that there is usually left a rudiment of the hyoid, supplying the pseudobranch, which is similar to the rudiment of the mandibular artery in Elasmobranchii. In Dipnoi the mandibular artery atrophies, but the hyoid is sometimes preserved (Protopterus), and sometimes lost.

In Fishes provided with a well developed air-bladder this organ receives arteries, which arise sometimes from the dorsal aorta, sometimes from the caeliac arteries, and sometimes from the dorsal section of the last (fourth) branchial trunk. The latter origin is found in Polypterus and Amia, and seems to have been inherited by the Dipnoi where the air-bladder forms a true lung.

The pulmonary artery of all the air-breathing Vertebrata is derived from the pulmonary artery of the Dipnoi.

In all the types above Fishes considerable changes are effected in the primitive arrangement of the arteries in the visceral arches.

In Amphibia the piscine condition is most nearly retained 2 . The mandibular artery is never developed, and the hyoid artery is imperfect, being only connected with the cephalic vessels and never directly joining the dorsal aorta. It is moreover developed later than the arteries of the true branchial arches behind. The subclavian arteries spring from the common trunks which unite to form the dorsal aorta.

In the Urodela there are developed, in addition to the hyoid,

1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but is distinctly figured in Lereboullet (No. 71).

2 In my account of the Amphibia, Gotte (No. 296) has been followed.

646 ARTERIES OF THE AMNIOTA.

four branchial arteries. The three foremost of these at first supply gills, and in the Perennibranchiate forms continue to do so through life. The fourth does not supply a gill, and very early gives off, as in the Dipnoi, a pulmonary branch.

The hyoid artery soon sends forward a lingual artery from its ventral end, and is at first continued to the carotid which grows forward from the dorsal part of the first branchial vessel.

In the Caducibranchiata, where the gills atrophy, the following changes take place. The remnant of the hyoid is continued entirely into the lingual artery. The first branchial is mainly continued into the carotid and other cephalic branches, but a narrow remnant of the trunk, which originally connected it with the dorsal aorta, remains, forming what is known as a ductus Botalli. A rete mirabile on its course is the remnant of the original gill.

The second and third branchial arches are continued as simple trunks into the dorsal aorta, and the blood from the fourth arch mainly passes to the lungs, but a narrow ductus Botalli still connects this arch with the dorsal aorta.

In the Anura the same number of arches is present in the embryo as in the Urodela, all four branchial arteries supplying branchiae, but the arrangement of the two posterior trunks is different from that in the Urodela. The third arch becomes at a very early period continued into a pulmonary vessel, a relativelynarrow branch connecting it with the second arch. The fourth arch joins the pulmonary branch of the third. At the metamorphosis the hyoid artery loses its connection with the carotid, and the only part of it which persists is the root of the lingual artery. The first branchial artery ceases to join the dorsal aorta, and forms the root of the carotid : the so-called carotid gland placed on its course is the remnant of the gill supplied by it before the metamorphosis.

The second artery forms a root of the dorsal aorta. The third, as in all the Amniota, now supplies the lungs, and also sends off a cutaneous branch. The fourth disappears. The connection of the pulmonary artery with both the third and fourth branchial arches in the embryo appears to me clearly to indicate that this artery was primitively derived from the fonrtli arc/i as in the Urodela, and that its permanent connection

ARTERIAL SYSTEM.

647

with the third arch in the Anura and in all the Amniota is secondary.

In the Amniota the metamorphosis of the arteries is in all cases very similar. Five arches, viz. the mandibular, hyoid, and three branchial arches are always developed (fig. 364), but, owing to the absence of branchiae, never function as branchial arteries. Of these the main parts of the first two, connecting the truncus arteriosus with the collecting trunk into which the arterial arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.

The anterior part of the collecting trunk into which these vessels fall is not obliterated when they disappear, but is on the contrary continued forwards as a vessel supplying the brain, homologous with that found in Fishes. It constitutes the internal carotid. Similarly the anterior part of the trunk from which the mandibular and hyoid arteries sprang is continued forwards as a small vessel 1 , which at first passes to the oral region and constitutes in Reptiles the lingual artery, homologous with the lingual artery of the Amphibia ; but in Birds and Mammals becomes more important, and is then known as the external carotid (fig. 125). By these changes the roots of the external and internal carotids spring respectively from the ventral and dorsal ends of the primitive third artery, i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and thus this arterial arch persists in all types as the common carotid,

FIG. 364. DIAGRAM OF THE ARRANGEMENT OF THE ARTERIAL ARCHES IN AN EMBRYO OF ONE OF THE

AMNIOTA. (From Gegenbaur ; after RATHKE.)

a. ventral aorta; a", dorsal aorta; ' 2 > 3> 4> 5- arterial arches ; c. carotid artery.

1 His (No. 232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery, and one from the hyoid artery, forming the lingual artery. The vessel from which they spring is the external carotid. These observations of His will very probably be found to hold true for other types.

6 4 8

ARTERIAL ARCHES OF THE AMNIOTA.

and the basal part of the internal carotid. The trunk connecting the third arterial arch with the system of the dorsal aorta persists in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but is lost in the remaining Reptiles and in Birds and Mammals (fig. 366 B, C, D). It disappears earliest in Mammals (fig. 365 C), later in Birds (fig. 365 B), and still later in the majority of Reptiles.

The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta.

In all Reptiles it persists on both sides (fig. 366 A and B), but with the division of the truncus arteriosus into three vessels

ad

FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after Rathke.)

The first two arches have disappeared in all three. In A and B the last three are still complete, but in C the last two are alone complete.

/. pulmonary artery springing from the fifth arch, but still connected with the system of the dorsal aorta by a ductus Botalli; c. external carotid; <'. internal carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of fore-limb.

one of these, i.e. that opening furthest to the left side of the ventricle (e and d), is continuous with the right fourth arch, and also with the common carotid arteries (c) ; while a second springing from the right side of the ventricle is continuous with the left fourth arch (Ji and f). The right and left divisions of the fourth arch meet however on the dorsal side of the oesophagus to give origin to the dorsal aorta (g).

In Birds (fig. 366 C) the left fourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains as

ARTERIAL SYSTEM.

649

the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous with all the systemic arteries. Thus it comes about that in Birds the right fourth arch (e) alone gives rise to the dorsal aorta.

In Mammals (fig. 366 D) the truncus arteriosus is only divided into two, but the left fourth arch (>), instead of the right, is that continuous with the dorsal aorta, and the right fourth arch (/) is only continued into the right vertebral and right subclavian arteries.

The fifth arch always gives origin to the pulmonary artery (fig. 365, /) and is continuous with one of the divisions of the truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and Birds (fig. 366 C, i] and probably in Crocodilia, the right and left pulmonary arteries spring respectively from the right and left fifth arches, and during the greater part of embryonic life the parts of the fifth arches between the origins of the pulmonary arteries and the system of the dorsal aorta are preserved as ductus Botalli. These ductus Botalli persist for life in the Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig. 366 D, m) only one of the fifth arches gives origin to the two pulmonary arteries, viz. that on the right side in Ophidia, and the left in Mammalia.

The ductus Botalli of the fifth arch (known in Man as the ductus arteriosus) of the side on which the pulmonary arteries are formed, may remain (e.g. in Man) as a solid cord connecting the common stem of the pulmonary aorta with the systemic aorta.

The main history of the arterial arches in the Amniota has been sufficiently dealt with, and the diagram, fig. 366, copied from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely remains for me to say a few words about the subclavian and vertebral arteries.

The subclavian arteries in Fishes usually spring from the trunks connecting the branchial veins with the dorsal aorta. This origin, which is also found in Amphibia, is typically found in the embryos of the Amniota. In the Lizards this origin persists through life, but both subclavians spring from the right

650

ARTERIAL ARCHES OF THE AMNIOTA.

side. In most other types the origin of the subclavians is carried upwards, so that they usually spring from a trunk common to them and the carotids (arteria anonyma) (Birds and some Mammals); or the left one, as in Man and some other Mammals, arises from the systemic aorta just beyond the carotids. Various further modifications in the origin of the subclavians of the same general nature are found in Mammalia, A 13

FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL

ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From Mivart ; after Rathke.)

A. a. internal carotid; b. external carotid ; c. common carotid; d. ductus Botalli between the third and fourth arches ; e. right aortic trunk ; /. subclavian ; g. dorsal aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli between the pulmonary artery and the system of the dorsal aorta.

B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic trunk; e. vertebral artery;/, left aortic trunk of dorsal aorta; h. pulmonary artery ; i. ductus Botalli of pulmonary artery.

C. a. internal carotid ; b. external carotid ; c. common carotid ; d. systemic aorta; e. fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal aorta; h, left subclavian (fourth arch of left side); i. pulmonary artery; k. and /. right and left ductus Botalli of pulmonary arteries.

D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k. right vertebral; /. continuation of right subclavian; in. pulmonary artery; n. ductus Botalli of pulmonary artery.

THE VENOUS SYSTEM.

6 5 I

but they need not be specified in detail. The vertebral arteries usually arise in close connection with the subclavians, but in Birds they arise from the common carotids.

BIBLIOGRAPHY of the Arterial System.

(496) H. Rathke. " Ueb. d. Entwick. d. Arterien vv. bei d. Saugethiere von d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(-197) H. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier." Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

TJie Venous System,.

The venous system, as it is found in the embryos of Fishes, consists in its earliest condition of a single large trunk, which traverses the splanchnic mesoblast investing the part of the alimentary tract behind the heart. This trunk is directly continuous in front with the heart, and underlies the alimentary canal through both its praeanal and postanal sections. It is shewn in section in fig. 367, v, and may be called the subintestinal vein. This vein has been found in the embryos of Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs parallel to the dorsal aorta above, into which it is sometimes continued behind (Teleostei, Ganoidei, etc.).

In Elasmobranch embryos the subintestinal vein terminates, as may be gathered from sections (fig. 368, v.cau), shortly before the end of the tail. The same series of sections also shews that at the cloaca, where the gut enlarges and comes in contact with the skin, this vein bifurcates, the two branches uniting into a single vein both in front of and behind the cloaca.

In most Fishes the anterior part of this vein atrophies, the caudal section alone remaining, but the anterior section of it persists in the fold of the intestine in Petromyzon, and also remains in the spiral valve of some Elasmobranchii. In Amphioxus, moreover, it forms, as in the embryos of higher types, the main venous trunk, though even here it is usually broken up into two or three parallel vessels.

It no doubt represents one of the primitive longitudinal trunks of the vermiform ancestors of the Chordata. The heart and the branchial artery constitute a specially modified anterior continuation of this vein. The

652

THE SUBINTESTINAL VEIN.

-p.o

rp.r.

dilated portal sinus of Myxine is probably also part of it ; and if this is really rhythmically contractile 1 the fact would be interesting as shewing that this quality, which is now localised in the heart, was once probably common to the subintestinal vessel for its whole length.

On the development of the cardinal veins (to be described below) considerable changes are effected in the subintestinal vein. Its postanal section, which is known in the adult as the caudal vein, unites with the cardinal veins. On this junction being effected retrogressive changes take place in the praeanal section of the original subintestinal vessel. It breaks up in front into a number of smaller vessels, the most important of which is a special vein, which lies in the fold of the spiral valve, and which is more conspicuous in some Elasmobranchii than in Scyllium, in which the development of the vessel has been mainly studied. The lesser of the two branches connecting it round the cloaca with the caudal vein first vanishes, and then the larger ; and the two posterior cardinals are left as the sole forward continuations of the caudal vein. The latter then becomes prolonged forwards, so that the two cardinals open into it some little distance in front of the hind end of the kidneys. By these changes, and by the disappearance of the postanal section of the gut, the caudal vein is made to appear as a supraintestinal and not, as it really is, a subintestinal vessel.

From the subintestinal vein there is given off a branch which supplies the yolk-sack. This leaves the subintestinal vein close

1 J. Miiller holds that this sack is not rhythmically contractile.

FIG. 367. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle plate; ;;//'. inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body; st. segmental tube ; sd. segmental duct ; sp.v. spiral valve; v. subintestinal vein ; p.o. primitive generative cells.

THE VENOUS SYSTEM.

653

to the liver. The liver, on its development, embraces the subintestinal vein, which then breaks up into a capillary system in the liver, the main part of its blood coming at this period from the yolk-sack.

The portal system is thus established from the subintestinal vein ; but is eventually joined by the various visceral, and sometimes by the genital, veins as they become successively developed.

The blood from the liver is brought back to the sinus venosus by veins known as the hepatic veins, which, like the hepatic capillary system, are derivatives of the subintestinal vessel.

There join the portal system in Myxinoids and many Teleostei a number of veins from the anterior abdominal walls, representing a commencement of the anterior abdominal or epigastric vein of higher types 1 .

In the higher Vertebrates the original subintestinal vessel never attains a full development, even in the embryo. It is represented by (i) the ductus

FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x. subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta; v.cait. caudal vein.

1 Stannius, Vergleich. Anat., p. 251.

654

THE CARDINAL VEINS.

venosus, which, like the true subintestinal vein, gives origin (in the Amniota) to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether the partial atrophy of the subintestinal vessel was primitively caused by the development of the cardinal veins, or for some other reason, it is at any rate a fact that in all existing Fishes the cardinal veins form the main venous channels of the trunk.

Their later development than the subintestinal vessel as well as their absence in Amphioxus, probably indicate that they became evolved, at any rate in their present form, within the Vertebrate phylum.

The embryonic condition of the venous system, with a single large subintestinal vein is, as has been stated, always modified by the development of a paired system of vessels, known as the cardinal veins, which bring to the heart the greater part of the blood from the trunk.

The cardinal veins appear in Fishes as four paired longitudinal trunks (figs. 363 and 369), two anterior (/) and two posterior (c). They unite into two transverse trunks on either side, known as the ductus Cuvieri (dc), which fall into the sinus venosus, passing from the body wall to the sinus by a lateral mesentery of the heart already spoken of (p. 627, fig. 352). The anterior pair, known as the anterior cardinal or jugular veins, bring to the heart the blood from the head and neck. They are placed one on each side above the level of the branchial arches (fig. 299, a.cv). The posterior cardinal veins lie immediately dorsal to the mesonephros (Wolfifian body), and are mainly supplied by the blood from this organ and from the walls of the body (fig. 275, c.a.v). In many forms (Cyclostomata, Elasmobranchii and many Teleostei) they unite posteriorly with the caudal veins in the manner already described, and in a large number of instances the connecting branch between the two systems, in its passage through the mesonephros, breaks up into a capillary network, and so gives rise to a renal portal system.

The vein from the anterior pair of fins (subclavian) usually unites with the anterior jugular vein.

j

FIG. 369. DIAGRAM OF THE PAIRED VENOUS SYSTEM

OF A FISH. (From Gegenbaur. )

j. jugular vein (anterior cardinal vein) ; c. posterior cardinal vein; //. hepatic veins ; sv. sinus venosus ; dc. ductus Cuvieri.

THE VENOUS SYSTEM. 655

The venous system of the Amphibia and Amniota always differs from that of Fishes in the presence of a new vessel, the vena cava inferior, which replaces the posterior cardinal veins; the latter only being present, in their piscine form, during embryonic life. It further differs from that of all Fishes, except the Dipnoi, in the presence of pulmonary veins bringing back the blood directly from the lungs.

In the embryos of all the higher forms the general characters of the venous system are at first the same as in Fishes, but with the development of the vena cava inferior the front sections of the posterior cardinal veins atrophy, and the ductus Cuvieri, remaining solely connected with the anterior cardinals and their derivatives, constitute the superior venae cavae. The inferior cava receives the hepatic veins.

Apart from the non-development of the subintestinal vein the visceral section of the venous system is very similar to that in Fishes.

The further changes in the venous system must be dealt with separately for each group.

Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior cardinal veins arise as in Pisces. From the former the internal jugular vein arises as a branch ; the external jugular constituting the main stem. The subclavian with its large cutaneous branch also springs from the system of the anterior cardinal. The common trunk formed by the junction of these three veins falls into the ductus Cuvieri.

The posterior cardinal veins occupy the same position as in Pisces, and unite behind with the caudal veins, which Gotte has shewn to be originally situated below the post-anal gut. The iliac veins unite with the posterior cardinal veins, where the latter fall into the caudal vein. The original piscine condition of the veins is not long retained. It is first of all disturbed by the development of the anterior part of the important unpaired venous trunk which forms in the adult the vena cava inferior. This is developed independently, but unites behind with the right posterior cardinal. From this point backwards the two cardinal veins coalesce for some distance, to give rise to the posterior section of the vena cava inferior, situated between the kidneys 1 . The anterior sections of the cardinal veins subsequently atrophy. The posterior part of the cardinal veins, from their junction with the vena cava inferior to the caudal veins, forms a rhomboidal figure. The iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitu 1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and cannot be considered as completely established.

656 VEINS OF THE SNAKE.

dinal vessel on the outer border of the kidneys, which receives transverse vertebral veins and transmits their blood to the kidneys, thus forming a renal portal system. The anterior limbs of the rhomboid formed by the cardinal veins soon atrophy, so that the blood from the hind limbs can only pass to the inferior vena cava through the renal portal system. The posterior parts of the two cardinal veins (uniting in the Urodela directly with the unpaired caudal vein) still persist. The iliac veins also become directly connected with a new vein, the anterior abdominal vein, which has meanwhile become developed. Thus the iliac veins become united with the system of the vena cava inferior through the vena renalis advehens on the outer border of the kidney, and with the anterior abdominal veins by the epigastric veins.

The visceral venous system begins with the development of two vitelline veins, which at first join the sinus venosus directly. They soon become enveloped in the liver, where they break up into a capillary system, which is also joined by the other veins from the viscera. The hepatic system has in fact the same relations as in Fishes. Into this system the anterior abdominal vein also pours itself in the adult. This vein is originally formed of two vessels, which at first fall directly into the sinus venosus, uniting close to their opening into the sinus with a vein from the truncus arteriosus. They become prolonged backwards, and after receiving the epigastric veins above mentioned from the iliac veins, and also veins from the allantoic bladder, unite behind into a single vessel. Anteriorly the right vein atrophies and the left continues forward the unpaired posterior section.

A secondary connection becomes established between the anterior abdominal vein and the portal system ; so that the blood originally transported by the former vein to the heart becomes diverted so as to fall into the liver. A remnant of the primitive connection is still retained in the adult in the form of a small vein, the so-called vena bulbi posterior, which brings the blood from the walls of the truncus arteriosus directly into the anterior abdominal vein.

The pulmonary veins grow directly from the heart to the lungs.

For our knowledge of the development of the venous system of the Amniota we are mainly indebted to Rathke.

Reptilia. As an example of the Reptilia the Snake may be selected, its venous system having been fully worked out by Rathke in his important memoir on its development (No. 300).

The anterior (external jugular) and posterior cardinal veins are formed in the embryo as in all other types (fig. 370, vj and vc] ; and the anterior cardinal, after giving rise to the anterior vertebral and to the cephalic veins, persists with but slight modifications in the adult ; while the two ductus Cuvieri constitute the superior venos cavas.

The two posterior cardinals unite behind with the caudal veins. They are placed in the usual situation on the dorsal and outer border of the kidneys.

THE VENOUS SYSTEM.

657

U FIG. 370. ANTERIOR PORTION OF THE VENOUS SYSTEM OF AN EMBRYONIC SNAKE. (From Gegenbaur; after Rathke.)

vc. posterior cardinal vein; vj. jugular vein; DC. ductus Cuvieri ; vu. allantoic vein ; v. ventricle ; ba. truncus arteriosus ; a. visceral clefts ; /. auditory vesicle.

With the development of the vena cava inferior, to be described below, the blood from the kidneys becomes mainly transported by this vessel to the heart ; and the section of the posterior cardinals opening into the ductus Cuvieri gradually atrophies, their posterior parts remaining however on the outer border of the kidneys as the vena? renales advehentes 1 .

While the front part of the posterior cardinal veins is undergoing atrophy, the intercostal veins, which originally poured their blood into the posterior cardinal veins, become also connected with two longitudinal veins the posterior vertebral veins which are homologous with the azygos and hemiazygos veins of Man ; and bear the same relation to the anterior vertebral veins that the anterior and posterior cardinals do to each other.

These veins are at first connected by trans verse anastomoses with the posterior cardinals, but, on the disappearance of the front part of the latter, the whole of the blood from the intercostal veins falls into the posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into the jugular vein.

The posterior vertebral veins are at first symmetrical, but after becoming connected by transverse anastomoses, the right becomes the more important of the two.

The vena cava inferior, though considerably later in its development than the cardinals, arises fairly early. It constitutes in front an unpaired trunk, at first very small, opening into the right allantoic vein, close to the heart. Posteriorly it is continuous with two veins placed on the inner border of the kidneys 2 .

The vena cava inferior passes through the dorsal part of the liver, and in doing so receives the hepatic veins.

The portal system is at first constituted by the vitelline vein, which is directly continuous with the venous end of the heart, and at first receives the two ductus Cuvieri, but at a later period unites with the left ductus.

1 Rathke's account of the vena renalis advehens is thus entirely opposed to that which Gotte gives for the Frog, but my own observations on the Lizard incline me to accept Rathke's statements, for the Amniota at any rate.

2 The vena cava inferior does not according to Rathke's account unite behind with the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Gb'tte questions the accuracy of Rathke's statements on this head, but my own observations are entirely in favour of Rathke's observations, and lend no support whatever to Gotte's views.

B. III.

658 VEINS OF THE CHICK.

It soon receives a mesenteric vein bringing the blood from the viscera, which is small at first but rapidly increases in importance.

The common trunk of the vitelline and mesenteric veins, which may be called the portal vein, becomes early enveloped by the liver, and gives off branches to this organ, the blood from which passes by the hepatic veins to the vena cava inferior. As the branches in the liver become more important, less and less blood is directly transported to the heart, and finally the part of the original vitelline vein in front of the liver is absorbed, and the whole of the blood from the portal system passes from the liver into the vena cava inferior.

The last section of the venous system to be dealt with is that of the anterior abdominal vein. There are originally, as in the Anura, two veins belonging to this system, which owing to the precocious development of the bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}.

These veins, running along the anterior abdominal wall, are formed somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. They unite with two epigastric veins (homologous with those in the Anura), which connect them with the system of the posterior cardinal veins. The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein. This vein at first receives the vena cava inferior close to the heart, but eventually the junction of the two takes place in the region of the liver, and finally the anterior abdominal vein (as it comes to be after the atrophy of the allantois) joins the portal system and breaks up into capillaries in the liver 1 .

In Lizards the iliac veins join the posterior cardinals, and so pour part of their blood into the kidneys ; they also become connected by the epigastric veins with the system of the anterior abdominal or allantoic vein. The subclavian veins join the system of the superior venae cavas.

The venous system of Birds and Mammals differs in two important points from that of Reptilia and Amphibia. Firstly the anterior abdominal vein is only a foetal vessel, forming during foetal life the allantoic vein ; and secondly a direct connection is established between the vena cava inferior and the veins of the hind limbs and posterior parts of the cardinal veins, so that there is no renal portal system.

Aves. The Chick may be taken to illustrate the development of the venous system in Birds.

On the third day, nearly the whole of the venous blood from the body of the embryo is carried back to the heart by two main venous trunks, the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on each side to form the short transverse ductus Cuvieri (DC), both of which unite with the sinus venosus close to the heart. As the head and neck continue to enlarge, and the wings become developed, the single anterior

1 The junction between the portal system and the anterior abdominal vein is apparently denied by Rathke (No. 300, p. 173), hut this must he an error on his part.

THE VENOUS SYSTEM.

659

V.C.L

cardinal or jugular vein (fig. 371, /), of each side, is joined by two new veins : the vertebral vein, bringing back blood from the head and neck, and the subclavian vein from the wing (W\

On the third day the posterior cardinal veins are the only veins which return the blood from the hinder part of the body of the embryo.

About the fourth or fifth day, however, the vena cava inferior (fig. 371, V.C.L) makes its appearance. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running backward in the middle line below the aorta, and speedily losing itself in the tissues of the Wolffian bodies. When the true kidneys are formed it also receives blood from them, and thenceforward enlarging rapidly becomes the channel by which the greater part of the blood from the hinder part of the body finds its way to the heart. In proportion as the vena cava inferior increases in size, the posterior cardinal veins diminish.

The blood originally coming to them from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins, similar to those in Reptilia, which are however placed dorsally to the heads of the ribs, and join the anterior vertebral veins. With their appearance the anterior parts of the posterior cardinals disappear. The blood from the hind limbs becomes transported directly through the kidney into the vena cava inferior, without forming a renal portal system 1 .

On the third day the course of the vessels from the yolk-sack is very simple. The two vitelline veins, of which the right is already the smaller, form the ductus venosus, from which, as it passes through the liver on its way to the heart, are given off the two sets of vena advehentes and vena revehentes (fig. 371).

With the appearance of the allantois on the fourth day, a new feature is introduced. From the ductus venosus there is given off a vein which quickly divides into two branches. These, running along the ventral walls of the body from which they receive some amount of blood, pass to the allantois. They are the allantoic veins (fig. 371, U] homologous with the anterior abdominal vein of the lower types. They unite in front to form a single vein, which becomes, by reason of the rapid growth of the allantois, very long. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates ; and, its two

FIG. 371. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK AT THE COMMENCEMENT OF THE FIFTH

DAY.

H. heart ; d. c. ductus Cuvieri. Into the ductus Cuvieri of each side fall/, the jugular vein, W. the vein from the wing, and c. the inferior cardinal vein ; S. V. sinus venosus ; Of. vitelline vein ; U. allantoic vein, which at this stage gives off branches to the bodywalls ; V.C.l. inferior vena cava ; /. liver.

The mode in which this is effected requires further investigation.

42 2

66o

VEINS OF THE CHICK.

branches becoming large and conspicuous, there still appear to be two main allantoic veins. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.

On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more developed, it acquires a distinct venous system, and its blood is returned by veins which form a trunk, the mesenteric vein (fig. 372, M") falling into the vitelline vein at its junction with the allantoic vein.

These three great veins, in fact, form a large common trunk, which enters at once into the liver, and which we may now call the portal vein (fig. 372, P. V}. This, at its entrance into the liver, partly breaks up into the vena advehentes, and partly continues as the ductus venosus (D.V} straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the venas revehentes, carrying back the blood which circulates through the hepatic capillaries, join the ductus venosus close to its exit from the liver. By the time however that the vena cava has become a large and important vessel it is found that the venae revehentes, or as we may now call them the hepatic veins, have shifted their embouchment, and now fall directly into that vein, the ductus venosus making a separate junction rather higher up (fig. 372).

This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels ; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes obliterated ; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the hepatic veins into the vena cava.

Although the allantoic (anterior abdominal) vein is obliterated in the adult, there is nevertheless established an anastomosis between the portal system and the veins bringing the blood from the limbs to the vena cava

FIG. 372. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THE LATER DAYS OF INCUBATION.

H. heart ; V.S.R. right vena cava superior; V.S.L. left vena cava superior. The two venas cavrc superiores are the original 'ductus Cuvieri,' they open into the sinus venosus. J. jugular vein; Su.V. anterior vertebral vein ; In. V. inferior vertebral vein ; W. subclavian; V.C.I, vena cava inferior; D. V. ductus venosus ; P. V. portal vein ; M. mesenteric vein bringing blood from the intestines into the portal vein ; O.f. vitelline vein ; U. allantoic vein. The three last mentioned veins unite together to form the portal vein ; /. liver.

THE VENOUS SYSTEM.

66l

inferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as the coccygeo-mesenteric vein, which joins the portal vein ; while at the same time the posterior pelvic veins are connected with the common iliac veins by a vessel which unites with them close to their junction with the coccygeo-mesenteric vein.

Mammalia. In Mammals the same venous trunks are developed in the embryo as in other types (fig. 373 A). The anterior cardinals or external jugulars form the primitive veins of the anterior part of the body, and the internal jugulars and anterior vertebrals are subsequently formed. The subclavians (fig. 373 A, j), developed on the formation of the anterior limbs, also pour their blood into these primitive trunks. In the lower Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc., the two ductus Cuvieri remain as the two superior venae cavae, but more usually an anastomosis arises between the right and left innominate veins, and eventually the whole of the blood of the left superior cava is carried to the right side, and there is left only a single superior cava (fig. 373 B and C).

F IG - 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF

MAMMALS (MAN). (From Gegenbaur.)

j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein.

A. Stage in which the cardinal veins have already disappeared. Their position is indicated by dotted lines.

B. Later stage when the blood from the left jugular vein is carried into the right to form the single vena cava superior ; a remnant of the left superior cava being however still left.

C. Stage after the left vertebral vein has disappeared; the right vertebral remaining as the azygos vein. The coronary vein remains as the last remnant of the left superior vena cava.

A small rudiment of the left superior cava remains however as the sinus coronartus and receives the coronary vein from the heart (figs. 373 C, cor and 374, cs).

The posterior cardinal veins form at first the only veins receiving the

662

THE VEINS OF MAMMALIA.

blood from the posterior part of the trunk and kidneys ; and on the development of the hind limbs receive the blood from them also.

As in the types already described an unpaired vena cava inferior becomes eventually developed, and gradually carries off a larger and larger portion of the blood originally returned by the posterior cardinals. It unites with the common stem of the allantoic and vitelline veins in front of the liver.

At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior (fig. 374, il}. These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins (fig. 374, hy). Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia.

Posterior vertebral veins, similar to those of Reptilia and Birds, are established in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior

FIG. 374. DIAGRAM OF THE CHIEF

VENOUS TRUNKS OF MAN. (From Gegenbaur.)

cs. vena cava superior ; s. subclavian vein ; ji. internal jugular ; je. external jugular ; az. azygos vein ; ha. hemiazygos vein ; c. clotted line shewing previous position of cardinal veins ; ci. vena cava inferior ; r. renal veins ; il. iliac ; hy. hypogastric veins ; h. hepatic veins.

The dotted lines shew the position of embryonic vessels aborted in the adult.

cardinal veins (fig. 373 A) 1 .

On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374, f), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins (Ay).

Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein (fig. 374, az), the persisting part of the left forming the hemiazygos vein (ha}.

The remainder of the venous system is formed in the embryo of the vitelline and allantoic veins, the former being eventually joined by the mesenteric vein so as to constitute the portal vein.

1 Rathke, as mentioned above, holds that in the Snake the front part of the posterior cardinals completely aborts. Further investigations are required to shew whether there really is a difference between Mammalia and Reptilia in this matter.

THE VENOUS SYSTEM. 663

The vitelline vein is the first part of this system established, and divides near the heart into two veins bringing back the blood from the yolk-sack (umbilical vesicle). The right vein soon however aborts.

The allantoic (anterior abdominal) veins are originally paired. They are developed very early, and at first course along the still widely open somatic walls of the body, and fall into the single vitelline trunk in front. The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.

The succeeding changes have been somewhat differently described by Kolliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii.

With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver.

The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava.

At the close of foetal life the allantoic vein becomes obliterated up to its place of entrance into the liver ; the ductus venosus becomes a solid cord the so-called round ligament and the whole of the venous blood is brought to the liver by the portal vein 1 .

Owing to the allantoic (anterior abdominal) vein having merely a fcetal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established.

BIBLIOGRAPHY of the Venous System.

(498) J. Marshall. "On the development of the great anterior veins." Phil. Trans., 1859.

(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Saugethieren." MeckeVs Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere." Bericht. Jib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke (Nos. 299, 300, and 301).

1 According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a secondary connection established in the latter part of foetal life.

664 LYMPHATIC SYSTEM.

Lymphatic System.

The lymphatic system arises from spaces in the general parenchyma of the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.

In all the true Vertebrata certain parts of the system form definite trunks communicating with the venous system ; and in the higher types the walls of the main lymphatic trunks become quite distinct.

But little is known with reference to the ontogeny of the lymphatic vessels, but they originate late in larval life, and have at first the form of simple intercellular spaces.

The lymphatic glands appear to originate from lymphatic plexuses, the cells of which produce lymph corpuscles. It is only in Birds and Mammals, and especially in the latter, that the lymphatic glands form definite structures.

The Spleen. The spleen, from its structure, must be classed with the lymphatic glands, though it has definite relations to the vascular system. It is developed in the mesoblast of the mesogastrium, usually about the same time and in close connection with the pancreas.

According to Miiller and Peremeschko the mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which uniting with like processes from other cells form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these cells take place at a later period to form the so-called Malpighian corpuscles of the spleen.

BIBLIOGRAPHY of Spleen.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad. Wiss., Vol. LVI. 1867.

Suprarenal ^bodies.

In Elasmobranch Fishes two distinct sets of structures are found, both of which have been called suprarenal bodies. As shewn in the sequel both of these structures probably unite in the higher types to form the suprarenal bodies.

One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentally arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they are

SUPRARENAL BODIES. 665

closely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body.

The second body consists of an unpaired column of cells placed between the dorsal aorta and unpaired caudal vein, and bounded on each side by the posterior parts of the kidney. I propose to call it the interrenal body. In front it overlaps the paired suprarenal bodies, but does not unite with them. It is formed of a series of well-marked lobules, etc. In the fresh state Leydig (No. 506) finds that "fat molecules form the chief mass of the body, and one finds freely imbedded in them clear vesicular nuclei." As may easily be made out from hardened specimens it is invested by a tunica propria, which gives off septa dividing it into well-marked areas filled with polygonal cells. These cells constitute the true parenchyma of the body. By the ordinary methods of hardening, the oil globules, with which they are filled in the fresh state, completely disappear.

The paired suprarenal bodies (Balfour, No. 292, pp. 242 244) are developed from the sympathetic ganglia. These ganglia, shewn in an early stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a glandular part. The former constitutes the sympathetic ganglia of the adult ; the latter the true paired suprarenal bodies. The interrenal body is however developed (Balfour, No. 292, pp. 245 247) from indifferent mesoblast cells between the two kidneys, in the same situation as in the adult.

The development of the suprarenal bodies in the Amniota has been most fully studied by Braun (No. 503) in the Reptilia.

In Lacertilia they consist of a pair of elongated yellowish bodies, placed between the vena renalis revehens and the generative glands.

They are formed of two constituents, viz. (i) masses of brown cells placed on the dorsal side of the organ, which stain deeply with chromic acid, like certain of the cells of the suprarenals of Mammalia, and (2) irregular cords, in part provided with a lumen, filled with fat-like globules l , amongst which are nuclei. On treatment with chromic acid the fat globules disappear, and the cords break up into bodies resembling columnar cells.

The dorsal masses of brown cells are developed from the sympathetic ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent mesoblast cells as a thickening in the lateral walls of the inferior vena cava, and the cardinal veins continuous with it. The observations of Brunn (No. 504) on the Chick, and Kolliker (No. 298, pp. 953955) n the Mammal, add but little to those of Braun. They shew that the greater part of the gland (the cortical substance) in these two types is derived from the mesoblast, and that the glands are closely connected with sympathetic ganglia ; while Kolliker also states that the posterior part of the organ is unpaired in the embryo rabbit of 1 6 or 17 days.

The structure and development of what I have called the interrenal body

1 These globules are not formed of a true fatty substance, and this is also probably true for the similar globules of the interrenal bodies of Elasmobranchii.

666 SUPRARENAL BODIES.

in Elasmobranchii so closely correspond with that of the mesoblastic part of the suprarenal bodies of the Reptilia, that I have very little hesitation in regarding them as homologous 1 ; while the paired bodies in Elasmobranchii, derived from the sympathetic ganglia, clearly correspond with the part of the suprarenals of Reptilia having a similar origin ; although the anterior parts of the paired suprarenal bodies of Fishes have clearly become aborted in the higher types.

In Elasmobranch Fishes we thus have (i) a series of paired bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part.

BIBLIOGRAPHY of the Suprarenal bodies,

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien. " Arbeit, a. d. zool.-zoot. Institut Wurzlttrg, Vol. V. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." Archiv f. mikr. Anat., Vol. VIII. 1872.

(505) Fr. Leydig. Untersiich. iib. Fische u. fieptilten. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

1 The fact of the organ being unpaired in Elasmobranchii and paired in the Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired in the Rabbit.

CHAPTER XXII.

THE MUSCULAR SYSTEM.

IN all the Ccelenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of ectodermal or entodermal epithelial cells (figs. 375 and 376 B). The elements provided with these processes, which were first discovered by Kleinenberg, are known as myo-epithelial cells. Their contractile parts may either be striated (fig. 376) or non-striated (fig. 375). In some instances the epithelial part of the cell may nearly abort, its nucleus alone remaining (fig. 376 A) ; and in this way a layer of muscles lying completely below the surface may be established.

There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myo-epithelial cells of this kind. The more important of these groups are the Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and the Vertebrata 1 .

While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myo-epithelial cells, the mode of evolution of the

1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amoeboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vasoperitoneal vesicles are provided with muscular tails.

FIG. 375. MYO-EPITHELIAL CELLS OF HYDRA. (From Gegenbaur ; after Kleinenberg.)

m. contractile fibres.

668 THE MUSCULAR FIBRES.

muscular system of other types is still very obscure. The muscles may arise in the embryo from amoeboid or indifferent cells, and the Hertwigs 1 hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology.

The voluntary muscular system of the CJiordata.

The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myo-epithelial type. The embryonic muscle-cells are at first simple epithelial cells, but

FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after O. and R. Hertwig.)

A. Muscle-cell from the circular fibres of the subumbrella.

B. Myo-epithelial cells from the base of a tentacle.

soon become spindle-shaped : part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon in fig. 57.

The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which,

1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881.

THE MUSCULAR SYSTEM.

669

t>r

except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma.

The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites.

It has already been stated (pp. 292 ^296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elasmobranchii it will be convenient to commence with this group. Each somite is composed of two layers a somatic and a splanchnic both formed of a single row of columnar cells. Between these two layers is a cavity, which is at first directly continuous with the general body cavity, of which indeed it merely forms a specialised part (fig. 377). Before long the cavity becomes however completely constricted off from the permanent body cavity.

Very early (fig. 377) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove (% 378) that the cells in this situation (mp') have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and produce

FIG. 377. TRANSVERSE SECTION THROUGH THETRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

nc. neural canal ; pr. posterior root of spinal nerve ; x. subnotochordal rod ; ao. aorta ; sc. somatic mesoblast ; sf>. splanchnic mesoblast ; mp. muscle-plate ; mp', portion of muscle-plate converted into muscle ; Vr. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract.

THE MUSCLE-PLATES.

a mass of cells, placed between the muscles and the notochord ( Vr\ These cells form the commencing vertebral bodies, and have at first (fig. 378) the same segmentation as the somites from which they sprang.

After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates ; since they become directly converted into the whole voluntary muscular system of the trunk (fig. 379, mp}.

According to the statements of Bambeke and Go'tte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates (fig. 79). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle- plate of other Vertebrata, and exhibit a similar segmentation.

Osseous Fishes are stated to agree with Amphibians in the development of their somites and muscular system 1 , but further observations on this point are required.

In Birds the horizontal splitting of the mesoblast extends at first to the dorsal summit of the mesoblastic plates, but after the isolation of the somites the split between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part to persist. The somites on the second day, as seen in a transverse section (fig. 115, P.?'.), are somewhat quadrilateral in form but broader than they are deep.

Each at that time consists of a somewhat thick cortex of radi

FlG. 378. HORIZONTALSECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLL1UM CONSIDERABLY YOUNGER THAN 28 F.

The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.

ch. notochord ; ep. epiblast ; Vr, rudiment of vertebral body ; mp. muscle- plate ; mp' . portion of muscle-plate already differentiated into longitudinal muscles.

1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal., Vol. XI.

THE MUSCULAR SYSTEM. 671

ating rather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or lateral as they are at this period) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the upper and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16). The muscle-plate when formed (fig. 117) is found to consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity ; and no sooner is the muscle-plate formed than the middle portion of the inner layer becomes converted into longitudinal muscles. The avian muscle-plates have, in fact, precisely the same constitution as those of Elasmobranchii. The central space is clearly a remnant of the vertebral portion of the body cavity, which, though it wholly or partially disappears in a previous stage, reappears again on the formation of the muscle-plate.

The remainder of the somite, after the formation of the muscle-plate, is of very considerable bulk ; the cells of the cortex belonging to it lose their distinctive characters, and the major part of it becomes the vertebral rudiment.

In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to the dorsal summit of the mesoblastic plates, and the dorsal portions of the plates with their contained cavities become divided into somites, and are then separated off from the ventral. The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost. The further development resembles that in Birds.

The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements, take place on quite the same type throughout the Vertebrata, yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are important common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (i) the extension of the body cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscleplates ; (2) the primitive division of the vertebral plate into an outer (somatic) and an inner (splanchnic) layer, and the formation of a large part of the voluntary muscular system out of the inner

THE MUSCLE-PLATES.

sp.c

layer, which in all cases is converted into muscles earlier than the outer layer.

The conversion of the muscle-plates into muscles. It

will be convenient to commence this subject with a description of the changes which take place in such a simple type as that of the Elasmobranchii.

At the time when the muscleplates have become independent structures they form flat two-layered oblong bodies enclosing a slit-like central cavity (fig. 379, mp). The outer or somatic wall is formed of simple epithelial -like cells. The inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity of the plates the epithelium forming the remainder of the layer commences to insert itself; so that between the first-formed muscle and the cavity of the muscle-plate there appears a thin layer of cells, not however continuous throughout.

When first formed the muscleplates, as viewed from the exterior, have nearly straight edges ; soon however they become bent in the middle, so that the edges have an obtusely angular form, the apex of the angle being directed forwards. They are so arranged that the anterior edge of the one plate fits into the posterior edge of the one in front. In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa.

The growth of the plates is very rapid, and their upper ends

FIG. 379. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate; mp' . inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; si. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; z/. subintestinal vein ; P.O. primitive generative cells.

THE MUSCULAR SYSTEM. 673

soon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, mp'). It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness by the addition of fresh spindle-shaped muscle-cells from the somatic layer as well as by the division of the already existing cells.

Thus both layers of the muscle-plate are concerned in forming the great longitudinal lateral muscles, though the splanchnic layer is converted into muscles very much sooner than the somatic 1 .

Each muscle-plate is at first a continuous structure, extending from the dorsal to the ventral surface, but after a time it becomes divided by a layer of connective tissue, which becomes developed nearly on a level with the lateral line, into a dorso-lateral and a ventro-lateral section. The ends of the muscle-plates continue for a long time to be formed of undifferentiated columnar cells. The complicated outlines of the inter-muscular septa become gradually established during the later stages of development, causing the well-known appearances of the muscles in transverse sections, which require no special notice here.

The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs (fig. 346). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs (fig. 380). Small portions of several muscle-plates (m.pl) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original dis 1 The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (vide fig. 57, mp) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles.

B. in. 43

674

THE MUSCLE-PLATES.

3,-n,

FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.

The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length ; whereas they join the spinal cord half-way between two posterior roots.

sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally directed nerve springing from posterior root; nip. muscle-plate; mp'. part of muscleplate already converted into muscles; vi.pl. part of muscle-plate which gives rise to the muscles of the limbs; /. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric) duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of hepatic duct with duodenum ; umc. umbilical canal.

THE MUSCULAR SYSTEM. 675

tinctness. There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscleplates themselves, after giving off buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds.

In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out.

It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish- like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out.

In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles.

The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive inter-muscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles.

In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kolliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.

432

676 THE HEAD-CAVITIES.

Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.

It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kolliker however believes that they have an independent origin.

The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscleplates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Gotte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs.

In the higher Vertebrata on the other hand the entrance of the muscleplates into the limbs has not been made out (Kolliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscleplates.

TJte Somites and muscular system of the head.

The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent to the somites of the trunk : they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii.

The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head ; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior extremity of the alimentary canal ; and are continuous behind with the general body-cavity (fig. 20 A, vp}. I propose calling them the head-cavities. The cavities of the two sides have no communication with each other.

Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft ; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections :

THE MUSCULAR SYSTEM.

677

vn~.

(i) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft ; (3) a section behind the first branchial cleft.

The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the mandibular arch.

As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole headcavity becomes on each side divided into (i) a premandibular section ; (2) a mandibular section (vide fig. 29 A, PP] > (3) a hyoid section ; (4) sections in each of the branchial arches.

The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells (fig. 381, ipp}. It is situated close to the eye, and presents a rounded or sometimes a triangular figure in section. The two halves of the cavity are prolonged ventralwards, and meet below the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them.

The next division of the head-cavity, which from its position

FIG. 381. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.

The section, owing to the cranial flexure, cuts both the foreand the hind-brain. It shews the premandibular and mandibular head-cavities ipp and ipp, etc. The section is moreover somewhat oblique from side to side.

fb. fore-brain ; /. lens of eye ; m. mouth ; pt. upper end of mouth, forming pituitary involution; lao. mandibular aortic arch; ipp. and ipp. first and second head-cavities; \vc. first visceral cleft; V. fifth nerve ; aim. auditory nerve ; VII. seventh nerve ; aa. dorsal aorta ; acv. anterior cardinal vein ; ch, notochord.

678 THE HEAD-CAVITIES.

may be called the mandibular cavity, presents a spatulate shape, being dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft (fig. 20, pp}. Like the previous space it is lined by a short columnar epithelium.

The mandibular aortic arch is situated close to its inner side (fig. 381, 2pp). After becoming separated from the lower part (Marshall), the upper part of the cavity atrophies about the time of the appearance of the external gills. Its lower part also becomes much narrowed, but its walls of columnar cells persist. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. In each of the remaining arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal dilated portion appears, however, to be present in the third or hyoid section alone (fig. 20), and even there disappears very soon, after being segmented off from the lower part (Marshall). The cavities in the posterior parts of the head become much reduced like those in its anterior part, though at rather a later period. FlG . 382 . HORIZONTAL

It has been shewn that the divi- SECTION THROUGH THE PENULTIMATE VISCERAL ARCH OF

sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS. with the exception of the anterior, e p. epiblast; vc. pouch of early become atrophied, not so how- hypoblast which will form the

walls of a visceral cleit ; //. CVer their walls. The cells forming segment of body-cavity in vis the walls both of the dorsal and ven- ceral arch ; aa ' aortic arch ' tral sections of these cavities become elongated, and finally become converted into muscles. Their exact history has not been followed in its details, but they almost unquestionably become the musculus contrictor superficialis and musculus interbranchialis 1 ; and probably also musculus levator mandibuli and other muscles of the front part of the head.

The anterior cavity close to the eye remains unaltered much longer than the remaining cavities.

1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zcltschrift, Vol. vn.

THE MUSCULAR SYSTEM.

679

Its further history is very interesting. In my original account of this cavity (No. 292, p. 208) I stated my belief that its walls gave rise to the eye-muscles, and the history of this process has been to some extent worked out by Marshall in his important memoir (No. 509).

Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus internus of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity.

Marshall brings evidence to shew that the rectus externus (as might be anticipated from its nerve supply) has no connection with the walls of the premandibular head-cavity, and finds that it arises close to the position originally occupied by the second and third cavities. Marshall has not satisfactorily made out the mode of development of the obliquus superior.

The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account of which has already been given at p. 461.

Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45, /ic\ the Newt (Osborn and Scott), and various Reptilia (Parker).

BIBLIOGRAPHY.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat. and Phys., Vol. vi. 1872.

(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.

(510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthierc." Silz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. . Entwick. d. Wirbelthiere. Berlin, 1879.

Vide 2^0 Gotte (No. 296), Kolliker (N o. 298), Balfour (No. 292), Huxley, etc.

CHAPTER XXIII.

EXCRETORY ORGANS.

EXCRETORY organs consist of coiled or branched and often ciliated tubes, with an excretory pore opening on the outer surface of the body, and as a rule an internal ciliated orifice placed in the body-cavity. In forms provided with a true vascular system, there is a special development of capillaries around the glandular part of the excretory organs. In many instances the glandular cells of the organs are filled with concretions of uric acid or some similar product of nitrogenous waste.

There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in the animal kingdom, but although there is not a little to be said for holding all these organs to be derived from some common prototype, the attempt to establish definite homologies between them is beset with very great difficulties.

Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and in the Rotifera excretory organs of a similar form to those of the Platyelminthes are also present.

These organs (Fraipont, No. 513) are more or less distinctly paired, and consist of a system of wide canals, often united into a network, which open on the one hand into a pair of large tubes leading to the exterior, and on the other into fine canals which terminate by ciliated openings, either in spaces between the connective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). The fine canals open directly into the larger ones, without first uniting into canals of an intermediate size.

EXCRETORY ORGANS.

68 1

The two large tubes open to the exterior, either by means of a median posteriorly placed contractile vesicle, or by a pair of vesicles, which have a ventral and anterior position. The former type is characteristic of the majority of the Trematoda, Cestoda. and Rotifera, and the latter of the Nemertea and some Trematoda. In the Turbellaria the position of the external openings of the system is variable, and in a few Cestoda (Wagner) there are lateral openings on each of the successive proglottides, in addition to the terminal openings. The mode of development of these organs is unfortunately not known.

Mollusca. In the Mollusca there are usually present two independent pairs of excretory organs one found in a certain number of forms during early larval life only 1 , and the other always present in the adult.

The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and possibly also in Paludina. It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum. It is purely a larval organ, disappearing before the close of the veliger stage. In the aquatic Pulmonata, where it is best developed, it consists on each side of a V-shaped tube, with a dorsally-placed apex, containing an enlargement of the lumen. There is a ciliated cephalic limb, lined by cells with concretions, and terminating by an internal opening near the eye, and a nonciliated pedal limb opening to the exterior 3 .

Two irreconcilable views are held as to the development of this system. Rabl (Vol. II. No. 268) and Hatschek hold that it is developed in the mesoblast ; and Rabl states that in Planorbis it is formed from the anterior mesoblast cells of the mesoblastic bands. A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while

1 I leave out of consideration an external renal organ found in many marine Gasteropod larvte, vide Vol. II. p. 280.

2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool. exfJr. et gener., Vol. VIII.

3 The careful observations of Fol seem to me nearly conclusive in favour of this limb having an external opening, and the statement to the reverse effect on p. 280 of Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be corrected.

682 POLYZOA.

the continuations of the two limbs are formed by perforated mesoblast cells.

According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventralwards and forwards. It thus acquires its V-shaped form.

In the terrestrial Pulmonata the provisional excretory organs are, according to Fol, formed as epiblastic invaginations, in the same way as those in the aquatic Pulmonata, but have the form of simple non-ciliated sacks, without internal openings.

The permanent renal organ of the Mollusca consists typically of a pair of tubes, although in the majority of the Gasteropoda one of the two tubes is not developed. It is placed considerably behind the provisional renal organ.

Each tube, in its most typical form, opens by a ciliated funnel into the pericardial cavity, and has its external opening at the side of the foot. The pericardial funnel leads into a glandular section of the organ, the lining cells of which are filled with concretions. This section is followed by a ciliated section, from which a narrow duct leads to the exterior.

As to the development of this organ the same divergence of opinion exists as in the case of the provisional renal organ.

Rabl's careful observations on Planorbis (Vol. II. No. 268) tend to shew that it is developed from a mass of mesoblast cells, near the end of the intestine. The mass becomes hollow, and, attaching itself to the epiblast on the left side of the anus, acquires an opening to the exterior. Its internal opening is not established till after the formation of the heart. Fol gives an equally precise account, but states that the first rudiment of the organ arises as a solid mass of epiblast cells. Lankester finds that this organ is developed as a paired invagination of the. epiblast in Pisidium, and Bobretzky also derives it from the epiblast in marine Prosobranchiata. In Cephalopoda on the other hand Bobretzky's observations (I conclude this from his figures) indicate that the excretory sacks of the renal organs are derived from the mesoblast.

Polyzoa. Simple excretory organs, consisting of a pair of ciliated canals, opening between the mouth and the anus, have

EXCRETORY ORGAN>.

68 3

been found by Hatschek and Joliet in the Entoproctous Polyzoa, and are developed, according to Hatschek, by whom they were first found in the larva, from the mesoblast

Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, with both peritoneal and external openings, are found in the Brachiopoda. They undoubtedly serve as genital ducts, but from their structure are clearly of the same nature as the excretory organs of the Chaetopoda described below. Their development has not been worked out.

Chaetopoda. Two forms of excretory organ have been met with in the Chaetopoda. The one form is universally or nearly universally present in the adult, and typically consists of a pair of coiled tubes repeated in every segment. Each tube has an internal opening, placed as a rule in the segment in front of that in which the greater part of the organ and the external opening are situated.

There are great variations in the structure of these organs, which cannot be dealt with here. It may be noted however that the internal opening may be absent, and that there may be several internal openings for each organ (Polynoe). In the Capitellidae moreover several pairs of excretory tubes have been shewn by Eisig (No. 512) to be present in each of the posterior segments.

The second form of excretory organ has as yet only been found in the larva of Polygordius, and will be more conveniently dealt with in connection with the development of the excretory system of this form.

There is still considerable doubt as to the mode of formation of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277), from his observations on the Oligochasta, holds that they develop as outgrowths of the epithelial layer covering the posterior side of the dissepiments, and secondarily become connected with the epidermis.

Hatschek finds that in Criodrilus they arise from a continuous linear thickening of the somatic mesoblast, immediately beneath the epidermis, and dorsal to the ventral band of longitudinal muscles. They break up into S-shaped cords, the anterior end of each of which is situated in front of a dissepiment, and is formed at first of a single large cell, while the posterior part is

684 CHvETOPODA.

continued into the segment behind. The cords are covered by a peritoneal lining, which still envelopes them, when in the succeeding stage they are carried into the body-cavity. They subsequently become hollow, and their hinder ends acquire openings to the exterior. The formation of their internal openings has not been followed.

Kleinenberg is inclined to believe that the excretory tubes take their origin from the epiblast, but states that he has not satisfactorily worked out their development.

The observations of Risig (No. 512) on the Capitellidae support Kowalevsky's view that the excretory tubes originate from the lining of the peritoneal cavity.

Hatschek (No. 514) has given a very interesting account of the development of the excretory system in Polygordius.

The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383, npli), and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excretory organ found in some Molluscan larvae (vide p. 68 1).

Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.

In its earliest stage the provisional excretory organ of Polygordius consists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS

, . , r 11-1 LARVA. (After Hatschek.)

each with an anterior funnel-like open- m _ moulh . ^ supraKBSO .

ing situated in the midst of the meSO- phageal ganglion ; nph. nephri11 11 . , dion ; ine.p. mesoblastic band;

blast cells, and a posterior external an _ anus 5 oL stomach . opening. The latter is placed immediately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube (fig. 383, np/i) and the two openings so formed may eventually become divided into five (fig. 384 A), all communicating by a single pore with the exterior.

When the posterior region of the embryo becomes segmented,

EXCRETORY ORGANS.

685

paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation.

From the point of junction of the two main branches of the larval kidney there grows backwards (fig. 384 B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established.

The excretory tubes in the second and succeeding segments are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube.

The tube may be continued with a sinuous course through

A A

A +

A.

Y

Y Y Y Y

J)

FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY SYSTEM OF POLYGORDIUS. (After Hatschek.)

several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the perma

686 GEPHYREA.

nent excretory tubes is diagrammatically represented in fig. 384 C and D.

The provisional excretory organ atrophies during larval life.

If Hatschek's account of the development of the excretory system of Polygordius is correct, it is clear that important secondary modifications must have taken place in it, because his description implies that there sprouts from the anterior excretory organ, while it has its own external opening, a posterior duct, which does not communicate either with the exterior or with the body-cavity! Such a duct could have no function. It is intelligible either (i) that the anterior excretory organ should lead into a longitudinal duct, opening posteriorly ; that then a series of secondary openings into the body-cavity should attach themselves to this, that for each internal opening an external should subsequently arise, and the whole break up into separate tubes ; or (2) that behind an anterior provisional excretory organ a series of secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.

Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one, similar in position and sometimes also in structure, to the provisional excretory organ of Polygordius, has so far only been found in the larvae of Echiurus and Bonellia.

In all the Gephyrea the so-called 'brown tubes' are apparently homologous with the segmented excretory tubes of Chaetopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a single highly modified tube in Bonellia, forming the oviduct and uterus ; a pair of tubes in the Gephyrea inermia, and two or three pairs in most Gephyrea armata, except Bonellia. Their development has not been studied.

In the Gephyrea armata there is always present a pair of posteriorly placed excretory organs, opening in the adult into the anal extremity of the alimentary tract, and provided with numerous ciliated peritoneal funnels. These organs were stated by Spengel to arise in Bonellia as outgrowths of the gut ; but in Echinrus Hatschek (No. 515) finds that they are developed from the somatic mesoblast of the terminal part of the trunk. They soon become hollow, and after attaching themselves to the epiblast on each side of the anus, acquire external openings. They are not at first provided with peritoneal funnels, but these parts of the organs become developed from a ring of cells at

EXCRETORY ORGANS.

687

their inner extremities ; and there is at first but a single funnel for each vesicle. The mode of increase of the funnels has not been observed, nor has it been made out how the organs themselves become attached to the hind-gut.

The provisional excretory organ of Echiurus is developed at an early larval stage, and is functional during the whole of larval life. It at first forms a ciliated tube on each side, placed in front of that part of the larva which becomes the trunk of the adult. It opens to the exterior by a fine pore on the ventral side, immediately in front of one of the mesoblastic bands, and appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal ciliated funnel. The primitively simple excretory organ becomes eventually highly complex by the formation of numerous branches, each ending in a slightly swollen extremity. These branches, in the later larval stages, actually form a network, and the inner end of each main branch divides into a bunch of fine tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.

In the larva of Bonellia Spengel has described a pair of provisional excretory tubes, opening near the anterior end of the body, which are probably homologous with the provisional excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).

Discophora. As in many of the types already spoken of, permanent and provisional excretory organs may be present in the Discophora. The former are usually segmentally arranged, and resemble in many respects the excretory tubes of the Chaetopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).

Bourne 1 has shewn that the cells surrounding the main duct in the medicinal Leech are perforated by a very remarkable network of ductules, and the structure of these organs in the Leech is so peculiar that it is permissible to state with due reserve their homology with the excretory organs of the Chaetopoda.

The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast.

1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of Micr. Science, Vol. XX. 1880.

688 ARTHROPODA.

There are found in the embryos of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which are not yet fully made out. In Nephelis they appear as one (according to Robin), or (according to Biitschli) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end of the U forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Fiirbringer is inclined from his own researches to believe that they open laterally. They contain a clear fluid.

In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement, from which a convoluted tube is continued for some distance backwards; the tube then turns forwards again, and after bending again upon itself opens to the exterior. The anterior part is broken up into a kind of labyrinthic network.

The provisional excretory organs of the Leeches cannot be identified with the anterior provisional organs of Polygordius and Echiurus.

Arthropoda. Amongst the Arthropoda Peripatus is the only form with excretory organs of the type of the segmental excretory organs of the Chsetopoda 1 .

These organs are placed at the bases of the feet, in the lateral divisions of the body-cavity, shut off from the main median division of the body-cavity by longitudinal septa of transverse muscles.

Each fully developed organ consists of three parts :

(i) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this, and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube

1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." Quart. J, of Micr. Science, Vol. XIX. 1879.

EXCRETORY ORGANS. 689

and at the other, as I believe, into the body cavity. This section becomes very conspicuous, in stained preparations, by the intensity with which the nuclei of its walls absorb the colouring matter.

In the majority of the Tracheata the excretory organs have the form of the so-called Malpighian tubes, which always (vide Vol. II.) originate as a pair of outgrowths of the epiblastic proctodaeum. From their mode of development they admit of comparison with the anal vesicles of the Gephyrea, though in the present state of our knowledge this comparison must be regarded as somewhat hypothetical.

The antennary and shell-glands of the Crustacea, and possibly also the so-called dorsal organ of various Crustacean larvae appear to be excretory, and the two former have been regarded by Claus and Grobben as belonging to the same system as the segmental excretory tubes of the Chaetopoda.

Nematoda. Paired excretory tubes, running for the whole length of the body in the so-called lateral line, and opening in front by a common ventral pore, are present in the Nematoda. They do not appear to communicate with the body cavity, and their development has not been studied.

Very little is known with reference either to the structure or development of excretory organs in the Echinodermata and the other Invertebrate types of which no mention has been so far made in this Chapter.

Excretory organs and generative ducts of the Craniata.

Although it would be convenient to separate, if possible, the history of the excretory organs from that of the generative ducts, yet these parts are so closely related in the Vertebrata, in some cases the same duct having at once a generative and a urinary function, that it is not possible to do so.

The excretory organs of the Vertebrata consist of three distinct glandular bodies and of their ducts. These are (i) a small glandular body, usually with one or more ciliated funnels opening into the body cavity, near the opening of which there projects into the body cavity a vascular glomerulus. It is situated very far forwards, and is usually known as the head 44

690 ELASMOBRANCHII.

kidney, though it may perhaps be more suitably called, adopting Lankester's nomenclature, the pronepliros. Its duct, which forms the basis for the generative and urinary ducts, will be called the segmented duct.

(2) The Wolffian body, which may be also called the mesonepJiros. It consists of a series of, at first, segmentally (with a few exceptions) arranged glandular canals (segmental tubes) primitively opening at one extremity by funnel-shaped apertures into the body cavity, and at the other into the segmental duct. This duct becomes in many forms divided longitudinally into two parts, one of which then remains attached to the segmental tubes and forms the Wolffian or mesonepJiric duct, while the other is known as the Milllerian dnct.

(3) The kidney proper or metanephros. This organ is only found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolrfian duct.

The above parts do not coexist in full activity in any living adult member of the Vertebrata, though all of them are found together in certain embryos. They are so intimately connected that they cannot be satisfactorily dealt with separately.

Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the same time it forms a convenient starting point for studying the modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros. The development of the Elasmobranch excretory system has been mainly studied by Semper and myself.

The first trace of the system makes its appearance as a knob of mesoblast, springing from the intermediate cell-mass near the level of the hind end of the heart (fig. 385 K,pd). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (fig. 385 B, pd). The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body cavity (fig. 421, sd) continuous with a lumen, which

EXCRETORY ORGANS.

691

makes its appearance in the column (fig. 386, sd). The knob forms the only structure which can be regarded as a rudiment of the pronephros.

spn

FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL

CLEFTS.

The sections illustrate the development of the segmental duct (pd) or primitive duct of the pronephros. In A (the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. In B is seen a section of the column which has grown backwards from the knob in A.

spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. subnotochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate; ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //. body cavity; ep. epiblast; al. alimentary canal.

While the lumen is gradually being formed, the segmental tubes of the mesonephros become established. They appear to arise as differentiations of the parts of the primitive lateral plates of mesoblast, placed between the dorsal end of the body cavity and the muscle-plate (fig. 386, st) 1 , which are usually known as the intermediate cell-masses.

The lumen of the segmental tubes, though at first very small, soon becomes of a considerable size. It appears to be established in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body cavity in the muscle-plates with the permanent body cavity. The lumen of each tube opens at its lower end into the dorsal part of the body cavity (fig. 386, st}, and each tube curls obliquely

1 In my original account of the development I held these tubes to be invaginations of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of my original statement from his investigations on the chick ; and from a re-examination of my specimens he arrived at the results stated above, and which I am now myself inclined to adopt.

442

692

ELASMOBRANCHII.

sp.c

backwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly.

One segmental tube makes its appearance for each somite (fig. 265), commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus. Soon after their formation the blind ends of the segmental tubes come in contact with, and open into the segmental duct, and each of them becomes divided into four parts. These are (i) a section carrying the peritoneal opening, known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and terminating in (4) a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epiblast and mesoblast, and assumes a position close to the epithelium lining the body cavity (fig. 380, sd}. The general features of the excretory organs at this period are diagrammatically represented in the woodcut (fig. 387). In this fig. pd is the segmental duct and o its abdominal opening; s.t points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body cavity, and the other into the segmental duct, which forms the only duct of the system, and communicates at its front end with the body cavity, and behind with the cloaca.

FIG. 386. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; nip, muscle-plate ; nip', inner layer of muscle-plate already converted into muscles ; Vr, rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.

EXCRETORY ORGANS. 693

The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line x, and in transverse section in figs. 388 and 389. The resulting ducts are (i) the Wolffian duct or mesonephric duct (wd\ dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Miillerian duct in the female, and the rudiments of this duct in the male. In the

FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN

ELASMOBRANCH EMBRYO.

pd. segmental duct. It opens at o into the body cavity and at its other extremity into the cloaca; x. line along which the division appears which separates the segmental duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.

female the formation of these ducts takes place (fig. 389) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Miillerian duct (pd}. A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen (fig. 389 A). The anterior part of the segmental duct is not divided, but remains continuous with the Mullerian duct, of which its anterior pore forms the permanent peritoneal opening 1 (fig. 387). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly

1 Five or six segmental tubes belong to the region of the undivided anterior part of the segmental duct, which forms the front end of the Mullerian duct ; but they appear to atrophy very early, without acquiring a definite attachment to the segmental duct.

694

ELASMOBRANCHIL

in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Miillerian duct, being shorter,

trd/

FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR I'ART OF THE SEGMENTAL DUCT OF A FEMALE EMBRYO OF SCYLLIUM CANICULA.

The figure shews how the segmental duct becomes split into the Wolffian or mesonephric duct above, and Miillerian duct or oviduct below.

wd. Wolffian or mesonephric duct; od. Miillerian duct or oviduct ; sd. segmental duct.

FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A

SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.

me. medullary canal; mp. muscle-plate; ch. notochord; ao. aorta; cav. cardinal vein; st. segmental tube. On the left side the section passes through the opening of a segmental tube into the body cavity. On the right this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; iv.d. Wolffian duct; m.d. Miillerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate; gr. the germinal ridge with the thickened germinal epithelium ; /. liver ; i. intestine with spiral valve.

and in the column of cells with which it is continuous being from the first incomplete.

The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening

EXCRETORY ORGANS.

695

into the Wolffian duct (fig. 390, px). The remainder of the vesicle becomes converted into a Malpighian body (mg}.

By the first of these changes 10^-4 M @W>f a tube is established connecting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the anterior part of the excretory organs in the adult, and most probably in the hinder part, yet it seems almost certain that the secondary and tertiary Malpighian bodies of the majority of segments are developed from its persisting blind end. Each of these

FIG. 390. LONGITUDINAL VERTICAL SECTION THROUGH PART OF THE MESONEPHROS OF AN EMBRYO OF SCYLLIUM.

The figure contains two examples of the budding of the vesicle of a segmental tube (which forms a Malpighian body in its own segment) to unite with the tubulus in the preceding segment close to its opening into the Wolffian (mesonephric) duct.

ge. epithelium of body-cavity; st. peritoneal funnel of segmental tube with its peritoneal opening; mg. Malpighian body; px. bud from Malphigian body uniting with preceding segment.

secondary and tertiary Malpighian bodies is connected with a convoluted tubulus (fig. 391, a.mg), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the

st.c

w.d

FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT. The figure shews a fibrous band passing from the primary to the secondary Malpighian bodies in two segments, which is the remains of the outgrowth from the primary Malpighian body.

sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Malpighian body; w.d. mesonephric (Wolffian) duct.

696 ELASMOBRANCI1II.

segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.

The third section of each tubulus becomes by continuous growth, especially in the hinder segments, very bulky and convoluted.

The general character of a slightly developed segment of the mesonephros at its full growth may be gathered from fig. 391. It commences with (i) a peritoneal opening, somewhat oval in form (st.d) and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (w.d}, opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one, two, or more (according to the size of the segment) accessory Malpighian bodies (a.mg) smaller than the one into which the segmental tube falls, eventually opens by (4) a narrowish collecting tube into the Wolffian duct at the posterior end of the segment. Each segment is probably completely isolated from the adjoining segments, and never has more than one peritoneal funnel and one communication with the Wolffian duct.

Up to this time there has been no distinction between the anterior and posterior tubuli of the mesonephros, which alike open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in Scyllium canicula), either in some species elongate, overlap, while at the same time their openings travel backward so that they eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian ducts, in the male; or in other species become modified, by a peculiar process of splitting from the Wolnian duct, so as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other species (fig. 392). In both cases the modified posterior kidney-segments are probably equivalent to the per

EXCRETORY ORGANS. 697

manent kidney or metanephros of the amniotic Vertebrates, and for this reason the numerous collecting tubes or single collecting tube, as the case may be, will be spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the permanent Wolffian body or mesonephros.

The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture, situated at the extremity of the median papilla behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings, become obliterated.

In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes. Branches appear to grow from the first three or four or more of them (though probably not from their peritoneal openings), which pass to the base of the testis, and there uniting into a longitudinal canal, form a network, and receive the secretion of the testicular ampullae (fig. 393, nf). These ducts, the vasa efferent ia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as the longitudinal canal of the Wolffian body (l.c\ from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian corpuscle. From the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian body, along which the semen is conveyed to the Wolffian duct (v.d). The Wolffian duct itself becomes much contorted and acts as vas deferens.

Figs. 392 and 393 are diagrammatic representations of the chief constituents of the adult urinogenital organs in the two sexes. In the adult female (fig. 392), there are present the following parts :

(1) The oviduct or Mullerian duct (m.d) split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body cavity, and behind the two oviducts have independent communications with the general cloaca.

(2) The mesonephric ducts (w.d), the other product of the

698

ELASMOBRANCHII.

segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior persisting segment of the mesonephros on each side, and unite behind to

FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT FEMALE ELASMOBRANCH.

m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are represented with openings into the body cavity, the posterior segmental tubes form the mesonephros ; ov. ovary.

open by a common papilla into the cloaca. The mesonephric duct receives the secretion of the anterior tubuli of the primitive mesonephros.

(3) The ureter which carries off the secretion of the kidney proper or metanephros. It is represented in my diagram in its most rare and differentiated condition as a single duct connected with the posterior segmental tubes.

(4) The segmental tubes (.$-./) some of which retain their

-S.t:

FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT MALE ELASMOBRANCH.

m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis; VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.

EXCRETORY ORGANS. 699

original openings into the body cavity, and others are without them. They are divided into two groups, an anterior forming the mesonephros or Wolffian body, which pours its secretion into the Wolffian duct ; and a posterior group forming a gland which is probably equivalent to the kidney proper of amniotic Craniata, and is connected with the ureter.

In the male the following parts are present (fig. 393):

(1) The Mlillerian duct (m.d], consisting of a small rudiment attached to the liver, representing the foremost end of the oviduct of the female.

(2) The mesonephric duct (w.d] which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (vd}. In the adult male its foremost part has a very tortuous course.

(3) The ureter (d\ which has the same fundamental constitution as in the female.

(4) The segmental tubes (s.t). The posterior tubes have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them to act as transporters of the semen.

Connected with the anterior tubes there are present (i) the vasa efferentia (VE], united on the one hand with (2) the central canal in the base of the testis (/), and on the other with the longitudinal canal of the Wolffian body (/<?). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in my diagram.

Apart from the absence of the pronephros the points which deserve notice in the Elasmobranch excretory system are (i) The splitting of the segmental duct into Wolffian (mesonephric) and Mullerian ducts. (2) The connection of the former with the mesonephros, and of the latter with the abdominal opening of the segmental duct which represents the pronephros of other types. (3) The fact that the Mullerian duct serves as oviduct, and the Wolffian duct as vas deferens. (4) The differentiation of a posterior section of the mesonephros into a special gland foreshadowing the metanephros of the Amniota.

/OO CYCLOSTOMATA.

Cyclostomata. The development of the excretory system amongst the Cyclostomata has only been studied in Petromyzon (Miiller, Furbringer, and Scott).

The first part of the system developed is the segmental duct. It appears in the embryo of about 14 days (Scott) as a solid cord of cells, differentiated from the somatic mesoblast near the dorsal end of the body cavity. This cord is at first placed immediately below the epiblast, and grows backwards by a continuous process of differentiation of fresh mesoblast cells. It soon acquires a lumen, and joins the cloacal section of the alimentary tract before the close of foetal life. Before this communication is established, the front end of the duct sends a process towards the body cavity, the blind end of which acquires a ciliated opening into the latter. A series of about four or five successively formed outgrowths from the duct, one behind the other, give rise to as many ciliated funnels opening into the body cavity, and each communicating by a more or less elongated tube with the segmental duct. These funnels, which have a metameric arrangement, constitute the pronephros, the whole of which is situated in the pericardial region of the body cavity.

On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into the body cavity, and covered by peritoneal epithelium. For a considerable period the pronephros constitutes the sole functional part of the excretory system.

A mesonephros is formed (Furbringer) relatively late in larval life, as a segmentally arranged series of solid cords, derived from the peritoneal epithelium. These cords constitute the rudiments of the segmental tubes. They are present for a considerable portion of the body cavity, extending backwards from a point shortly behind the pronephros. They soon separate from the peritoneal epithelium, become hollowed out into canals, and join the segmental duct. At their blind extremity (that originally connected with the peritoneal epithelium) a Malpighian body is formed.

The pronephros is only a provisional excretory organ, the atrophy of which commences during larval life, and is nearly completed when the Ammoccete has reached 180 mm. in length.

EXCRETORY ORGANS. 70 1

Further changes take place in connection with the excretory system on the conversion of the Ammoccete into the adult.

The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This cloaca also communicates by two apertures (abdominal pores) with the body cavity. The generative products are carried into the cloaca by these pores ; so that their transportation outwards is not performed by any part of the primitive urinary system. The urinogenital cloaca is formed by the separation of the portion of the primitive cloaca containing the openings of the segmental ducts from that connected with the alimentary tract.

The mesonephros of the Ammoccete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).

In Myxine the excretory system consists (i) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which the pronephros is composed open on the ventral side of the anterior portion of the segmental duct, which in old individuals is cut off from the posterior section of the duct. On the dorsal side of the portion of the segmental duct belonging to the pronephros there are present a small number of diverticula, terminating in glomeruli : they are probably to be regarded as anterior segmental tubes. (2) Of a mesonephros, which commences a considerable distance behind the pronephros, and is formed of straight extremely simple segmental tubes opening into the segmental duct (fig. 385).

The excretory system of Myxine clearly retains the characters of the system as it exists in the larva of Petromyzon.

Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by a duct, the nature of which is somewhat doubtful, but which is probably homologous with the mesonephric duct of other types.

The system commences in the embryo (Rosenberg, Oellacher, Gotte, Furbringer) with the formation of a groove-like fold of the somatic layer of peritoneal epithelium, which becomes gradually constricted into a canal; the process of constriction commencing in the middle and extending in both directions. The canal does not however close anteriorly, but remains open to the body cavity, thus giving rise to a funnel equivalent to the pronephric funnels of Petromyzon and Myxine. On the inner side of this

702

TELEOSTEI.

funnel there is formed a glomerulus, projecting into the body

cavity ; and at the same time that

this is being formed the anterior end

of the canal becomes elongated and

convoluted. The above structures

constitute a pronephros, while the

posterior part of the primitive canal

forms the segmental duct.

The portion of the body cavity with the glomerulus and peritoneal funnel of the pronephros (fig. 395, po) soon becomes completely isolated from the remainder, so as to form a closed cavity (gl). The development of the mesonephros does not take place till long after that of the pronephros. The segmental tubes which form it are stated by Fiirbringer to arise from solid ingrowths of peritoneal epithelium, developed successively from before backwards, but Sedgwick informs me that they arise as differentiations of the mesoblastic cells near the peritoneal epithelium. They soon become hollow, and unite with the segmental duct. Malpighian bodies are developed on their median portions. They grow very greatly in length, and become much convoluted, but the details of this process have not been followed out.

The foremost segmental tubes are situated close behind the pronephros, while the hindermost are in many cases developed in the post-anal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney of the adult, and the mesonephros the remainder ; the so-called caudal portion, where present, being derived (?) from the postanal segmental tubes.

In some cases the cephalic portion of the kidneys is absent

FIG. 394. PORTIONS OF THE MESONEPHROS OF MYXINE. (From Gegenbaur; after J. Miiller.)

a. segmental duct ; b. segmental tube; c. glomerulus ; d. afferent, e. efferent artery.

B represents a portion of A highly magnified.

EXCRETORY ORGANS. 703

in the adult, which probably implies the atrophy of the pronephros ; in other instances the cephalic portion of the kidneys is the only part developed. Its relation to the embryonic proncphros requires however further elucidation.

In the adult the ducts in the lower part of the kidneys lie as a rule on their outer borders, and almost invariably open into a

pr

FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT PARTS TEN DAYS BEFORE HATCHING.

pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x. subnotochordal rod ; al. alimentary tract.

urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few instances there is a common urinogenital pore.

In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It appears to me most probable, from the analogy of Lepidostcus, to be described in the next section, that these ducts are split off from the primitive segmental duct, and correspond with the Miillerian ducts of Elasmobranchii, etc. ; though on this point we have at present no positive embryological evidence (vide general considerations at the end of the Chapter). In the female Salmon and the male and female Eel the generative products are carried to the exterior by abdominal pores. It is possible that this may represent a primitive condition, though it

704

GANOIDEI.

is more probably a case of degeneration, as is indicated by the presence of ducts in the male Salmon and in forms nearly allied to the Salmonidae.

The coexistence of abdominal pores and generative ducts in Mormyrus appears to me to demonstrate that the generative ducts in Teleostei cannot be derived from the coalescence of the investment of the generative organs with the abdominal pores.

Ganoidei. The true excretory gland of the adult Ganoidei resembles on the whole that of Teleostei, consisting of an elongated band on each side the mesonephros an anterior dilatation of which probably represents the pronephros.

There is in both sexes a Mullerian duct, provided, except in Lepidosteus, with an abdominal funnel, which is however situated relatively very far back in the abdominal cavity. The Mullerian ducts appear to serve as generative canals in both sexes. In Lepidosteus they are continuous with the investment of the generative glands, and thus a relation between the generative ducts and glands, very similar to that in Teleostei, is brought about.

Posteriorly the Mullerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each side is clearly the primitive segmental duct. It receives the secretion of a certain number of the posterior mesonephric tubules, and usually unites with its fellow to form a kind of bladder, opening by a single pore into the cloaca, behind the anus. The duct which receives the secretion of the anterior mesonephric tubules is the true mesonephric or Wolffian duct.

The development of the excretory system, which has been partially worked out in Acipenscr and Lepidosteus 1 , is on the whole very similar to that in the Teleostei. The first portion of the system to

FIG. 396. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION.

me. medullary cord ; ms. mesoblast ; sg. segmental duct ; ch. notochord ; .r. subnotochordal rod; hy. hypoblast.

1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and myself.

EXCRETORY ORGANS.

705

be formed is the segmental duct. In Lepidosteus this duct is formed as a groove-like invagination of the somatic peritoneal epithelium, precisely as in Teleostei, and shortly afterwards forms a duct lying between the mesoblast and the epiblast (fig. 396, sg}. In Acipenser (Salensky) however it is formed as

FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER

EMBRYO. (After Salensky.)

Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. notochord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic plate.

a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, Wg).

In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one 1 peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n. At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus.

The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvae of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late.

The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each

1 I have not fully proved this point, but have never found more than one opening.

B. III.

45

GANOIDEI.

side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct 1 . The posterior segmental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct.

In later stages (larvae of 7 10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are es

sjy.c

mjo

pr.n

FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A

LARVA OF ACIPENSER 5 MM. IN LENGTH.

st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ; b.c. body cavity.

tablished, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These

1 Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, or in situ, could not be determined.

EXCRETORY ORGANS. 707

funnels have been noticed by Furbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Mullerian ducts has not been worked out.

Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Mullerian ducts serving as genital ducts in both sexes.

Amphibia. In Amphibia (Gotte, Furbringer) the development of the excretory system commences, as in Teleostei, by the formation of the segmental duct from a groove formed by a fold of the somatic layer of the peritoneal epithelium, near the dorsal border of the body cavity (fig. 399, u). The anterior end of the groove is placed immediately behind the branchial region. Its posterior part soon becomes converted into a canal by a constriction which commences a short way from the front end of the groove, and thence extends backwards. This canal at first ends blindly close to the cloaca, into which however it soon opens.

The anterior open part of the groove in front of the constriction (fig. 399, n] becomes differentiated into a longitudinal duct, which remains in open communication with the body cavity by two (many Urodela) three (many Anura) or four (Cceciliidae) canals. This constitutes the dorsal part of the pronephros. The ventral part of the gland is formed from the section of the duct immediately behind the longitudinal canal. This part grows in length, and, assuming an S-shaped curvature, becomes placed on the ventral side of the first formed part of the pronephros. By continuous growth in a limited space the convolutions of the canal of the pronephros become more numerous, and the complexity of the gland is further increased by the outgrowth of blindly ending diverticula.

At the root of the mesentery, opposite the peritoneal openings of the pronephros, a longitudinal fold, lined by peritoneal epithelium, and attached by a narrow band of tissue, makes its appearance. It soon becomes highly vascular, and constitutes a glomerulus homologous with that in Petromyzon and Teleostei.

452

AMPHIBIA.

a*'

The section of the body cavity which contains the openings of the pronephros and the glomerulus, becomes dilated, and then temporarily shut off from the remainder. At a later period it forms a special though not completely isolated compartment. For a long time the pronephros and its duct form the only excretory organs of larval Amphibia. Eventually however the formation of the mesonephros commences, and is followed by the atrophy of the pronephros. The mesonephros is composed, as in other types, of a series of segmental tubes, but these, except in Cceciliidae, no longer correspond in number with the myotomes, but are in all instances more numerous. Moreover, in the posterior part of the mesonephros in the Urodeles, and through the whole length of the gland in other types, secondary and tertiary segmental tubes are formed in addition to the primary tubes.

FIG. 399. TRANSVERSE SECTION THROUGH A VERY YOUNG TADPOLE OF BOMBINATOR AT THE LEVEL OF THE ANTERIOR END OF THE YOLK-SACK. (After

Gotte.)

a. fold of epiblast continuous with the dorsal fin; is", neural cord; m. lateral muscle; as 1 . outer layer of muscle-plate; s. lateral plate of mesoblast ; b. mesentery ; u. open end of the segmental duct, which forms the pronephros ; f. alimentary tract ; f. ventral diverticulum which becomes the liver; e. junction of yolk cells and hypoblast cells ; d. yolk cells.

The development of the mesonephros commences in Salamandra (Fiirbringer) with the formation of a series of solid cords, which in the anterior myotomes spring from the peritoneal epithelium on the inner side of the segmental duct, but posteriorly arise independently of this epithelium in the adjoining mesoblast. Sedgwick informs me that in the

Frog the segmental tubes are throughout developed in the mesoblast, independently of the peritoneal epithelium. These cords next become detached from the peritoneal epithelium (in so far as they are primitively united to it), and after first assuming a vesicular form, grow out into coiled tubes, with a median limb the blind end of which assists in forming a Malpighian body, and a lateral limb which comes in contact with and opens into the segmental duct, and an intermediate portion connecting the two. At the junction of the median with the intermediate portion, and therefore at the neck of the Malpighian body, a canal grows out in a ventral direction, which meets the

EXCRETORY ORGANS. 709

peritoneal epithelium, and then develops a funnel-shaped opening into the body cavity, which subsequently becomes ciliated. In this way the peritoneal funnels which are present in the adult are established.

The median and lateral sections of the segmental tubes become highly convoluted, and the separate tubes soon come into such close proximity that their primitive distinctness is lost.

The first fully developed segmental tube is formed in Salamandra maculata in about the sixth myotome behind the pronephros. But in the region between the two structures rudimentary segmental tubes are developed.

The number of primary segmental tubes in the separate myotomes of Salamandra is as follows :

In the 6th myotome (i.e. the first with a true

segmental tube) 12 segmental tubes

yth roth myotome 23

IIth ... 34

I2th 3 4 or 4 5

I3th y> 45

1 3th i6th 56

It thus appears that the segmental tubes are not only more numerous than the myotomes, but that the number in each myotome increases from before backwards. In the case of Salamandra there are formed in the region of the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out of independent solid cords, which arise in the mesoblast dorsally to the tubes already established.

The secondary segmental tubes appear to develop out of these cords exactly in the same way as the primary ones, except that they do not join the segmental duct directly, but unite with the primary segmental tubes shortly before the junction of the latter with the segmental duct. In this way compound segmental tubes are established with a common collecting tube, but with numerous Malpighian bodies and ciliated peritoneal openings. The difference in the mode of origin of these compound tubes and of those in Elasmobranchii is very striking.

The later stages in the development of the segmental tubes have not been studied in the other Amphibian types.

In Cceciliidas the earliest stages are not known, but the tubes present in the adult (Spengel) a truly segmental arrangement, and in the young each of them is single, and provided with only a single peritoneal funnel. In the adult however many of the segmental organs become compound, and may have as many as twenty funnels, etc. Both simple and compound segmental tubes occur in all parts of the mesonephros, and are arranged in no definite order.

In the Anura (Spengel) all the segmental tubes are compound, and an enormous number of peritoneal funnels are present on the ventral surface, but it has not yet been definitely determined into what part of the segmental tubes they open.

710 AMPHIBIA.

Before dealing with the further changes of the Wolffian body it is necessary to return to the segmental duct, which, at the time when the pronephros is undergoing atrophy, becomes split into a dorsal Wolffian and ventral Mullerian duct. The process in Salamandra (Fiirbringer) has much the same character as in Elasmobranchii, the Mullerian duct being formed by the gradual separation, from before backwards, of a solid row of cells from the ventral side of the segmental duct, the remainder of the duct constituting the Wolffian duct. During the formation of the Mullerian duct its anterior part becomes hollow, and attaching itself in front to the peritoneal epithelium acquires an opening into the body cavity. The process of hollowing is continued backwards pari passu with the splitting of the segmental duct. In the female the process is continued till the Mullerian duct opens, close to the Wolffian duct, into the cloaca. In the male the duct usually ends blindly. It is important to notice that the abdominal opening of the Mullerian duct in the Amphibia (Salamandra) is a formation independent of the pronephros, and placed slightly behind it ; and that the undivided anterior part of the segmental duct (with the pronephros) is not, as in Elasmobranchii, united with the Mullerian duct, but remains connected with the Wolffian duct.

The development of the Mullerian duct has not been satisfactorily studied in other forms besides Salamandra. In Cceciliidae its abdominal opening is on a level with the anterior end of the Wolffian body. In other forms it is usually placed very far forwards, close to the root of the lungs (except in Proteus and Batrachoseps, where it is placed somewhat further back), and some distance in front of the Wolffian body.

The Mullerian duct is always well developed in the female, and serves as oviduct. In the male it does not (except possibly in Alytes) assist in the transportation of the genital products, and is always more or less rudimentary, and in Anura may be completely absent.

After the formation of the Mullerian duct, the Wolffian duct remains as the excretory channel for the Wolffian body, and, till the atrophy of the pronephros, for this gland also. Its anterior section, in front of the Wolffian body, undergoes a more or less complete atrophy.

The further changes of the excretory system concern (i) the junction in the male of the anterior part of the Wolffian body with the testis ; (2) certain changes in the collecting tubes of the

EXCRETORY ORGANS.

711

posterior part of the mesonephros. The first of these processes results in the division of the Wolffian body into a sexual and a non-sexual part, and in Salamandra and other Urodeles the division corresponds with the distribution of the simple and compound segmental tubes.

Since the development of the canals connecting the testes with the sexual part of the Wolffian body has not been in all points satisfactorily elucidated, it will be convenient to commence with a description of the adult arrangement of the parts (fig. 400 B). In most instances a non-segmental system of canals the vasa effcrentia (ve) coming from the testis, fall into a canal known as the longitudinal canal of the Wolffian body, from which there pass off transverse canals, which fall into, and are equal in number to, the primary Malpighian bodies of the sexual part of the gland. The spermatozoa, brought to the Malpighian bodies, are thence transported along the segmental tubes to the Wolffian duct, and so to the exterior. The system of canals connecting the testis with the Malpighian bodies is known as the testicular network. The number of segmental tubes connected with the testis varies very greatly. In Siredon there are as many as from 30 32 (Spengel).

The longitudinal canal of the Wolffian body is in rare instances (Spelerpes, etc.) absent, where the sexual part of the Wolffian body is slightly developed. In the Urodela the testes are united with the anterior part of the Wolffian body. In the Cceciliidas the junction takes place in an homologous part of the Wolffian body, but, owing to the development of the anterior segmental tubes, which are rudimentary in the Urodela, it is situated some way behind the front end. Amongst the Anura the connection of the testis with the tubules of the Wolffian body is subject to considerable variations. In Bufo cinereus the normal Urodele type is preserved, and in Bombinator the same arrangement is found in a rudimentary condition, in that there are transverse trunks from the longitudinal canal of the Wolffian body, which end blindly, while the semen is carried into the Wolffian duct by canals in front of the Wolffian body. In Alytes and Discoglossus the semen is carried away by a similar direct continuation of the longitudinal canal in front of the Wolffian body, but there are no rudimentary transverse canals passing into the Wolffian body, as in Bombinator. In Rana the transverse ducts which pass off from the longitudinal canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian bodies, enter directly into the collecting tubes near their opening into the Wolffian duct.

712 AMPHIBIA.

In most Urodeles the peritoneal openings connected with the primary generative Malpighian bodies atrophy, but in Spelerpes they persist. In the Cceciliidie they also remain in the adult state.

With reference to the development of these parts little is known except that the testicular network grows out from the primary Malpighian bodies, and becomes united with the testis. Embryological evidence, as well as the fact of the persistence of the peritoneal funnels of the generative region in the adults of some forms, proves that the testicular network is not developed from the peritoneal funnels.

Rudiments of the testicular network are found in the female Cceciliidae and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and of transverse canals passing from this to the Malpighian bodies, together with some branches passing into the mesovarium.

Amongst the Urodela the collecting tubes of the hinder non-sexual part of the Wolffian body, which probably represents a rudimentary metanephros, undergo in the male sex a change similar to that which they usually undergo in Elasmobranchii. Their points of junction with the Wolffian duct are carried back to the hindermost end of the duct (fig. 400 B), and the collecting tubes themselves unite together into one or more short ducts (ureters) before joining the Wolffian duct.

In Batrachoseps only the first collecting tube becomes split off in this way ; and it forms a single elongated ureter which receives all the collecting tubes of the posterior segmental tubes. In the female and in the male of Proteus, Menobranchus, and Siren the collecting tubes retain their primitive transverse course and open laterally into the Wolffian duct. In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the cloaca.

The urinary bladder of the Amphibia is an outgrowth of the ventral wall of the cloacal section of the alimentary tract, and is homologous with the allantois of the amniotic Vertebrata.

The subjoined diagram (fig. 400) of the urogenital system of Triton illustrates the more important points of the preceding description.

In the female (A) the following parts are present :

(1) The Mullerian duct or oviduct (od) derived from the splitting of the segmental duct.

(2) The Wolffian duct (sug) constituting the portion of the segmental duct left after the formation of the Mullerian duct.

(3) The mesonephros (r), divided into an anterior sexual part

EXCRETORY ORGANS.

7'3

connected with a rudimentary testicular network, and a posterior part. The collecting tubes from both parts fall transversely into the Wolffian duct.

(4) The ovary (ov).

(5) The rudimentary testicular network.

In the male (B) the following parts are present :

(1) The functionless though fairly developed Miillerian duct (;).

(2) The Wolffian duct (sug).

(3) The mesonephros (r) divided into a true sexual part, through the segmental tubes of which the semen passes, and a non-sexual part. The collecting tubes of the latter do not enter the Wolffian duct directly, but bend obliquely backwards and only fall into it close to its cloacal aperture, after uniting to form one or two primary tubes (ureters).

(4) The testicular network (ve) consisting of (i) transverse ducts from the testes, falling into (2) the longitudinal canal of the Wolffian body, from which (3) transverse canals are again given off to the Malpighian bodies.

Amniota. The amniotic Vertebrata agree, so far as is known, very closely amongst themselves in the formation of the urinogenital system.

The most characteristic feature of the system is the full development of a metanephros, which constitutes the functional kidney on the atrophy of the mesonephros or Wolffian body, which is a purely embryonic organ. The first part of the system to develop is a duct, which is usually spoken of as the Wolffian duct, but which is really the homologue of the seg

FIG. 400. DIAGRAM OF THE URINOGENITAL SYSTEM OF TRITON. (From Gegenbaur ; after Spengel.)

A. Female. B. Male. r. mesonephros, on the surface of which numerous peritoneal funnels are visible ; sug. mesonephric or Wolffian duct; od. oviduct (Miillerian duct); in. Miillerian duct of male ; ve. vasa efferentia of testis ; t. testis ; ov. ovary ; up. urinogenital pore.

714 AMNIOTA.

mental duct. It apparently develops in all the Amniota nearly on the Elasmobranch type, as a solid rod, primarily derived from the somatic mesoblast of the intermediate cell mass (fig. 401 W.d}\

The first trace of it is visible in an embryo Chick with eight somites, as a ridge projecting from the intermediate cell mass towards the epiblast in the region of the seventh somite. In the course of further development it continues to constitute such a ridge as far as the eleventh somite (Sedgwick), but from this point it grows backwards in the space between the epiblast and mesoblast In an embryo with fourteen somites a small lumen has appeared in its middle part and in front it is connected with rudimentary Wolffian tubules, which develop in continuity with it (Sedgwick). In the succeeding stages the lumen of the duct gradually extends backwards and forwards, and the duct itself also passes inwards relatively to the epiblast (fig. 402). Its hindend elongates till it comes into connection with, and opens into, the cloacal section of the hind-gut' 2 .

It might have been anticipated that, as in the lower types, the anterior end of the segmental duct would either open into the body cavity, or come into connection with a pronephros. Neither of these occurrences takes place, though in some types (the Fowl) a structure, which is probably the rudiment of a pronephros, is developed ; it does not however appear till a later stage, and is then unconnected with the segmental duct. The next part of the system to appear is the mesonephros or Wolffian body.

This is formed in all Amniota as a series of segmental tubes, which in Lacertilia (Braun) correspond with the myotomes, but in Birds and Mammalia are more numerous.

In Reptilia (Braun, No. 542), the mesonephric tubes develop as segmentally-arranged masses on the inner side of the Wolffian duct, and appear to be at first united with the peritoneal epithelium. Each mass soon becomes an oval vesicle, probably opening for a very short period into the

1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as developing from a groove opening to the peritoneal cavity, which subsequently becomes constricted into a duct. I have never met with specimens such as those figured by these authors.

2 The foremost extremity of the segmental duct presents, according to Gasser, curious irregularities and an anterior completely isolated portion is often present.

EXCRETORY ORGANS.

715

peritoneal cavity by a peritoneal funnel. The vesicles become very early detached from the peritoneal epithelium, and lateral outgrowths from them give rise to the main parts of the segmental tubes, which soon unite with the segmental duct.

In Birds the development of the segmental tubes is more complicated 1 .

The tubules of the Wolffian body are derived from the intermediate cell mass, shewn in fig. 401, between the upper end of the body cavity and the

g.o.

FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN

EMBRYO CHICK OF 45 HOURS.

M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ; p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. bloodvessel.

muscle-plate. In the Chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment the intermediate cell mass becomes detached from the peritoneal epithelium at certain points, remaining attached to it at other points, there being several such to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords (fig. 402, st] which soon unite with the segmental duct (wd}. Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel.

1 Correct figures of the early stages of these structures were first given by Kolliker, but the correct interpretation of them and the first satisfactory account of the development of the excretory organs of Birds was given by Sedgwick (No. 549).

716

AMNIOTA.

In the Duck the attachment of the intermediate cell mass to the peritoneal epithelium is prolonged further back than in the Chick.

In the foremost segmental tubes, which never reach a very complete development, the peritoneal funnels widen considerably, while at the same time they acquire a distinct lumen. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity (fig. 403, gl). There is thus formed a series of free peritoneal glomeruli belonging to the anterior Wolfnan tubuli 1 . These tubuli become however early aborted.

In the case of the remaining tubules developed from the S-shaped cords the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the segmental duct. Their blind extremities constitute the rudiments of Malpighian bodies.

am

FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH

ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.

am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.

1 These external glomeruli were originally mistaken by me (No. 539) for the glomeralus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No. 550).

EXCRETORY ORGANS.

717

In the posterior part of the Wolffian body of the Chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks up into oval vesicles similar to those of the Reptilia, which form the rudiments of the segmental tubes.

Secondary and tertiary segmental tubules are formed in the Chick, on the dorsal side of the primary tubules, as direct differentiations of the mesoblast. They open independently into the Wolffian duct.

In Mammalia the segmental tubules (Egli) are formed as solid masses in the same situation as in Birds and Reptiles. It is not known whether they are united with the peritoneal epithelium. They soon become oval vesicles, which develop into complete tubules in the manner already indicated.

After the establishment of the Wolffian body there is formed in both sexes in all the Amniota a duct, which in the female becomes the oviduct, but which is functionless and disappears more or less completely in the male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Mullerian duct of

FIG. 403. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK OF ABOUT IOO H.

gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao. aorta ; me. mesentery. The segmental tube, and the connection between the external and internal parts of the glomerulus are not shewn in this figure.

FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS). (After Balfour and Sedgwick. )

A is the nth section of the series. B i 5th

C i8th ,, ,,

gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.

7 i8

AMNIOTA.

the Ichthyopsida. In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick).

The pronephros, as I shall call it, consists of a slightly convoluted longitudinal canal with three or more peritoneal openings. In the earliest condition, it consists of three successive open involutions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body cavity, and is situated some considerable distance behind the front end of the Wolfifian duct.

In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium,

FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times. m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes place; a. thickened portion of the germinal epithelium in which the primitive germinal cells C and o are lying ; E. modified mesoblast which will form the stroma of the ovary ; WK. Wolffian body ; y. Wolffian duct.

EXCRETORY ORGANS. 719

and develop a lumen. The condition of the structure at this stage is illustrated by fig. 404, representing three transverse sections through two grooves, and through the ridge connecting them.

The pronephros may in fact now be described as a slightly convoluted duct, opening into the body cavity by three groovelike apertures, and continuous behind with the rudiment of the true Miillerian duct.

The stage just described is that of the fullest development of the pronephros. In it, as in all the previous stages, there appear to be only three main openings into the body cavity ; but in some sections there are indications of the possible presence of one or two additional rudimentary grooves.

In an embryo not very much older than the one last described the pronephros atrophies as such, its two posterior openings vanishing, and its anterior opening remaining as the permanent opening of the Miillerian duct.

The pronephros is an extremely transitory structure, and its development and atrophy are completed between the QOth and i2Oth hours of incubation.

The position of the pronephros in relation to the Wolffian body is shewn in fig. 405, which probably passes through a region between two of the peritoneal openings. As long as the pronephros persists, the Mullerian duct consists merely of a very

FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour

and Sedgwick.)

In A the terminal portion of the duct is quite distinct ; in B it has united with the walls of the Wolffian duct.

md. Mullerian duct ; Wd. Wolffian duct.

72O AMNIOTA.

small rudiment, continuous with the hindermost of the three peritoneal openings, and its solid extremity appears to unite with the walls of the Wolffian duct.

After the atrophy of the pronephros, the Miillerian duct commences to grow rapidly, and for the first part of its course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (fig. 406). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus precisely similar to that of the Miillerian duct in Elasmobranchii and Amphibia.

This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer walls of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolrfian duct, it may grow backwards from cells derived from that duct.

The Miillerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so.

The mode of growth of the Miillerian duct in the posterior part of its course will best be understood from the following description quoted from the paper by Sedgwick and myself.

"A few sections before its termination the Miillerian duct appears as a well-defined oval duct lying in contact with the wall of the Wolffian duct on the one hand and the germinal epithelium on the other. Gradually, however, as we pass backwards, the Miillerian duct dilates ; the external wall of the Wolffian duct adjoining it becomes greatly thickened and pushed in in its middle part, so as almost to touch the opposite wall of the duct, and so form a bay in which the Miillerian duct lies. As soon as the Miillerian duct has come to lie in this bay its walls lose their previous distinctness of outline, and the cells composing them assume a curious vacuolated appearance. No well-defined line of separation can any longer be traced between the walls of the Wolffian duct and those of the Miillerian, but between the two is a narrow clear space traversed by an irregular network of fibres, in some of the meshes of which nuclei are present.

The Miillerian duct may be traced in this condition for a considerable number of sections, the peculiar features above described becoming more and more marked as its termination is approached. It continues to dilate and attains a maximum size in the section or so before it disappears. A lumen may be observed in it up to its very end, but is usually irregular in outline and frequently traversed by strands of protoplasm. The Miillerian

EXCRETORY ORGANS. 721

duct finally terminates quite suddenly, and in the section immediately behind its termination the Wolffian duct assumes its normal appearance, and the part of its outer wall on the level of the Miillerian duct conies into contact with the germinal epithelium."

Before describing the development of the Mullerian duct in other Amniotic types it will be well to say a few words as to the identifications above adopted. The identification of the duct, usually called the Wolffian duct, with the segmental duct (exclusive of the pronephros) appears to be morphologically justified for the following reasons : (i) that it gives rise to part of the Mullerian duct as well as to the duct of the Wolffian body ; behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian body, before the Mullerian duct originates from it. (3) That it develops in a manner strikingly similar to that of the segmental duct of various lower forms.

With reference to the pronephros it is obvious that the organ identified as such is in many respects similar to the pronephros of the Amphibia. Both consist of a somewhat convoluted longitudinal canal, with a certain number of peritoneal openings ;

The main difficulties in the homology are :

(1) the fact that the pronephros in the Bird is not united with the segmental duct ;

(2) the fact that it is situated behind the front end of the Wolffian body. It is to be remembered in connection with the first of these difficulties

that in the formation of the Mullerian duct in Elasmobranchii the anterior undivided extremity of the primitive segmental duct, with the peritoneal opening, which probably represents the pronephros, is attached to the Mullerian duct, and not to the Wolffian duct ; though in Amphibia the reverse is the case. To explain the discontinuity of the pronephros with the segmental duct it is only necessary to suppose that the segmental duct and pronephros, which in the Ichthyopsida develop as a single formation, develop in the Bird as two independent structures a far from extravagant supposition, considering that the pronephros in the Bird is undoubtedly quite functionless.

With reference to the posterior position of the pronephros it is only necessary to remark that a change in position might easily take place after the acquirement of an independent development, and that the shifting is probably correlated with a shifting of the abdominal opening of the Mullerian duct.

The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota. The Mullerian duct is also usually stated to develop as a groove of the peritoneal epithelium, shewn in the Lizard in fig. 354, md., which is continued backward as a primitively solid rod in the space between B. ill. 46

722

AM N IOTA.

the Wolffian duct and peritoneal epithelium, without becoming attached to the Wolffian duct.

On the formation of the Miillerian duct, the duct of the mesonephros becomes the true mesonephric or Wolffian duct.

After these changes have taken place a new organ of great importance makes its appearance. This organ is the permanent kidney, or metanephros.

Metanephros. The mode of development of the metanephros has as yet only been satisfactorily elucidated in the Chick (Sedgwick, No. 549). The ureter and the collecting tubes of the kidney are developed from a dorsal outgrowth of the hinder part of the Wolffian duct. The outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of the Wolffian body.

This mass of mesoblastic cells may be called the metanephric blastema. Sedgwick, of the accuracy of whose account I have satisfied myself, has shewn that in the Chick it is derived from the intermediate cell mass of the region of about the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion of the intermediate cell mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema ; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, places itself on the inner side of the ureter in the position just described.

In the subsequent development of the kidney collecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.

The process just described appears to me to prove that the kidney of the A mniota is a specially differentiated posterior section of the primitive mesonephros.

According to the view of Remak and Kolliker the outgrowths from the ureter give rise to the whole of the tubuli uriniferi and the capsules of the Malpighian bodies, the mesoblast around them forming blood-vessels, etc. On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in

EXCRETORY ORGANS. 723

accordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formed in situ in the adjacent mesoblast.

Braun (No. 542) has arrived at the conclusion that in the Lacertilia the tissue, out of which the tubuli of the metanephros are formed, is derived from irregular solid ingrowths of the peritoneal epithelium, in a region behind the Wolffian body, but in a position corresponding to that in which the segmental tubes take their origin. These ingrowths, after separating from the peritoneal epithelium, unite together to form a cord into which the ureter sends the lateral outgrowths already described. These outgrowths unite with secreting tubuli and Malpighian bodies, formed in situ. In Lacertilia the blastema of the kidney extends into a postanal region. Braun's account of the origin of the metanephric blastema does not appear to me to be satisfactorily demonstrated.

The ureter does not long remain attached to the Wolffian duct, but its opening is gradually carried back, till (in the Chick between the 6th and 8th day) it opens independently into the cloaca.

Of the further changes in the excretory system the most important is the atrophy of the greater part of the Wolffian body, and the conversion of the Wolffian duct in the male sex into the vas deferens, as in Amphibia and the Elasmobranchii.

The mode of connection of the testis with the Wolffian duct is very remarkable, but may be derived from the primitive arrangement characteristic of Elasmobranchii and Amphibia.

In the structures connecting the testis with the Wolffian body two parts have to be distinguished, (i) that equivalent to the testicular network of the lower types, (2) that derived from the segmental tubes. The former is probably to be found in peculiar outgrowths from the Malpighian bodies at the base of the testes.

These were first discovered by Braun in Reptilia, and consist in this group of a series of outgrowths from the primary (?) Malpighian bodies along the base of the testis : they unite to form an interrupted cord in the substance of the testis, from which the testicular tubuli (with the exception of the seminiferous cells) are subsequently differentiated. These outgrowths, with the exception of the first two or three, become detached from the Malpighian bodies. Outgrowths similar to those in the male are found in the female, but subsequently atrophy.

Outgrowths homologous with those found by Braun have

46 2

724 AMNIOTA.

been detected by myself (No. 555) in Mammals. It is not certain to what parts of the testicular tubuli they give rise, but they probably form at any rate the vasa recta and rete vasculosum.

In Mammals they also occur in the female, and give rise to cords of tissue in the ovary, which may persist through life.

The comparison of the tubuli, formed out of these structures, with the Elasmobranch and Amphibian testicular network is justified in that both originate as outgrowths from the primary Malpighian bodies, and thence extend into the testis, and come into connection with the true seminiferous stroma.

As in the lower types the semen is transported from the testicular network to the Wolffian duct by parts of the glandular tubes of the Wolffian body. In the case of Reptilia the anterior two or three segmental tubes in the region of the testis probably have this function. In the case of Mammalia the vasa efferentia, i.e. the coni vasculosi, appear, according to the usually accepted view, to be of this nature, though Banks and other investigators believe that they are independently developed structures. Further investigations on this point are required. In Birds a connection between the Wolffian body and the testis appears to be established as in the other types. The Wolffian duct itself becomes, in the males of all Amniota, the vas deferens and the convoluted canal of the epididymis the latter structure (except the head) being entirely derived from the Wolffian duct.

In the female the Wolffian duct atrophies more or less completely.

In Snakes (Braun) the posterior part remains as a functionless canal, commencing at the ovary, and opening into the cloaca. In the Gecko (Braun) it remains as a small canal joining the ureter ; in Blindworms a considerable part of the canal is left, and in Lacerta (Braun) only interrupted portions.

In Mammalia the middle part of the duct, known as Gaertner's canal, persists in the females of some monkeys, of the pig and of many ruminants.

The Wolffian body atrophies nearly completely in both sexes ; though, as described above, part of it opposite the testis persists as the head of the epididymis. The posterior part of the gland from the level of the testis may be called the sexual part of the gland, the anterior part forming the non-sexual part.

EXCRETORY ORGANS. 725

The latter, i.e. the anterior part, is first absorbed ; and in some Reptilia the posterior part, extending from the region of the genital glands to the permanent kidney, persists till into the second year.

Various remnants of the Wolffian body are found in the adults of both sexes in different types. The most constant of them is perhaps the part in the female equivalent to the head of the epididymis and to parts also of the coiled tube of the epididymis, which may be called, with Waldeyer, the epoophoron 1 . This is found in Reptiles, Birds and Mammals ; though in a very rudimentary form in the first-named group. Remnants of the anterior non-sexual part of the Wolffian bodies have been called by Waldeyer parepididymis in the male, and paroophoron in the female. Such remnants are not (Braun) found in Reptilia, but are stated to be found in both male and female Birds, as a small organ consisting of blindly ending tubes with yellow pigment. In some male Mammals (including Man) a parepididymis is found on the upper side of the testis. It is usually known as the organ of Giraldes.

The Mlillerian duct forms, as has been stated, the oviduct in the female. The two ducts originally open independently into the cloaca, but in the Mammalia a subsequent modification of this arrangement occurs, which is dealt with in a separate section. In Birds the right oviduct atrophies, a vestige being sometimes left. In the male the Miillerian ducts atrophy more or less completely.

In most Reptiles and in Birds the atrophy of the Miillerian ducts is complete in the male, but in Lacerta and Anguis a rudiment of the anterior part has been detected by Leydig as a convoluted canal. In the Rabbit (Kolliker) 2 and probably other Mammals the whole of the ducts probably disappears, but in some Mammals, e.g. Man, the lower fused ends of the Miillerian ducts give rise to a pocket opening into the urethra, known as the uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the rudiments are more considerable, and may be continued into horns homologous with the horns of the uterus (Weber).

The hydatid of Morgani in the male is supposed (Waldeyer) to represent the abdominal opening of the Fallopian tube in the female, and therefore to be a remnant of the Miillerian duct.

Changes in the lower parts of the urinogenital ducts in the Amniota.

The genital cord. In the Monodelphia the lower part of the Wolffian ducts becomes enveloped in both sexes in a special

1 This is also called parovarium (His), and Rosenmiiller's organ.

2 Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but his account is by no means satisfactory, and its presence is distinctly denied by Kolliker.

726

AMNIOTA.

cord of tissue, known as -the genital cord (fig. 407, gc), within the lower part of which the MUllerian ducts are also enclosed. In the male the MUllerian ducts in this cord atrophy, except at their distal end where they unite to form the uterus masculinus. The Wolffian ducts, after becoming the vasa deferentia, remain for some time enclosed in the common cord, but afterwards separate from each other. The seminal vesicles are outgrowths of the vasa deferentia.

In the female the Wolffian ducts within the genital cord atrophy, though rudiments of them are for a long time visible or even permanently persistent. The lower parts of the MUllerian ducts unite to form the vagina and body of the uterus. The junction commences in the middle and extends forwards and backwards ; the stage with a median junction being retained permanently in Marsupials.

The urinogenital sinus and external generative organs. In all the Amniota, there open at first into the common cloaca the alimentary canal dorsally, the allantois ventrally, and the Wolffian and MUllerian ducts and ureters laterally. In Reptilia and Aves the embryonic condition is retained. In both groups the allantois serves as an embryonic urinary bladder, but while it atrophies in Aves, its stalk dilates to form a permanent urinary bladder in Reptilia. In Mammalia the dorsal part of the cloaca with the alimentary tract becomes first of all partially constricted off from the ventral, which then forms a urinogenital sinus (fig. 407, ug). In the course of development the urinogenital sinus becomes, in all Mammalia but the Ornithodelphia, completely separated from the intestinal cloaca, and the two parts obtain separate external openings. The ureters (fig. 407, 3) open higher up than the other ducts into the stalk of the allantois which dilates to form the bladder (4). The stalk connecting the bladder with the ventral wall of the body constitutes the urachus, and loses its lumen before the close of embryonic life. The part of the stalk of the allantois below the openings of the ureters narrows to form the urethra, which opens together with the Wolffian and MUllerian ducts into the urinogenital cloaca.

In front of the urinogenital cloaca there is formed a genital prominence (fig. 407, cp), with a groove continued from the

EXCRETORY ORGANS. 727

urinogenital opening ; and on each side a genital fold (&). In the male the sides of the groove on the prominence coalesce together, embracing between them the opening of the urinogenital cloaca ; and the prominence itself gives rise to the penis,

FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.)

The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are seen from the front.

3. ureter; 4. urinary bladder ; 5. urachus; of. genital ridge (ovary or testis) ; W. left Wolffian body ; x. part at apex from which coni vasculosi are afterwards developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of Wolffian and Mullerian ducts bound up in a common sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from which the labia majora or scrotum are developed.

along which the common urinogenital passage is continued. The two genital folds unite from behind forwards to form the scrotum.

In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis : the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus.

728 GENERAL CONCLUSIONS.

General conclusions and Summary.

Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvae of all other forms.

This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct the segmental duct are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Fiirbringer.

The duct of the pronephros is always developed prior to the gland, and there are two types according to which its development may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).

It is quite certain that the second of these processes is not a true record of the evolution of 'the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain.

The mode of development of the pronephros itself is apparently partly dependent on that of its duct. In Petromyzon, where the duct does not at first communicate with the body cavity, the pronephros is formed as a series of outgrowths from the duct, which meet the peritoneal epithelium and open into the body cavity ; but in other instances it is derived from the anterior open end of the groove which gives rise to the segmental duct. The open end of this groove may either remain single

EXCRETORY ORGANS. 729

(Teleostci, Ganoidei) or be divided into two, three or more apertures (Amphibia). The main part of the gland in either case is formed by convolutions of the tube connected with the peritoneal funnel or funnels. The peritoneal funnels of the pronephros appear to be segmentally arranged.

The pronephros is distinguished from the mesonephros by developmental as well as structural features. The most important of the former is the fact that the glandular tubules of which it is formed are always outgrowths of the segmental duct ; while in the mesonephros they are always or almost always 1 formed independently of the duct.

The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros; unless the structures found in Myxine are to be regarded as such. Functionally the place of such Malpighian bodies is taken by the vascular peritoneal ridge spoken of in the previous pages as the glomerulus.

That this body is really related functionally to the pronephros appears to be indicated (i) by its constant occurrence with the pronephros and its position opposite the peritoneal openings of this body ; (2) by its atrophy at the same time as the pronephros ; (3) by its enclosure together with the pronephridian stoma in a special compartment of the body-cavity in Teleostei and Ganoids, and its partial enclosure in such a compartment in Amphibia.

The pronephros atrophies more or less completely in most types, though it probably persists for life in the Teleostei and Ganoids, and in some members of the former group it perhaps forms the sole adult organ of excretion.

The cause of its atrophy may perhaps be related to the fact that it is situated in the pericardial region of the body-cavity, the dorsal part of which is aborted on the formation of a closed pericardium ; and its preservation in Teleostei and Ganoids may on this view be due to the fact that in these types its peritoneal funnel and its glomerulus are early isolated in a special cavity.

Mesonephros. The mesonephros is in all instances composed of a series of tubules (segmental tubes) which are developed independently of the segmental duct. Each tubule is

1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an exception to the general rule that there is no outgrowth from the segmental or metanephric duct to meet the segmental tubes.

730 GENERAL CONCLUSIONS.

typically formed of (i) a peritoneal funnel opening into (2) a Malpighian body, from which there proceeds (3) a coiled glandular tube, finally opening by (4) a collecting tube into the segmental duct, which constitutes the primitive duct for the mesonephros as well as for the pronephros.

The development of the mesonephridian tubules is subject to considerable variations.

(1) They may be formed as differentiations of the intermediate cell mass, and be from the first provided with a lumen, opening into the body-cavity, and directly derived from the section of the body-cavity present in the intermediate cell mass; the peritoneal funnels often persisting for life (Elasmobranchii).

(2) They may be formed as solid cords either attached to or independent of the peritoneal epithelium, which after first becoming independent of the peritoneal epithelium subsequently send downwards a process, which unites with it and forms a peritoneal funnel, which may or may not persist (Acipenser, Amphibia).

(3) They may be formed as in the last case, but acquire no secondary connection with the peritoneal epithelium (Teleostei, Amniota). In connection with the original attachment to the peritoneal epithelium, a true peritoneal funnel may however be developed (Aves, Lacertilia).

Physiological considerations appear to shew that of these three methods of development the first is the most primitive. The development of the tubes as solid cords can hardly be primary.

A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii. It is on the one hand difficult to understand why, if the openings are homologous in the two types, the original peritoneal attachment should be obliterated in Amphibia, only to be shortly afterwards reacquired. On the other hand it is still more difficult to understand what physiological gain there could be, on the assumption of the non-homology of the openings, in the replacement of the primary opening by a secondary opening exactly similar to it. Considering the great variations in development which occur in undoubtedly homologous parts I incline to the view that the openings in the two types are homologous.

EXCRETORY ORGANS.

731

In the majority of the lower Vertebrata the mesonephric tubes have at first a segmental arrangement, and this is no doubt the primitive condition. The coexistence of two, three, or more of them in a single segment in Amphibia, Aves and Mammalia has recently been shewn, by an interesting discovery of Eisig, to have a parallel amongst Chaetopods, in the coexistence of several segmental organs in a single segment in some of the Capitellidae.

In connection with the segmental features of the mesonephros it is perhaps worth recalling the fact that in Elasmobranchii as well as other types there are traces of segmental tubes in some of the postanal segments. In the case of all the segmental tubes a Malpighian body becomes established close to the extremity of the tube adjoining the peritoneal opening, or in an homologous position in tubes without such an opening. The opposite extremity of the tube always becomes attached to the segmental duct.

In many of the segments of the mesonephros, especially in the hinder ones, secondary and tertiary tubes become developed in certain types, which join the collecting canals of the primary tubes, and are provided, like the primary tubes, with Malpighian bodies at their blind extremities.

There can it appears to me be little or no doubt that the secondary tubes in the different types are homodynamous if not homologous. Under these circumstances it is surprising to find in what different ways they take their origin. In Elasmobranchii a bud sprouts out from the Malpighian body of one segment, and joins the collecting tube of the preceding segment, and subsequently, becoming detached from the Malpighian body from which it sprouted, forms a fresh secondary Malpighian body at its blind extremity. Thus the secondary tubes of one segment are formed as buds from the segment behind. In Amphibia (Salamandra) and Aves the secondary tubes develop independently in the mesoblast. These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below.

Before leaving the mesonephros it may be worth while putting forward some hypothetical suggestions as to its origin and relation to the pro

732 GENERAL CONCLUSIONS.

nephros, leaving however the difficult questions as to the homology of the segmental tubes with the segmental organs of Chastopods for subsequent discussion.

It is a peculiarity in the development of the segmental tubes that they at first end blindly, though they subsequently grow till they meet the segmental duct with which they unite directly, without the latter sending out any offshoot to meet them 1 . It is difficult to believe that peritoneal infundibula ending blindly and unprovided with some external orifice can have had an excretory function, and we are therefore rather driven to suppose that the peritoneal infundibula which become the segmental tubes were either from the first provided each with an orifice opening to the exterior, or were united with the segmental duct. If they were from the first provided with external openings we may suppose that they became secondarily attached to the duct of the pronephros (segmental duct), and then lost their external openings, no trace of these structures being left, even in the ontogeny of the system. It would appear to me more probable that the pronephros, with its duct opening into the cloaca, was the only excretory organ of the unsegmented ancestors of the Chordata, and that, on the elongation of the trunk and its subsequent segmentation, a series of metameric segmental tubes became evolved opening into the segmental duct, each tube being in a sort of way serially homologous with the primitive pronephros. With the segmentation of the trunk the latter structure itself may have acquired the more or less definite metameric arrangement of its parts.

Another possible view is that the segmental tubes may be modified derivatives of posterior lateral branches of the pronephros, which may at first have extended for the whole length of the body-cavity. If there is any truth in this hypothesis it is necessary to suppose that, when the unsegmented ancestor of the Chordata became segmented, the posterior branches of the primitive excretory organ became segmentally arranged, and that, in accordance with the change thus gradually introduced in them, the time of their development became deferred, so as to accord to a certain extent with the time of formation of the segments to which they belonged. The change in their mode of development which would be thereby introduced is certainly not greater than that which has taken place in the case of segmental tubes, which, having originally developed on the Elasmobranch type, have come to develop as they do in the posterior part of the mesonephros of Salamandra, Birds, etc.

Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the

1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement.

EXCRETORY ORGANS. 733

excretory organs is perhaps found permanently in Cyclostomata 1 and transitorily in the embryos of most forms.

At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. 626).

The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards ; so that primitively the segmental duct must have served as efferent duct both for the generative products and the pronepJiric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Amphibia).

The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two a ventral Mullerian duct and a dorsal Wolffian duct.

The Mullerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii ; but the generative function of the Mullerian duct gradually impressed itself more and more upon the embryonic development, so that, in the course of time, the Mullerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii ; while in Aves it has been carried even further, and it seems possible that in some Amniota the Mullerian and segmental

1 It is by no means certain that the transportation outwards of the genital products by the abdominal pores in the Cyclostomata may not be the result of degeneration.

734 GENERAL CONCLUSIONS.

ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fiirbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products ; this function being, no doubt, eventually restricted to one of them.

Three different types of development of the abdominal opening of the Mullerian duct have been observed.

In Amphibia (Salamandra) the permanent opening of the Mullerian duct is formed independently, some way behind the pronephros.

In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Mullerian duct, and no true pronephros appears to be formed.

In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Mullerian duct.

These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland.

In Elasmobranchii, on the other hand, the functional opening is formed at a period when we should expect the pronephros to develop. This state is very possibly the result of a differentiation by which the pronephros gradually ceased to become developed, but one of its peritoneal openings remained as the abdominal aperture of the Mullerian duct. Aves, finally, appear to have become differentiated along a third line ; since in their ancestors the anterior (?) pore of the head-kidney appears to have become specialised as the permanent opening of the Mullerian duct.

The Mullerian duct is usually formed in a more or less complete manner in both sexes. In Ganoids, where the separation between it and the Wolffian duct is not completed to the cloaca, and in the Dipnoi, it probably serves to carry off the generative products of both sexes. In other cases however only the female

EXCRETORY ORGANS.

735

products pass out by it, and the partial or complete formation of the Mullerian duct in the male in these cases needs to be explained. This may be done either by supposing the Ganoid arrangement to have been the primitive one in the ancestors of the other forms, or, by supposing characters acquired primitively by the female to have become inherited by both sexes.

It is a question whether the nature of the generative ducts of Teleostei can be explained by comparison with those of Ganoids. The fact that the Mullerian ducts of the Teleostean Ganoid Lepidosteus attach themselves to the generative organs, and thus acquire a resemblance to the generative ducts of Teleostei, affords a powerful argument in favour of the view that the generative ducts of both sexes in the Teleostei are modified Mullerian ducts. Embryology can however alone definitely settle this question.

In the Elasmobranchii, Amphibia, and Amniota the male products are carried off by the Wolffian duct, and they are transported to this duct, not by open peritoneal funnels of the mesonephros, but by a network of ducts which sprout either from a certain number of the Malpighian bodies opposite the testis (Amphibia, Amniota), or from the stalks connecting the Malpighian bodies with the open funnels (Elasmobranchii). After traversing this network the semen passes (except in certain Anura) through a variable number of the segmental tubes directly to the Wolffian duct. The extent of the connection of the testis with the Wolffian body is subject to great variations, but it is usually more or less in the anterior region. Rudiments of the testicular network have in many cases become inherited by the female.

The origin of the connection between the testis and Wolffian body is still very obscure. It would be easy to understand how the testicular products, after falling into the body-cavity, might be taken up by the open extremities of some of the peritoneal funnels, and how such open funnels might have groove-like prolongations along the mesorchium, which might eventually be converted into ducts. Ontogeny does not however altogether favour this view of the origin of the testicular network. It seems to me nevertheless the most probable view which has yet been put forward.

The mode of transportation of the semen by means of the mesonephric tubules is so peculiar as to render it highly improbable that it was twice acquired, it becomes therefore necessary to suppose that the Amphibia and

736 GENERAL CONCLUSIONS.

Amniota inherited this mode of transportation of the semen from the same ancestors as the Elasmobranchii. It is remarkable therefore that in the Ganoidei and Dipnoi this arrangement is not found.

Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the Miillerian duct serving for both sexes is the primitive arrangement, and the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary condition, which has originated at a stage in the evolution of the Vertebrata when some of the segmental tubes had begun to serve as the efferent ducts of the testis, and has resulted in consequence of a degeneration of the latter structures. Although the second alternative is the more easy to reconcile with the affinities of the Ganoid and Elasmobranch types, as indicated by the other features of their organization, I am still inclined to accept the former ; and consider that the incomplete splitting of the segmental duct in Ganoidei is a strong argument in favour of this view.

Metanephros. With the employment of the Wolffian duct to transport the semen there seems to be correlated (i) a tendency of the posterior segmental tubes to have a duct of their own, in which the seminal and urinary fluids cannot become mixed, and (2) a tendency on the part of the anterior segmental tubes to lose their excretory function. The posterior segmental tubes, when connected in this way with a more or less specialised duct, have been regarded in the preceding pages as constituting a metanephros.

This differentiation is hardly marked in the Anura, but is well developed in the Urodela and in the Elasmobranchii ; and in the latter group has become inherited by both sexes. In the Amniota it culminates, according to the view independently arrived at by Semper and myself, (i) in the formation of a completely distinct metanephros in both sexes, formed however, as shewn by Sedgwick, from the same blastema as the Wolffian body, and (2) in the atrophy in the adult of the whole Wolffian body, except the part uniting the testis and the Wolffian duct.

The homology between the posterior metanephridian section of the Wolffian body, in Elasmobranchii and Urodela, and the kidney of the Amniota, is only in my opinion a general one, i.e. in both cases a common cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more or less similar differentiation of parts.

Fiirbringer has urged against Semper's and my view that no satisfactory proof of it has yet been offered. This proof has however, since Fiirbringer wrote his paper, been supplied by Sedgwick's observations. The development of the kidney in the Amniota is no doubt a direct as opposed to a phylogenetic development ; and the substitution of a direct for

EXCRETORY ORGANS. 737

a phylogenetic development has most probably been rendered possible by the fact that the anterior part of the mesonephros continued all the while to be unaffected and to remain as the main excretory organ during foetal life.

The most serious difficulty urged by Fiirbringer against the homology is the fact that the ureter of the metanephros develops on a type of its own, which is quite distinct from the mode of development of the ureters of the metanephros of the Ichthyopsidan forms. It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish.

Comparison of the Excretory organs of the Chordata and Invertebrata.

The structural characters and development of the various forms of excretory organs described in the preceding pages do not appear to me to be sufficiently distinctive to render it possible to establish homologies between these organs on a satisfactory basis, except in closely related groups.

The excretory organs of the Platyelminthes are in many respects similar to the provisional excretory organ of the trochosphere of Polygordius and the Gephyrea on the one hand, and to the Vertebrate pronephros on the other ; and the Platyelminth excretory organ with an anterior opening might be regarded as having given origin to the trochosphere organ, while that with a posterior opening may have done so for the Vertebrate pronephros 1 .

Hatschek has compared the provisional trochosphere excretory organ of Polygordius to the Vertebrate pronephros, and the posterior Chastopod segmental tubes to the mesonephric tubes ; the latter homology having been already suggested independently by both Semper and myself. With reference to the comparison of the pronephros with the provisional excretory organ of Polygordius there are two serious difficulties :

(1) The pronephric (segmental) duct opens directly into the cloaca, while the duct of the provisional trochosphere excretory organ opens anteriorly, and directly to the exterior.

(2) The pronephros is situated within the segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts ; while the provisional trochosphere organ is placed in front of the segmented region of the trunk, and is in no way segmented.

The comparison of the mesonephric tubules with the segmental excretory organs of the Chaetopoda, though not impossible, cannot be satisfactorily admitted till some light has been thrown upon the loss of the supposed external openings of the tubes, and the origin of their secondary connection with the segmental duct.

1 This suggestion has I believe been made by Fiirbringer. B. III. 47

738 BIBLIOGRAPHY.

Confining our attention to the Invertebrata it appears to me fairly clear that Hatschek is justified in holding the provisional trochosphere excretory organs of Polygordius, Echiurus and the Mollusca to be homologous. The atrophy of all these larval organs may perhaps be due to the presence of a well-developed trunk region in the adult (absent in the larva), in which excretory organs, probably serially homologous with those present in the anterior part of the larva, became developed. The excretory organs in the trunk were probably more conveniently situated than those in the head, and the atrophy of the latter in the adult state was therefore brought about, while the trunk organs became sufficiently enlarged to serve as the sole excretory organs.

BIBLIOGRAPHY OF THE EXCRETORY ORGANS. Invertebrata.

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Stat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d. Cesto'ides." Archives de Biologic, Vol. I. 1880.

(514) B. Hatschek. "Studien lib. Entwick. d. Anneliden." Arbeit, a. d. zool. Instit. Wien, Vol. I. 1878.

(515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d. zool. Instit. Wien, Vol. in. 1880.

EXCRETORY ORGANS OF VERTEBRATA. General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." yournal of Anat. and Phys., Vol. X. 1876.

(517) Max. Furbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meek el. Zur Morphol. d. Hani- u. Geschlechtnverkz.d. Wirbelthiere, etc. Halle, 1848.

(519) Joh. Miiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Rathke. " Beobachtungen u. Betrachtungen u. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825.

(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875 (522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.

1 The papers of Furbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.

BIBLIOGRAPHY. 739

ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat., Vol. XI. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k. Ak. Wiss. Berlin, 1845.

(525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschri/t, Vol. IX. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. n. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Entivicklung d. Teleostierniere. Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersitchungen ii. die mdnnlichen Geschleehts- und Harnwerkzeuge d. nackten Amphibien. Dorpat, 1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Hisiologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wiirzburg. Vol. III. 1876.

(538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit. f. wiss. Zool., Vol. IV.

Vide also Gotte (No. 296).

Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of a head -kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878.

(540 ) Banks. On the Wolffian bodies of the fatus and their remains in the adult. Edinburgh, 1864.

472

74O BIBLIOGRAPHY.

(541) Th. Bornhaupt. Untersuchungen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. iv. 1877.

(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d. WolfFschen Ganges im Hiihnerei." Mini. Acad. Imp. Petersbourg, vn. Series, Vol. xxvil. 1880.

(544) Th. Egli. Beitrage zur Anat. und Entwick. d. Geschlechtsorgane. Inaug. Diss. Zurich, 1876.

(545) E. Gasser. Beitrage zur Entwicklungsgeschichte d. Allantois, der Milllcr'schen Gange u. des Afters. Frankfurt, 1874.

(546) E. Gasser. "Beob. iib. d. Entstehung d. Wolff schen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Hiihnerembryonen." Sitz. d. GeseU. zur Befdrderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchting iiber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.

(550) A. Sedgwick. "On the development of the structure known as the glomerulus of the head-kidney in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. xiv. 1879.

(553) E. H. Weber. Zusdtze z. Lehre von Baue u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298).

CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS.

GENERATIVE ORGANS.

THE structure and growth of the ovum and spermatozoon were given in the first chapter of this work, but their derivation from the germinal layers was not touched on, and it is this subject with which we are here concerned. If there are any structures whose identity throughout the Metazoa is not open to doubt these structures are the ovum and spermatozoon ; and the constancy of their relations to the germinal layers would seem to be a crucial test as to whether the latter have the morphological importance usually attributed to them.

The very fragmentary state of our knowledge of the origin of the generative cells has however prevented this test being so far very generally applied.

Porifera. In the Porifera the researches of Schulze have clearly demonstrated that both the ova and the spermatozoa take their origin from indifferent cells of the general parenchyma, which may be called mesoblastic. The primitive germinal cells of the two sexes are not distinguishable ; but a germinal cell by enlarging and becoming spherical gives rise to an ovum ; and by subdivision forms a sperm-morula, from the constituent cells of which the spermatozoa are directly developed.

Ccelenterata. The greatest confusion prevails as to the germinal layer from which the male and female products are derived in the Ccelenterata 1 .

1 E. van Beneden (No. 556) was the first to discover a different origin for the generative products of the two sexes in Hydractinia, and his observations have led to numerous subsequent researches on the subject. For a summary of the observations on the Hydroids vide Weismann (No. 560).

742 CCELENTERATA.

The following apparent modes of origin of these products have been observed.

(1) The generative products of both sexes originate in the ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free Gonophores of Hydromedusae, the Siphonophora, and probably the Ctenophora.

(2) The generative products of both sexes originate in the entoderm (hypoblast) : Plumularia and Sertularella, amongst the Hydroids, and the. whole of the Acraspeda and Actinozoa.

(3) The male cells are formed in the ectoderm, and the female in the entoderm : Gonothyraea, Campanularia, Hydractinia, Clava.

In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Ccelenterata have led, it would seem to be necessary either to hold that there is no definite homology between the germinal layers in the different forms of Ccelenterata, or to offer some satisfactory explanation of the behaviour of the genital products, which would not involve the acceptance of the first alternative.

Though it can hardly be said that such an explanation has yet been offered, some observations of Kleinenberg (No. 557) undoubtedly point to such an explanation being possible.

Kleinenberg has shewn that in Eudendrium the ova migrate freely from the ectoderm into the endoderm, and vice versa ; but he has given strong grounds for thinking that they originate in the ectoderm. He has further shewn that the migration in this type is by no means an isolated phenomenon.

Since it is usually only possible to recognise generative elements after they have advanced considerably in development, the mere position of a generative cell, when first observed, can afford, after what Kleinenberg has shewn, no absolute proof of its origin. Thus it is quite possible that there is really only one type of origin for the generative cells in the Ccelenterata.

Kleinenberg has given reasons for thinking that the migration of the ova into the entoderm may have a nutritive object. If this be so, and there are numerous facts which shew that the position of generative cells is often largely influenced by their nutritive requirements, it seems not impossible

GENERATIVE ORGANS. 743

that the endodermal position of the generative organs in the Actinozoa and acraspedote Medusre may have arisen by a continuously earlier migration of the generative cells from the ectoderm into the endoderm ; and that the migration may now take place at so early a period of the development, that we should be justified in formally holding the generative products to be endodermal in origin.

\Ve might perhaps, on this view, formulate the origin of the generative products in the Ccelenterata in the following way :

Both ova and spermatozoa primitively originated in the ectoderm, but in order to secure a more complete nutrition the cells which give rise to them exhibit in certain groups a tendency to migrate into the endoderm. This migration, which may concern the generative cells of one or of both the sexes, takes place in some cases after the generative cells have become recognisable as such, and very probably in other cases at so early a period that it is impossible to distinguish the generative cells from indifferent embryonic cells.

Very little is known with reference to the origin of the generative cells in the triploblastic Invertebrata.

Chaetopoda and Gephyrea. In the Chaetopoda and Gephyrea, the germinal cells are always developed in the adult from the epithelial lining of the body cavity ; so that their origin from the mesoblast seems fairly established.

If we are justified in holding the body cavity of these forms to be a derivative of the primitive archenteron (vide pp. 356 and 357) the generative cells may fairly be held to originate from a layer which corresponds to the endoderm of the Ccelenterata 1 .

Chaetognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Biitschli, has been recently treated with great detail by O. Hertwig 2 .

The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore. These cells soon divide, and at the same time pass out of the hypoblast, and enter the archenteric cavity (fig. 408 - A, ge). The division into four cells, which is not satisfactorily represented ifl my diagram, takes place in such a way that two

1 The Hertwigs (No. 271) state that in their opinion the generative cells arise from the lining of the body cavity in all the forms whose body cavity is a product of the archenteron. We do not know anything of the embryonic development of the generative organs in the Echinodermata, but the adult position of the generative organs in this group is very unfavourable to the Hertwigs' view.

2 O. Hertwig, Die Chcetognathen. Jena, 1880

744

CH^ETOGNATHA.

cells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the

FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after

Biitschli, and B after Kowalevsky.) The three embryos are represented in the same positions.

A. Represents the gastrula stage.

B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.

C. Represents a later stage, in which the mouth involution (in) has become continuous with the alimentary tract, and the blastopore is closed.

///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs.

ovaries, one half of each primitive cell thus forming an ovary, and the other a testis.

FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal

surface. B, from the side. (After Biitschli.)

m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; <.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchnopleure ; ge. generative organs.

GENERATIVE ORGANS.

745

When the archenteric cavity is divided into a median alimentary tract, and two lateral sections forming the body cavity, the generative organs are placed in the common vestibule into which both the body cavity and alimentary cavity at first open (fig. 408).

The generative organs long retain their character as simple cells. Eventually (fig. 409) the two ovaries travel forwards, and apply themselves to the body walls, while the two testes also become separated by a backward prolongation of the median alimentary tract.

On the formation of the transverse septum dividing the tail from the body, the ovarian cells lie immediately in front of this septum, and the testicular cells in the region behind it.

Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek finds that the generative organs originate from a pair of specially large mesoblast cells, situated in the space between the stomach and the floor of the vestibule. The two cells undergo changes, which have an obvious resemblance to those of the generative cells of the Chsetognatha. They become surrounded by an investment of mesoblast cells, and divide so as to form two masses. Each of these masses at a later period separates into an anterior and a posterior part. The former becomes the ovary, the latter the testis.

Nematoda. In the Nematoda the generative organs are derived from the division of a single cell which would appear to be mesoblastic 1 .

Insecta. The generative cells have been observed at a very early embryonic stage in several insect forms (Vol. II. p. 404), but the observations so far recorded with reference to them do not enable us to determine with certainty from which of the germinal layers they are derived.

Crustacea. In Moina, one of the Cladocera, Grobben 2 has shewn that the generative organs are derived from a single cell, which becomes differentiated during the segmentation. This cell, which is in close contiguity with the cells from which both the mesoblast and hypoblast originate, subsequently divides ;

1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.

2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil. Wien. Vol. II. 1879.

746

CHORDATA.

sp.c

but at the gastrula stage, and after the mesoblast has become formed, the cells it gives rise to are enclosed in the epiblast, and do not migrate inwards till a later stage. The products of the division of the generative cell subsequently divide into two masses. It is not possible to assign the generative cell of Moina to a definite germinal layer. Grobben, however, thinks that it originates from the division of a cell, the remainder of which gives rise to the hypoblast.

Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs. 405 C and 4io,/0). They appear to be derived from the epithelial cells amongst which they lie ; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium.

It is at first impossible to distinguish the germinal cells which will become ova from those which will become spermatozoa.

The former however remain within the peritoneal epithelium (fig. 41 1), and become converted into ova in a manner more particularly described in Vol. II. pp. 54 59.

The history of the primitive germinal cells in the male has not been so adequately worked out as in the female.

The fullest history of them is that given by Semper (No. 559) for the Elasmobranchii, the general accuracy of which I can fully support ;

FIG. 410. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal cord ; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate ; mp'. inner layer of- muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; i>.o. primitive generative cells.

GENERATIVE ORGANS.

747

though with reference to certain stages in the history further researches are still required 1 .

In Elasmobranchii the male germinal cells, instead of remaining in the germinal epithelium, migrate into the adjacent stroma, accompanied I believe by some of the indifferent epithelial cells. Here they increase in number, and give rise to masses of variable form, composed partly of true germinal cells, and partly of smaller cells with deeply staining nuclei, which are, I believe, derived from the germinal epithelium.

FIG. 411. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OK SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.

These masses next break up into ampullae, mainly formed of germinal cells, and each provided with a central lumen ; and these ampullae attach themselves to tubes derived from the smaller cells, which are in their turn continuous with the testicular network. The spermatozoa are developed from the cells forming the walls of the primitive ampulla;; but the process of their formation does not concern us in this chapter.

In the Reptilia Braun has traced the passage of the primitive germinal cells into the testicular tubes, and I am able to confirm his observations on this point : he has not however traced their further history.

1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring my own observations into accord with his as to the structure of the Elasmobranch testis.

MODE OF EXIT OF GENITAL PRODUCTS.

In Mammalia the evidence of the origin of the spermatospores from the germinal epithelium is not quite complete, but there can be but little doubt of its occurrence 1 .

In Amphioxus Langerhans has shewn that the ova and spermatozoa are derived from similar germinal cells, which may be compared to the germinal epithelium of the Vertebrata. These cells are however segmentally arranged as separate masses (vide Vol. II. p. 54).

BIBLIOGRAPHY.

(554) G. Balbiani. Lemons s. la generation des Vcrlebrcs. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart, J. of Micr. Science, Vol. xvm.

(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit. f. -wiss. Zool., Vol. xxxv. 1881.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.zoot. Inslit. Wilrzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a. d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Fitffcalso O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

GENITAL DUCTS.

The development and evolution of the generative ducts is as yet very incompletely worked out, but even in the light of our present knowledge a comparative review of this subject brings to light features of considerable interest, and displays a fruitful field for future research.

In the Ccelenterata there are no generative ducts.

In the Hydromedusae and Siphonophora the generative products are liberated by being dehisced directly into the surrounding medium ; while in the Acraspeda, the Actinozoa and the Ctenophora, they are dehisced into parts of the gastrovascular system, and carried to the exterior through the mouth.

The arrangement in the latter forms indicates the origin of

1 An entirely different view of the origin of the sperm cells has been adopted by Balbiani, for which the reader is referred to his Memoir (No. 554).

GENITAL DUCTS.

749

the methods of transportation of the genital products to the exterior in many of the higher types.

It has been already pointed out that the body cavity in a very large number of forms is probably derived from parts of a gastrovascular system like that of the Actinozoa.

When the part of the gastrovascular system into which the generative products were dehisced became, on giving rise to the body cavity, shut off from the exterior, it would be essential that some mode of transportation outwards of the generative products should be constituted.

In some instances simple pores (probably already existing at the time of the establishment of a closed body cavity) become the generative ducts. Such seems probably to have been the case in the Chaetognatha (Sagitta) and in the primitive Chordata.

In the latter forms the generative products are sometimes dehisced into the peritoneal cavity, and thence transported by the abdominal pores to the exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus they pass by dehiscence into the atrial cavity, and thence through the gill slits and by the mouth, or by the abdominal pore (?) to the exterior. The arrangement in Amphioxus and the Teleostei is probably secondary, as possibly also is that in the Cyclostomata ; so that the primitive mode of exit of the generative products in the Chordata is still uncertain. It is highly improbable that the generative ducts of the Tunicata are primitive structures.

A better established and more frequent mode of exit of the generative products when dehisced into the body cavity is by means of the excretory organs. The generative products pass from the body cavity into the open peritoneal funnels of such organs, and thence through their ducts to the exterior. This mode of exit of the generative products is characteristic of the Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata, and probably also of the Mollusca. It is moreover quite possible that it occurs in the Polyzoa, some of the Arthropoda, the Platyelminthes and some other types.

The simple segmental excretory organs of the Polychaeta, the Gephyrea and the Brachiopoda serve as generative canals, and in many instances they exhibit no modification, or but a very slight one, in connection with their secondary generative

750 DERIVATION FROM EXCRETORY ORGANS.

function ; while in other instances, e.g. Bonellia, such modification is very considerable.

The generative ducts of the Oligochaeta are probably derived from excretory organs. In the Terricola ordinary excretory organs are present in the generative segments in addition to the generative ducts, while in the Limicola generative ducts alone are present in the adult, but before their development excretory organs of the usual type are found, which undergo atrophy on the appearance of the generative ducts (Vedjovsky).

From the analogy of the splitting of the segmental duct of the Vertebrata into the Miillerian and Wolffian ducts, as a result of a combined generative and excretory function (vide p. 728), it seems probable that in the generative segments of the Oligochasta the excretory organs had at first both an excretory and a generative function, and that, as a secondary result of this double function, each of them has become split into two parts, a generative and an excretory. The generative part has undergone in all forms great modifications. The excretory parts remain unmodified in the Earthworms (Terricola), but completely abort on the development of the generative ducts in the Limicola. An explanation may probably be given of the peculiar arrangements of the generative ducts in Saccocirrus amongst the Polychaeta (vide Marion and Bobretzky), analogous to that just offered for the Oligochaeta.

The very interesting modifications produced in the excretory organs of the Vertebrata by their serving as generative ducts were fully described in the last chapter ; and with reference to this part of our subject it is only necessary to call attention to the case of Lepidosteus and the Teleostei.

In Lepidosteus the Mullerian duct appears to have become attached to the generative organs, so that the generative products, instead of falling directly into the body cavity and thence entering the open end of a peritoneal funnel of the excretory organs, pass directly into the Mullerian duct without entering the body cavity. In most Teleostei the modification is more complete, in that the generative ducts in the adult have no obvious connection with the excretory organs.

The transportation of the male products to the exterior in all the higher Vertebrata, without passing into the body cavity, is in principle similar to the arrangement in Lepidosteus.

The above instances of the peritoneal funnels of an excretory organ becoming continuous with the generative glands, render it highly probable that there may be similar instances amongst the In vertebrata.

GENITAL DUCTS.

751

As has been already pointed out by Gegenbaur there are many features in the structure of the genital ducts in the more primitive Mollusca, which point to their having been derived from the excretory organs. In several Lamellibranchiata 1 (Spondylus, Lima, Pecten) the generative ducts open into the excretory organs (organ of Bojanus), so that the generative products have to pass through the excretory organ on their way to the exterior. In other Lamellibranchiata the genital and excretory organs open on a common papilla, and in the remaining types they are placed close together.

In the Cephalopoda again the peculiar relations of the generative organs to their ducts point to the latter having primitively had a different, probably an excretory, function. The glands are not continuous with the ducts, but are placed in special capsules from which the ducts proceed. The genital products are dehisced into these capsules and thence pass into the ducts.

In the Gasteropoda the genital gland is directly continuous with its duct, and the latter, especially in the Pulmonata and Opisthobranchiata, assumes such a complicated form that its origin from the excretory organ would hardly have been suspected. The fact however that its opening is placed near that of the excretory organ points to its being homologous with the generative ducts of the more primitive types.

In the Discophora, where the generative ducts are continuous with the glands, the structure both of the generative glands and ducts points to the latter having originated from excretory organs.

It seems, as already mentioned, very possible that there are other types in which the generative ducts are derived from the excretory organs. In the Arthropoda for instance the generative ducts, where provided with anteriorly placed openings, as in the Crustacea, Arachnida and the Chilognathous Myriapoda, the Pcecilopoda, etc., may possibly be of this nature, but the data for deciding this point are so scanty that it is not at present possible to do more than frame conjectures.

The ontogeny of the generative ducts of the Nematoda and

1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d. Thierreichs, Vol. in. p. 404.

752 DERIVATION FROM EXCRETORY ORGANS.

the Insecta appears to point to their having originated independently of the excretory organs.

In the Nematoda the generative organs of both sexes originate from a single cell (Schneider, Vol. I. No. 390).

This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (i) a superficial investing layer, and (2) an axial portion.

In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube.

In the male the superficial layer, which gives rise tc the epithelium of the vas deferens, is only formed at the hinder ond of the original column. In other respects the development takes place as in the female.

In the Insecta again the evidence, though somewhat conflicting, indicates that the generative ducts arise very much as in Nematodes, from the same primitive mass as the generative organs. In both of these types it would seem probable that the generative organs were primitively placed in the body cavity, and attached to the epidermis, through a pore in which their products passed out ; and that, acquiring a tubular form, the peripheral part of the gland gave rise to a duct, the remainder constituting the true generative gland. It is quite possible that the generative ducts of such forms as the Platyelminthes may have had a similar origin to those in Insecta and Nematoda, but from the analogy of the Mollusca there is nearly as much to be said for regarding them as modified excretory organs.

In the Echinodermata nothing is unfortunately known as to the ontogeny of the generative organs and ducts. The structure of these organs in the adult would however seem to indicate that the most primitive type of echinoderm generative organ consists of a blind sack, projecting into the body cavity, and opening by

GENITAL DUCTS. 753

a pore to the exterior. The sack is lined by an epithelium, continuous with the epidermis, the cells of which give rise to the ova or spermatozoa. The duct of these organs is obviously hardly differentiated from the gland ; and the whole structure might easily be derived from the type of generative organ characteristic of the Hydromedusae, where the generative cells are developed from special areas of the ectoderm, and, when ripe, pass directly into the surrounding medium.

If this suggestion is correct we may suppose that the generative ducts of the Echinodermata have a different origin to those of the majority of 1 the remaining triploblastica.

Their ducts have been evolved in forms in which the generative products continued to be liberated directly to the exterior, as in the Hydromedusae ; while those of other types have been evolved in forms in which the generative products were first transported, as in the Actinozoa, into the gastrovascular canals 2 .

1 It would be interesting to have further information about Balanoglossus.

2 These views fit in very well with those already put forward in Chapter xm. on the affinities of the Echinodermata.

B. III.

48

CHAPTER XXV.

THE ALIMENTARY CANAL AND ITS APPENDAGES, IN THE CHORDATA.

THE alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (i) the mesenteron lined by hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and (3) the proctodaeum or anal section lined like the stomodaeum by epiblast.

Mesenteron.

The early development of the epithelial wall of the mesenteron has already been described (Chapter XI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig. 420, ne). It often remains for a long time widely open in the middle towards the yolk-sack.

It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292 300).

The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Gotte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod.

MESENTERON.

755

It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk.

The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line (fig. 412, r), or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord (fig. 413, *).

FIG. 412. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.

df. dorsal fin ; sp.c. spinal cord ; //. body cavity ; sp. splanchnic layer of mesoblast ; so. somatic layer of mesoblast; mp'. portion of splanchnic mesoblast commencing to be differentiated into muscles ; ch. notochord ; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract ; al. alimentary tract.

FIG. 413. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

nc. neural canal ; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp'. portion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract.

In the hindermost part of the body its mode of formation differs somewhat from that above described. In this part the alimentary wall is' very thick, and undergoes no special growth prior to the formation of the subnotochordal rod ; on the contrary, a small linear portion of the wall becomes scooped out along the median dorsal line, and eventually separates from the remainder as the rod in question. In the trunk the splitting off of the rod takes place from before backwards, so that the anterior part of it is formed before the posterior.

The section of the subnotochordal rod in the head would appear to develop in the same way as that in the trunk, and the splitting off from the throat proceeds from before backwards.

482

756 MESENTERY.

On the formation of the dorsal aorta, the subnotochordal rod becomes separated from the wall of the gut and the aorta interposed between the two (fig. 367, *).

When the subnotochordal rod attains its fullest development it terminates anteriorly some way in front of the auditory vesicle, though a little behind the end of the notochord ; posteriorly it extends very nearly to the extremity of the tail and is almost co-extensive with the postanal section of the alimentary tract, though it does not reach quite so far back as the caudal vesicle (fig. 424, b x). Very shortly after it has attained its maximum size it begins to atrophy in front. We may therefore conclude that its atrophy, like its development, takes place from before backwards. During the later embryonic stages not a trace of it is to be seen. It has also been met with in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in all of which it appears to develop in fundamentally the same way as in Elasmobranchii. In Acipenser it appears to persist in the adult as the subvertebral ligament (Bridge, Salensky). It has not yet been found in a fully developed form in any amniotic Vertebrate, though a thickening of the hypoblast, which may perhaps be a rudiment of it, has been found by Marshall and myself in the Chick (fig. 1 10, x).

Eisig has instituted an interesting comparison between it and an organ which he has found in a family of Chaetopods, the Capitellidas. In these forms there is a tube underlying the alimentary tract for nearly its whole length, and opening into it in front, and probably behind. A remnant of such a tube might easily form a rudiment like the subnotochordal rod of the Ichthyopsida, and as Eisig points out the prolongation into the latter during its formation of the lumen of the alimentary tract distinctly favours such a view of its original nature. We can however hardly suppose that there is any direct genetic connection between Eisig's organ in the Capitellidas and the subnotochordal rod of the Chordata.

Splanchnic mesoblast and mesentery- The mesentcron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer cpithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance.

ALIMENTARY CANAL. 757

Coincidently with their differentiation the connective tissuestratum of the peritoneum becomes established.

The Mesentery. Prior to the splanchnic mesoblast growing round the alimentary tube above, the attachment of the latter structure to the dorsal wall of the body is very wide. On the completion of this investment the layer of mesoblast suspending the alimentary tract becomes thinner, and at the same time the alimentary canal appears to be drawn downwards and away from the vertebral column.

In what may be regarded as the thoracic division of the general pleuroperitoneal space, along that part of the alimentary canal which will form the oesophagus, this withdrawal is very slight, but it is very marked in the abdominal region. In the latter the at first straight digestive canal comes to be suspended from the body above by a narrow flattened band of mesoblastic tissue. This flattened band is the mesentery, shewn commencing in fig. 117, and much more advanced in fig. 1 19, M. It is covered on either side by a layer of flat cells, which form part of the general peritoneal epithelioid lining, while its interior is composed of indifferent tissue.

The primitive simplicity in the arrangement of the mesentery is usually afterwards replaced by a more complicated disposition, owing to the subsequent elongation and consequent convolution of the intestine and stomach.

The layer of peritoneal epithelium on the ventral side of the stomach is continued over the liver, and after embracing the liver, becomes attached to the ventral abdominal wall (fig. 380). Thus in the region of the liver the body cavity is divided into two halves by a membrane, the two sides of which are covered by the peritoneal epithelium, and which encloses the stomach dorsally and the liver ventrally. The part of the membrane between the stomach and liver is narrow, and constitutes a kind of mesentery suspending the liver from the stomach : it is known to human anatomists as the lesser omentum.

The part of the membrane connecting the liver with the anterior abdominal wall constitutes the fa lei form or suspensory ligament of the liver. It arises by a secondary fusion, and is not a remnant of a primitive ventral mesentery (vide pp. 624 and 625).

758 MESENTERY.

The mesentery of the stomach, or mesogastrium, enlarges in Mammalia to form a peculiar sack known as the greater omentum.

The mesenteron exhibits very early a trifold division. An anterior portion, extending as far as the stomach, becomes separated off as the respiratory division. On the formation of the anal invagination the portion of the mesenteron behind the anus becomes marked off as the postanal division, and between the postanal section and the respiratory division is a middle portion forming an intestinal and cloacal division.

The respiratory division of the mesenteron.

This section of the alimentary canal is distinguished by the fact that its walls send out a series of paired diverticula, which meet the skin, and after a perforation has been effected at the regions of contact, form the branchial or visceral clefts.

In Amphioxus the respiratory region extends close up to the opening of the hepatic diverticulum, and therefore to a position corresponding with the commencement of the intestine in higher types. In the craniate Vertebrata the number of visceral clefts has become reduced, but from the extension of the visceral clefts in Amphioxus, combined with the fact that in the higher Vertebrata the vagus nerve, which is essentially the nerve of the branchial pouches, supplies in addition the walls of the oesophagus and stomach, it may reasonably be concluded, as has been pointed out by Gegenbaur, that the true respiratory region primitively included the region which in the higher types forms the oesophagus and stomach.

In Ascidians the respiratory sack is homologous with the respiratory tract of Amphioxus.

The details of the development of the branchial clefts in the different groups of Vertebrata have already been described in the systematic part of this work.

In all the Ichthyopsida the walls of a certain number of clefts become folded ; and in the mesoblast within these folds a rich capillary network, receiving its blood from the branchial arteries, becomes established. These folds constitute the true internal gills.

ALIMENTARY CANAL.

759

In addition to internal gills external branchial processes covered by epiblast are placed on certain of the visceral arches in the larva of Polypterus, Protopterus and many Amphibia. The external gills have probably no genetic connection with the internal gills.

The so-called external gills of the embryos of Elasmobranchii are merely internal gills prolonged outwards through the gill clefts.

The posterior part of the primitive respiratory division of the mesenteron becomes, in all the higher Vertebrata, the oesophagus and stomach. With reference to the development of these parts the only point worth especially noting is the fact that in Elasmobranchii and Teleostei their lumen, though present in very young embryos, becomes at a later stage completely filled up, and thus the alimentary tract in the regions of the oesophagus and stomach becomes a solid cord of cells (fig. 23 A, ces)\ as already suggested (p. 61) it seems not impossible that this feature may be connected with the fact that the cesophageal region of the throat was at one time perforated by gill clefts.

In addition to the gills two important organs, viz. the thyroid body and the lungs, take their origin from the respiratory region of the alimentary tract.

Thyroid body. In the Ascidians the origin of a groovelike diverticulum of the ventral wall of the branchial sack, bounded by two lateral folds, and known as the endostyle or hypopharyngeal groove, has already been described (p. 18). This groove remains permanently open to the pharyngeal sack,

FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA

OF PETROMYZON. (From Gegenbaur ; after Calberla.)

o. mouth ; 6. olfactory pit ; v. septum between stomodteum and mesenteron ; h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle.

760

THE THYROID BODY.

and would seem to serve as a glandular organ secreting mucus. As was first pointed out by W. Miiller there is present in Amphioxus a very similar and probably homologous organ, known as the hypopharyngeal groove.

In the higher Vertebrata this organ never retains its primitive condition in the adult state. In the larva of Petromyzon there is, however, present a ventral groove-like diverticulum of the throat, extending from about the second to the fourth visceral cleft. This organ is shewn in longitudinal section in fig. 414, h, and in transverse section in fig. 415, and has been identified by W. Muller (Nos. 565 and 566) with the hypopharyngeal groove of Amphioxus and Ascidians. It does not, however, long retain its primitive condition, but its opening becomes gradually reduced to a pore, placed between the third and fourth of the permanent clefts (fig. 416, tli). This opening is retained throughout the Ammoccete condition, but the organ becomes highly complicated, with paired anterior and posterior horns and a median spiral portion. In the adult the connection with the pharynx is obliterated, and the organ is partly absorbed and partly divided up into a series of glandular follicles, and eventually forms the thyroid body.

From the consideration of the above facts W. Muller was led to the conclusion tJiat the tJiyroid body of the Craniata was derived from the endostyle or Jiypopharyngeal groove. In all the higher Vertebrata the thyroid body arises as a diverticulum of the ventral wall of the throat in the region either of the mandibular or hyoid arches (fig. 417, Tk}, which after being segmented off becomes divided up into follicles.

In Elasmobranch embryos it appears fairly early as a diverticulum from the ventral surface of the throat in the region of the niandibular arc/i, extending from the border of the mouth to the point where the ventral aorta divides into the two aortic branches of the mandibular arch (fig. 417, Th}.

FIG. 415. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH THE BRANCHIAL REGION OF YOUNG LARV.K OF PETROMYZON. (From Gegenbaur ; after Calberla.)

d. branchial region of throat.

ALIMENTARY CANAL.

761

Somewhat later it becomes in Scyllium and Torpedo solid, though still retaining its attachment to the wall of the oesophagus. It continues to grow in length, and becomes divided up into a number of solid branched lobules separated by connective tissue septa. Eventually its connection with the throat becomes lost, and the lobules develop a lumen. In Acanthias the lumen of the gland is retained (W. Miiller) till after its detachment from the

-- "

Pti

FIG. 416. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4 '8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. The letter tv pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.

c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ; mb. mid-brain ; cb, cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.

throat. It preserves its embryonic position through life. In Amphibia it originates, as in Elasmobranchii, from the region of the mandibular arch ; but when first visible it forms a double epithelial wall connecting the throat with the nervous layer of the epidermis. It subsequently becomes detached from the epidermis, and then has the usual form of a diverticulum from the throat. In most Amphibians it becomes divided into two lobes, and so forms a paired body. The peculiar connection between the thyroid diverticulum and the epidermis in Amphibia has been noted by Gotte in Bombinator, and by Scott and Osborn in Triton. It is not very easy to see what meaning this connection can have.

In the Fowl (W. Miiller) the thyroid body arises at the end of the second or beginning of the third day as an outgrowth from the hypoblast of the throat, opposite the point of origin of the anterior arterial arch. This outgrowth becomes by the fourth day a solid mass of cells, and by the fifth ceases to be connected with the epithelium of the throat, becoming at the same time bilobed. By the seventh day it has travelled somewhat backwards, and the two lobes have completely separated from each other. By

762

THE THYROID BODY.

the ninth day the whole is invested by a capsule of connective tissue, which sends in septa dividing it into a number of lobes or solid masses of cells, and by the sixteenth day it is a paired body composed of a number of hollow branched follicles, each with a ' membrana propria,' and separated from each other by septa of connective tissue. It finally travels back to the point of origin of the carotids.

Amongst Mammalia the thyroid arises in the Rabbit (Kolliker) and Man (His) as a hollow diverticulum of the throat at the bifurcation of the foremost pair of aortic arches. It soon however becomes solid, and is eventually detached from the throat and comes to lie on the ventral side of the larynx or windpipe. The changes it undergoes are in the main similar to those in the lower Vertebrata. It becomes partially constricted into two lobes, which remain however united by an isthmus 1 . The fact that the thyroid sometimes arises in the region of the first and sometimes in that of the second cleft is probably to be explained

Tli

FIG. 417. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT THE LEVEL OF THE AUDITORY INVOLUTION.

Th. rudiment of thyroid body ; aup. auditory pit ; aim. ganglion of auditory nerve ; iv. v. roof of fourth ventricle ; a.c.v. anterior cardinal vein ; aa. aorta ; f.aa aortic trunk of mandibular arch ; //. head cavity of mandibular arch ; Ivc. alimentary pouch which will form the first visceral cleft.

by its rudimentary character.

The Thymus gland. The thymus gland may conveniently be dealt with here, although its origin is nearly as obscure as its function. It has usually been held to be connected with the lymphatic system. Kolliker was the first to shew that this view was probably erroneous, and he attempted to prove that it was derived in the Rabbit from the walls of one of the visceral clefts, mainly on the ground of its presenting in the embryo an epithelial character.

1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a pair of epithelial vesicles, which are developed as outgrowths of the walls of the first pair of visceral clefts. He attempts to explain the contradictory observations of other embryologists by supposing that they have mistaken the ventral ends of visceral pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair of visceral clefts, at a much later period than would appear from the observations of His and Kolliker. In view of the comparative development of this organ it is difficult to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain the supposed errors of his predecessors is certainly not capable of being applied in the case of Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that the method of investigation by transverse sections, which has been usually employed, is less liable to error than that by longitudinal sections which he has adopted.

ALIMENTARY CANAL. 763

Stieda (No. 569) has recently verified Kolliker's statements. He finds that in the Pig and the Sheep the thymus arises as a paired outgrowth from the epithelial remnants of a pair of visceral clefts. Its two lobes may at first be either hollow (Sheep) or solid (Pig), but eventually become solid, and unite in the median line. Stieda and His hold that in the adult gland, the so-called corpuscles of Hassall are the remnants of the embryonic epithelial part of the gland, and that the lymphatic part of it is of mesoblastic origin ; but Kolliker believes the lymphatic cells to be direct products of the embryonic epithelial cells.

The posterior visceral clefts in the course of their atrophy give rise to various more or less conspicuous bodies of a pseudo-glandular nature, which have been chiefly studied by Remak 1 .

Swimming bladder and lungs. A swimming bladder is present in all Ganoids and in the vast majority of Teleostei. Its development however is only imperfectly known.

In the Salmon and Carp it arises, as was first shewn by Von Baer, as an outgrowth of the alimentary tract, shortly in front of the liver. In these forms it is at first placed on the dorsal side and slightly to the right, and grows backwards on the dorsal side of the gut, between the two folds of the mesentery.

The absence of a pneumatic duct in the Physoclisti would appear to be due to a post-larval atrophy.

In Lepidosteus the air-bladder appears to arise, as in the Teleostei, as an invagination of the dorsal wall of the oesophagus.

In advanced embryos of Galeus, Mustelus and Acanthias, MikluchoMaclay detected a small diverticulum opening on the dorsal side of the oesophagus, which he regards as a rudiment of a swimming bladder. This interpretation must however be regarded as somewhat doubtful.

The lungs. The lungs originate in a nearly identical way in all the Vertebrate forms in which their development has been observed. They are essentially buds or processes of the ventral wall of the primitive oesophagus.

At a point immediately behind the region of the visceral clefts the cavity of the alimentary canal becomes compressed laterally, and at the same time constricted in the middle, so that its transverse section (fig. 418 i) is somewhat hourglass-shaped, and shews an upper or dorsal chamber d, joining on to a lower or ventral chamber / by a short narrow neck.

1 For details on these organs vide Kolliker, Entwicklungsgeschichte, p. 88 1.

764

THE LUNGS.

The hinder end of the lower tube enlarges (fig. 418 2), and then becomes partially divided into two lobes (fig. 418 3). All these parts at first freely communicate, but the two lobes, partly by their own growth, and partly by a process of constriction, soon become isolated posteriorly; while in front they open into the lower chamber of the oesophagus (fig. 422).

By a continuation forwards of the process of constriction the lower chamber of the oesophagus, carrying with it the two lobes above mentioned, becomes gradually transformed into an independent tube, opening in front by a narrow slit-like aperture into the oesophagus. The single tube in front is the rudiment of the trachea and larynx, while the two diverticula behind become (fig. 419, Ig) the bronchial tubes and lungs.

While the above changes are taking place in the hypoblastic walls of the alimentary tract, the splanchnic mesoblast surrounding these structures becomes very much thickened ; but otherwise bears no marks of the internal changes which are going on, so that the above formation of the lungs and trachea cannot be seen from the surface. As the paired diverticula of the lungs grow backwards, the mesoblast around them takes however the form of two lobes, into which they gradually bore their way.

There do not seem to be any essential differences in the mode of formation of the above structures in the types so far observed, viz. Amphibia, Aves and Mammalia. Writers differ as to whether the lungs first arise as

FlG. 418. FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE LUNGS.

(After Gotte.)

a. mesoblast; b. hypoblast; d. cavity of digestive canal ; /. cavity of the pulmonary diverticulum.

In (i) the digestive canal has commenced to be constricted into an upper and lower canal ; the former the true alimentary canal, the latter the pulmonary tube; the two tubes communicate with each other in the centre.

In (2) the lower (pulmonary) tube has become expanded.

In (3) the expanded portion of the tube has become constricted into two tubes, still communicating with each other and with the digestive canal.

In (4) these are completely separated from each other and from the digestive canal, and the mesoblast has also begun to exhibit externally changes corresponding to the internal changes which have been going on.

ALIMENTARY CANAL.

765

re

paired diverticula, or as a single diverticulum ; and as to whether the rudiments of the lungs are established before those of the trachea. If the above account is correct it would appear that any of these positions might be maintained. Phylogenetically interpreted the ontogeny of the lungs appears however to imply that this organ was first an unpaired structure and has become secondarily paired, and that the trachea was relatively late in appearing.

The further development of the lungs is at first, in the higher types at any rate, essentially similar to that of a racemose gland. From each primitive diverticulum numerous branches are given off In Aves and Mammalia (fig. 355) they are mainly confined to the dorsal and lateral parts. These branches penetrate into the surrounding mesoblast and continue to give rise to secondary and tertiary branches. In the meso

At

FIG. 419. SECTION THROUGH THE CARDIAC REGION OF AN EMBRYO OF LACERTA MURALIS OF 9 MM. TO SHEW THE MODE OF FORMATION OF THE PERICARDIAL CAVITY.

ht. heart ; pc . pericardial cavity ; al. alimentary tract; Ig. lung; /. liver; pp. body cavity; md. open end of Mullerian duct; wd. Wolffian duct ; vc. vena cava inferior ; ao. aorta; ch. notochord; me, medullary cord.

blast around them numerous capillaries make their appearance, and the further growth of the bronchial tubes is supposed by Boll to be due to the mutual interaction of the hitherto passive mesoblast and of the hypoblast.

The further changes in the lungs vary somewhat in the different forms.

The air sacks are the most characteristic structures of the avian lung. They are essentially the dilated ends of the primitive diverticula or of their main branches.

In Mammalia (Kolliker, No. 298) the ends of the bronchial tubes become dilated into vesicles, which may be called the primary air-cells. At first, owing to their development at the ends of the bronchial branches, these are confined to the surface of the lungs. At a later period the primary air-cells divide each into two or three parts, and give rise to secondary air-cells, while at the same time the smallest bronchial tubes, which continue all the while to divide, give rise at all points to fresh air-cells. Finally the bronchial tubes cease to become more branched, and the air-cells belonging to each minute lobe come in their further growth to open into a common chamber.

766 THE CLOACA.

Before the lungs assume their function the embryonic air-cells undergo a considerable dilatation.

The trachea and larynx. The development of the trachea and larynx does not require any detailed description. The larynx is formed as a simple dilatation of the trachea. The cartilaginous structures of the larynx are of the same nature as those of the trachea.

It follows from the above account that the whole pulmonary structure is the result of the growth by budding of a system of branched hypoblastic tubes in the midst of a mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium of the tubes, and the mesoblast providing the elastic, muscular, cartilaginous, vascular, and other connective tissues of the tracheal and bronchial walls.

There can be no doubt that the lungs and air-bladder are homologous structures, and the very interesting memoir of Eisig on the air-bladder of the Chaetopoda 1 shews it to be highly probable that they are the divergent modifications of a primitive organ, which served as a reservoir for gas secreted in the alimentary tract, the gas in question being probably employed for respiration when, for any reason, ordinary respiration by the gills was insufficient.

Such an organ might easily become either purely respiratory, receiving its air from the exterior, and so form a true lung ; or mainly hydrostatic, forming an air-bladder, as in Ganoidei and Teleostei.

It is probable that in the Elasmobranchii the air-bladder has become aborted, and the organ discovered by Micklucho-Maclay may perhaps be a last remnant of it.

The middle division of the mesenteron. The middle division of the mesenteron, forming the intestinal and cloacal region, is primitively a straight tube, the intestinal region of which in most Vertebrate embryos is open below to the yolksack.

Cloaca. In the Elasmobranchii, the embryos of which probably retain a very primitive condition of the mesenteron, this region is not at first sharply separated from the postanal section behind. Opposite the point where the anus will even 1 H. Eisig, " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden." Mittheil. a. d. zool. Station z. Neafel, Vol. II. 1881.

ALIMENTARY CANAL.

767

tually appear a dilatation of the mesenteron arises, which comes in contact with the external skin (fig. 28 E, an}. This dilatation becomes the hypoblastic section of the cloaca. It communicates behind with the postanal gut (fig. 424 D), and in front with the intestine ; and may be defined as the dilated portion of the alimentary tract which receives the genital and urinary ducts and opens externally by the proctodczum.

In Acipenser and Amphibia the cloacal region is indicated as a ventral diverticulum of the mesenteron even before the closure of the blastopore. It is shewn in the Amphibia at an early stage in fig. 73, and at a later period, when in contact with the skin at the point where the anal invagination is about to appear, in fig. 420.

FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF

BOMBINATOR. (After Gotte.)

m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; ch. notochord ; pn. pineal gland.

In the Sauropsida and Mammalia the cloaca appears as a dilatation of the mesenteron, which receives the opening of the allantois almost as soon as the posterior part of the mesenteron is established.

The eventual changes which it undergoes have been already dealt with in connection with the urinogenital organs.

Intestine. The region in front of the cloaca forms the intestine. In certain Vertebrata it nearly retains its primitive character as a straight tube ; and in these types its anterior part is characterised by the presence of a peculiar fold, which in a highly specialised condition is known as the spiral valve. This structure appears in its simplest form in Ammocoetes. It

768 THE INTESTINE.

there consists of a fold in the wall of the intestine, giving to the lumen of this canal a semilunar form in section, and taking a half spiral.

In Elasmobranchii a similar fold to that in Ammoccetes first makes its appearance in the embryo. This fold is from the first not quite straight, but winds in a long spiral round the intestine. In the course of development it becomes converted into a strong ridge projecting into the lumen of the intestine (fig. 388, /). The spiral it makes becomes much closer, and it thus acquires the form of the adult spiral valve. A spiral valve is also found in Chimaera and Ganoids. No rudiment of such an organ is found in the Teleostei, the Amphibia, or the higher Vertebrata.

The presence of this peculiar organ appears to be a very primitive Vertebrate character. The intestine of Ascidians exhibits exactly the same peculiarity as that of Ammoccetes, and we may probably conclude from embryology that the ancestral Chordata were provided with a straight intestine having a fold projecting into its lumen, to increase the area of the intestinal epithelium.

In all forms in which there is not a spiral valve, with the exception of a few Teleostei, the intestine becomes considerably longer than the cavity which contains it, and therefore necessarily more or less convoluted.

The posterior part usually becomes considerably enlarged to form the rectum or in Mammalia the large intestine.

In Elasmobranchii there is a peculiar gland opening into the dorsal side of the rectum, and in many other forms there is a caecum at the commencement of the rectum or of the large intestine.

In Teleostei, the Sturgeon and Lepidosteus there opens into the front end of the intestine a number of caecal pouches known as the pancreatic caeca. In the adult Sturgeon these pouches unite to form a compact gland, but in the embryo they arise as a series of isolated outgrowths of the duodenum.

Connected with the anterior portion of the middle region of the alimentary canal, which may be called the duodenum, are two very important and constant glandular organs, the liver and the pancreas.

ALIMENTARY CANAL.

769

ITlf

The liver. The liver is the earliest formed and largest glandular organ in the embryo.

It appears in its simplest form in Amphioxus as a single unbranched diverticulum of the alimentary tract, immediately behind the respiratory region, which is directed forwards and placed on the left side of the body.

In all true Vertebrata the gland has a much more complicated structure. It arises as a ventral outgrowth of the duodenum (fig. 420, /). This outgrowth may be at first single, and then grow out into two lobes, as in Elasmobranchii (fig. 421) and Amphibia, or have from the first the form of two somewhat unequal diverticula, as in Birds (fig. 422), or again as in the Rabbit (Kolliker) one diverticulum may be first formed, and a second one appear somewhat later. The hepatic diverticula, whatever may be their primitive form, grow into a special thickening of the splanchnic mesoblast.

From the primitive diverticula there are soon given off a number of hollow buds (fig. 421) which rapidly increase in length and number, and form the so-called hepatic cylinders. They soon anastomose and unite together, and so constitute an irregular network. Coincidently with the formation of the hepatic network the united vitelline and visceral vein or veins (u.v\ in their passage through the liver, give off numerous branches, and gradually break up into a plexus of channels which form a secondary network amongst the hepatic cylinders. In Amphibia these channels are stated by Gotte to be lacunar, but in Elasmobranchii, and probably Vertebrata generally, they arc from the first provided with distinct though delicate walls. B. in. 49

FIG. 421. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ; cav. cardinal vein; ua. vitelline artery ; nv. vitelline vein united with subintestinal vein ; al. duodenum ; /. liver ; sd. opening of segmental duct into the body-cavity ; mp. muscle-plate ; urn. umbilical canal.

770

THE LIVER.

It is still doubtful whether the hepatic cylinders are as a rule hollow or solid. In Elasmobranchii they are at first provided with a large lumen, which though it becomes gradually smaller never entirely vanishes. The same seems to hold good for Amphibia and some Mammalia. In Aves the lumen of the cylinders is even from the first much more difficult to see, and the cylinders are stated by Remak to be solid, and he has been followed in this matter by Kolliker. In the Rabbit also Kolliker finds the cylinders to be solid.

The embryonic hepatic network gives rise to the parenchyma of the adult liver, with which in its general arrangement it closely agrees. The blood-channels are at first very large, and have a very irregular arrangement ; and it is not till comparatively late that the hepatic lobules with their characteristic vascular structures become established.

The biliary ducts are formed either from some of the primitive hepatic cylinders, or, as would seem to be the case in Elasmobranchii and Birds (fig. 422), from the larger diverticula of the two primitive outgrowths.

The gall-bladder is so inconstant, and the arrangement of the ducts opening into the intestine so variable, that no general statements can be made about them. In Elasmobranchii the primitive median diverticulum (fig. 421) gives rise to the ductus choledochus. Its anterior end dilates to form a gall-bladder.

In the Rabbit a ductus choledochus is formed by a diverticulum from the intestine at the point of insertion of the two primitive lobes. The gall-bladder arises as a diverticulum of the right primitive lobe.

The liver is relatively very large during embryonic life and has, no doubt, important functions in connection with the circulation.

r

FIG. 422. DIAGRAM OF THE DIGESTIVE TRACT OF A CHICK UPON THE FOURTH DAY. (After Gotte.)

The black line indicates the hypoblast. The shaded part around it is the splanchnic mesoblast.

Ig. lung ; st. stomach ; p. pancreas ; /. liver.

ALIMENTARY CANAL.

771

The pancreas. So far as is known the development of the pancreas takes place on a very constant type throughout the series of craniate Vertebrata, though absent in some of the Teleostean fishes and Cyclostomata, and very much reduced in most Teleostei and in Petromyzon.

It arises nearly at the same time as the liver in the form of a hollow outgrowth from the dorsal side of the intestine nearly opposite but slightly behind the hepatic outgrowth (fig. 422, /). It soon assumes, in Elasmobranchii and Mammalia, somewhat the form of an inverted funnel, and from the expanded dorsal part of the funnel there grow out numerous hollow diverticula into the passive splanchnic mesoblast.

As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The funnel-shaped receptacle loses its origi nal form, and elongating, assumes the character of a duct.

From the above mode of development it is clear that the glandular cells of the pancreas are derived from the hypoblast.

Into the origin of the varying arrangements of the pancreatic ducts it is not possible to enter in detail. In some cases, e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a division of the primitive gland and duct. In other cases, e.g. the Bird, a second diverticulum springs from the alimentary tract. In a large number of instances the primitive condition with a single duct is retained.

Postanal section of the mesenteron. In the embryos of all the Chordata there is a section of the mesenteron placed behind the anus. This section invariably atrophies at a comparatively early period of embryonic life ; but it is much better developed in the lower forms than in the higher. At its posterior extremity it is primitively continuous with the neural tube (fig. 420), as was first shewn by Kowalevsky.

The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore.

In the Tunicata the section of the mesenteron, which in all probability corresponds to the postanal gut of the Vertebrata, is that immediately

492

772 POSTANAL SECTION OF THE MESENTERON.

following the dilated portion which gives rise to the branchial cavity

and permanent intestine. It has already

been shewn that from the dorsal and

lateral portions of this section of the

primitive alimentary tract the notochord

and muscles of the Ascidian tadpole are

derived. The remaining part of its walls

forms a solid cord of cells (fig. 423, al'},

which either atrophies, or, according to

Kowalevsky, gives rise to blood-vessels.

In Amphioxus the postanal gut, FIG. 423. TRANSVERSE OPTICAL

.hough distinctly developed, is no, very % long, and atrophies at a comparatively (After Kowalevsky.) early period. The sect i on ; s f rom an embryo of

In Elasmobranchii this section of the the same age as fig. 8 iv.

alimentary tract is very well developed, ch - notochord ; nc neural 1 canal ;

. , , me. mesoblast ; of. hypoblast of and persists for a considerable period of ta ji <

embryonic life. The following is a history of its development in the genus Scyllium.

Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.

The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast.

In the succeeding stages, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about -J of that of the whole alimentary tract.

Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.

Posteriorly (A) there is present a terminal vesicle (alv) '25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (nc) ; to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B al}. Its average diameter is about '084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (x), but this does not extend as far back as the terminal vesicle.

The thick-walled stalk of the vesicle is connected with the cloacal section

ALIMENTARY CANAL.

773

of the alimentary tract by a very narrow thin-walled tube (C of). This for the most part has a fairly uniform calibre, and a diameter of not more than 035 mm. Its walls are formed of flattened epithelial cells. At a point not far from the cloaca it becomes smaller, and its diameter falls to -03 mm. In

cl.al

FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc . neural canal ; al. postanal gut ; alv. caudal vesicle of postanal gut ; x. subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta; v.cau, caudal vein.

front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (D al}.

Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.

The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.

In Teleostei the vesicle at the end of the tail, discovered by Kupffer,

774 THE STOMOD/EUM.

(fig- 34> hyv) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii.

In Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewh in the embryo of Bombinator in fig. 420.

Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period

FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypoblast ; p.a.g, postanal gut ; pr. remains of primitive streak folded in on the ventral side ; al. allantois ; me. splanchnic mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kolliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425, p.a.g.} and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms.

The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter xii., p. 323.

The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract.

In Ascidians this pit is placed on the dorsal surface (fig. 9, o), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically

THE STOMOD/EUM.

775

(vide p. 5), but further observations on its development are required.

In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the forebrain (fig. 426), and is deeper in Petromyzon (fig. 416, ;) than in any other known form.

From the primary buccal cavity or stomodaeum there grows out the pituitary pit (fig. 426, pt\ the development of which has already been described (p. 435).

The wall separating the stomodaeum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original limits of the primitive buccal cavity become obliterated ; while a secondary buccal cavity, partly lined by hypoblast and partly by epiblast, becomes established.

This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the posterior nares.

In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity.

In the Amniota further important changes take place.

In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427, /), and the two plates, meeting in the middle line, form a horizontal septum dividing the front part of the primitive buccal cavity into a dorsal respiratory section (), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. The

FIG. 426. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO.

r.unpaired rudimentofthecerebral hemispheres \pn. pineal gland ; /w.infundibulum ; //.ingrowth from mouth to form the pituitary body ; mb. mid-brain ; cb. cerebellum ; ch. notochord; al. alimentary tract; Zaa. artery of mandibular arch.

THE TEETH.

two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.

An internasal septum (fig. 427, e) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.

In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate.

The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the posterior part of the buccal cavity.

The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth ; but the latter alone will engage our attention here.

The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth.

As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis. The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter.

FIG. 427. DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE TRUE MOUTH BELOW. (From Gegenbaur.)

p. palatine plate of superior maxillary process; m. permanent mouth ; n. posterior part of nasal passage; e. internasal septum.

Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoid

THE STOMOD/KUM.

777

and branchial arches ; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region.

The teeth are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla.

The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.

Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior ; so that it becomes constituted of (i) an external layer of columnar cells, and (2) a central core of rounded cells ; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a papilla of the subepithelial connective tissue (p] which has in the meantime been developed.

From the above epithelial structure, which may be called the enamel organ, and from the papilla it covers, which maybe spoken of as the dental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule ; known as the dental capsule.

Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar transformation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a somewhat different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papilla? which project into the highly vascular tissue of the dental sheath. Between

FIG. 428. DIAGRAM SHEWING THE DEVELOPMENT OF THE TEETH. (From Gegenbaur.)

p. dental papilla ; e. enamel organ.

778 THE PROCTOD/EUM.

the epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana praeformativa.

The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium.

The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine while the inner portion remains as the pulp.

The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.

By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.

The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.

The general development of the enamel organs and dental papillae is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.

In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured (fig. 428).

The arrangement represented in fig. 428, in which successive enamel organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth.

The Proctodceuni.

In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communication

THE PROCTOD/EUM.

779

with the exterior by means of an epiblastic invagination, constituting a proctodseum.

This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodseum ; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication.

The mode of formation of the proctodaeum, which is in general extremely simple, is illustrated by fig. 420 an.

In most forms the original boundary between the cpiblast of the proctodaeum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication.

FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy, hypoblast ; p.a.g. postanal gut ; pr. remains of primitive streak folded in on the ventral side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

In Birds the formation of the proctodseum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa Fabricii.

The proctodseum first appears when the folding off of the tail end of the embryo commences (fig. 429, an} and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut.

The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodaeum. The actual perforation of the septum between the proctodeeum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of fcetal life, and the approxi

780 BIBLIOGRAPHY.

mation of the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces.

The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctodaeum, with which the bursa Fabricii communicates.

BIBLIOGRAPHY. Alimentary Canal and its appendages.

(561) B. Afanassiew. "Ueber Bau u. Entwicklung d. Thymus d. Saugeth." Archivf. mikr. Anat. Bd. xiv. 1877.

(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Hiihnerembryonen." Archivf. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beilrdge zur Entivicklungsgeschichle d. Darmkanah im Hiihnchen. 1867.

(565) W. Millie r. "Ueber die Entwickelung der Schilddriise." Jenaische Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaische Zeitschrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische Untcrsuchungen. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. iib. d. Entwick. d. Glandula Thymus, Glandula thyroidea,u. Glandula car otica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad. Petrop. 1766.

(571) H. Wolfler. Ueb. d. Entwick. u. d. Bau d. Schilddriise. Berlin, 1880. Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of Micros. Science, Vol. in. 1855.

(573) R. Owen. Odontography . London, 1840 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative. London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros. Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker's Histology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.

INDEX TO VOLUME III.

Abdominal muscles, 675

Abdominal pore, 626, 749

Acipenser, development of, 102; affinities of, 1 1 8 ; comparison of gastrula of, 279 ; pericardial cavity of, 627

Actinotrocha, 373

Air-bladder of Teleostei, 77; Lepidosteus, 117; blood supply of, 645 ; general account of, 763 ; homologies of, 766

Alciope, eye of, 480

Alisphenoid region of skull, 569

Alimentary canal and appendages, development of, 754

Alimentary tract ofAscidia, 18; Molgula, 22; Pyrosoma, 24; Salpa, 31 ; Elasmobranchii, 52; Teleostei, 75; Petromyzon, 93, 97; Acipenser, no; Amphibia, 129, 136; Chick, 167; respiratory region of, 754; temporary closure of oesophageal region of, 759

Allantois, development of in Chick, 191, 198; blood-vessels of in Chick, 193; Lacerta, 205, 209; early development of in Rabbit, 229, of Guinea-pig, 264; origin of, 309. See also ' Placenta ' and 'Bladder

Alternation of generations in Ascidians, origin of, 35 ; in Botryllus, 35 ; Pyrosoma, 36; Salpa, 36; Doliolum, 36

Alytes, branchial chamber of, 136; yolksack of, 139; branchiae, 141 ; Miillerian duct of, 710

Amblystoma, ovum of, 120; larva of, 142,

H3

Amia, ribs of, 561

Ammocoetes, 95; metamorphosis of, 97;

eye of, 498 Amnion, early development of in Chick,

185; later history of in Chick, 196;

Lacerta, 204, 210; Rabbit, 229; origin

of, 3.07. 39

Amphibia, development of, 120; viviparous, 121; gastrula of, 277; suctorial mouth of, 317; cerebellum of, 426; infundibulum of, 431; pineal gland of, 433; cerebrum of, 439; olfactory lobes of, 444; nares of, 553; notochord and its sheath, 548; vertebral column of, 554; ribs of, 561 ; branchial arches of, 574; mandibular and hyoid arches of, 582 ; columella of, 582 ; pectoral girdle of, 605; pelvic girdle of, 607; limbs of, 619; heart of, 638; arterial system of, f>45 ; venous system of, 655 ; excretory

system of, 707 ; vasa efierentia of, 711; liver of, 769; postanal gut of, 774; stomodaeum of, 778

Amphiblastula larva of Porifera, 344

Amphioxus, development of, i ; gastrula of, 275 ; formation of mesoblast of, 292 ; development of notochord of, 293; head of, 314; spinal nerves of, 461; olfactory organ of, 462 ; venous system of, 651; transverse abdominal muscle f> 673; generative cells of, 748; liver of, 769; postanal gut of, 772; stomodaeum of, 777

Amphistylic skulls, 578

Angular bone, 594

Anterior abdominal vein, 653

Anura, development of, 121; epiblast of, 125; mesoblast of, 128; notochord of, 128; hypoblast of, 129; general growth of embryo of, 131; larva of, 134; vertebral column of, 556 ; mandibular arch of, 584

Anus of Amphioxus, 7 ; Ascidia, 18; Pyrosoma, 28 ; Salpa, 31 ; Elasmobranchii, 57; Amphibia, 130, 132; Chick, 167; primitive, 324

Appendicularia, development of, 34

Aqueductus vestibuli, 519

Aqueous humour, 497

Arachnida, nervous system of, 409; eye of, 481

Area, embryonic, of Rabbit, 218; epiblast

of, 219; origin of embryo from, 228

area opaca of Chick, 150; epiblast,

hypoblast, and mesoblast of, 159 area pellucida of Chick, 150; of Lacerta, 202

area vasculosa of Chick, 194; mesoblast of, 1 60; of Lizard, 209; Rabbit, 228, 229

Arteria centralis retinas, 503

Arterial system of Petromyzon, 97; constitution of in embryo, 643 ; of Fishes, 644; of Amphibia, 645; of Amniota, 647

Arthropoda, head of, 313 ; nervous system of, 409 ; eye of, 480 ; excretory organs of, 688

Articular bone of Teleostei, 581 ; of Sauropsida, 588

Ascidia, development of, 9

Ascidians. See 'Tunicata'

Ascidiozooids, 25

Atrial cavity of Amphioxus, 7; Ascidia, 18; Pyrosoma, 24

7 82

INDEX.

Atrial pore of Amphioxus, 7; Ascidia, 20; Pyrosoma, 28 ; Salpa, 32

Auditory capsules, ossifications in, 595, 59.6

Auditory involution of Elasmobranchii, 57; Teleostei, 73; Petromyzon, 89, 92; Acipenser, 106; Lepidosteus, 114; Amphibia, 127; Chick, 170

Auditory nerve, development of, 459

Auditory organs, of Ascidia, 15; of Salpa, 31; of Ammocoetes, 98; Ganoidei, 108, 114; of Amphibia, 127; of Aves, 170; general development of, 512; of aquatic forms, 512; of land forms, 513; of Ccelenterata, 513; of Mollusca, 515; of Crustacea, 516; of Vertebrata, 517; of Cyclostomata, 89, 92, 518; of Teleostei, Lepidosteus and Amphibia, 518; of Mammalia, 519; accessory structures of, 527; ofTunicata, 528

Auriculo-ventricular valves, 642

Autostylic skulls, 579

Aves, development of, 145; cerebellum of, 426; midbrain of, 427; infundibulum of, 431; pineal gland of, 434; pituitary body of, 436; cerebrum of, 439 ; olfactory lobes of, 444 ; spinal nerves of, 449 ; cranial nerves of, 455 ; vagus of, 458; glossopharyngeal of, 458; vertebral column of, 557; ossification of vertebral column of, 558; branchial arches of, 572, 573; pectoral girdle of, 603; pelvic girdle of, 608; heart of, 637 ; arterial system of, 647 ; venous system of, 658; muscle-plates of, 670; excretory organs of, 714; mesonephros of, 715; pronephros of, 718; Miillerian duct of, 718, 720; nature of pronephros of, 721 ; connection of Miillerian duct with Wolffian in, 720 ; kidney of, 722; lungs of, 764; liver of, 769; postanal gut of, 774

Axolotl, 142, 143; ovum of, 120; midbrain of, 427; mandibular arch of, 583

Basilar membrane, 524

Basilar plate, 565

Basipterygium, 612

Basisphenoid region of skull, 569

Bilateral symmetry, origin of, 373-376

Bile duct, 770

Bladder, Amphibia, 131 ; of Amniota, 726

Blastodermic vesicle, of Rabbit, first development of, 217; of 7th day, 222; Guinea-pig, 263; meaning of, 291

Blastoderm of Pyrosoma, 24; Elasmobranchii, 41; Chick, 150; Lacerta 202

Blastopore, of Amphioxus, 2; of Ascidia, II ; Elasmobranchii, 42, 54, 62 ; Petromyzon, 87; Acipenser, 104 ; Amphibia, 125, 130; Chick, 153; Rabbit, 216; true Mammalian, 226; comparative history of closure of, 284, 288; summary of fate of, 340; relation of to primitive anus, 324

Blood-vessels, development of, 633

Body cavity, of Ascidia, 2 1 ; Molgula, 2 1 ; Salpa, 31; Elasmobranchii, 47 ; of Teleostei, 75 ; Petromyzon, 94 ; Chick, 169; development of in Chordata, 325; views on origin of, 356 360, 377; of Invertebrata, 623; of Chordata, 624; of head, 676

Bombinator, branchial chamber of, 136; vertebral column of, 556

Bonellia, excretory organs of, 687

Bones, origin of cartilage bones, 542 ; origin of membrane bones, 543; development of, 543; homologies of membrane bones, 542 ; homologies of cartilage bones, 545

Brachiopoda, excretory organs of, 683 ; generative ducts of, 749

Brain, of Ascidia, IT, 15; Elasmobranchii, 56, 59, 60; Teleostei, 77; Petromyzon, 89, 92 ; Acipenser, 105 ; Lepidosteus, 113; early development of in Chick, 170; flexure of in Chick, 175; later development of in Chick, 176; Rabbit, 229, general account of development of, 419; flexureof, 420; histogeny of, 422

Branchial arches, prseoral, 570; disappearance of posterior, 573; dental plates of in Teleostei, 574; relation of to head cavities, 571 ; see ' Visceral arches'

Branchial chamber of Amphibia, 136

Branchial clefts, of Amphioxus, 7 ; of Ascidia, 18, 20; Molgula, 23; Salpa, 32; of Elasmobranchii, 57, 59 01; Teleostei, 77; Petromyzon, 91, 96; Acipenser, 105; Lepidosteus, 114, 116; Amphibia, 132, 133; Chick, 178; Rabbit, 231; praeoral, 312, 318; of Invertebrata, 326; origin of, 326

Branchial rays, 574

Branchial skeleton, development of, 572, 592; of Petromyzon, 96, 312, 571; of Ichthyopsida, 572; dental plates of in Teleostei, 574; relation of to head cavities, 572

Branchiae, external of Elasmobranchii, 6r, 62; of Teleostei, 77; Acipenser, 107; Amphibia, 127, 133, 135

Brood-pouch, of Salpa, 29 ; Teleostei, 68, Amphibia, 12 1

Brown tubes of Gephyrea, 686

Bulbus arteriosus, of Pishes, 638 ; Amphibia, 639

Bursa Fabricii, 167, 779

Canalis auricularis, 639 Canalis reuniens, 521 Capitellidre, excretory organs of, 683 Carcharias, placenta of, 66 Cardinal vein, 652 Carnivora, placenta of, 250 Carpus, development of, 620 Cartilage bones of skull, 595 ; homologies of, 595

INDEX.

783

Cat, placenta of, 250

Caudal swellings of Elasmobranchii, 46,

55; Teleostei, 72; Chick, 162, 170 Cephalic plate of Elasmobranchii, 55 Cephalochorda, development of, i Cephalopoda, eyes of, 473 477 Cerebellum, Petromyzon, 93; Chick, 176;

general account of development of, 424,

425

Cerebrum of Petromyzon, 93, 97; Chick, 175 ; general development of, 429, 438; transverse fissure of, 443 Cestoda, excretory organs of, 68 1 Cetacea, placenta, 255 Chtetognatha, nervous system of, 349; eye of, 479 ; generative organs of, 743 ; generative ducts of, 749 Chcetopoda, head of, 313; eyes of, 479; excretory organs of, 683; generative organs of, 743 ; generative ducts of, 749 Charybdnea, eye of, 472 Cheiroptera, placenta of, 244 Cheiropterygium, 618; relation of to ich thyopterygium, 621

Chelonia, development of, 210; pectoral girdle of, 603 ; arterial system of, 649 Chick, development of, 145 ; general growth of embryo of, 1 70 ; rotation of embryo of, 173; fcetal membranes of, 185; epiblast of, 150, 166; optic nerve and choroid fissure of, 500

Chilognatha, eye of, 481

Chilopoda, eye of, 481

Chimasra, lateral line of, 539 ; vertebral column of, 548; nares of, 533

Chiromantis, oviposition of, 121

Chorda tympani, development of, 460

Chordata, ancestor of, 311; branchial system of, 312; evidence from Ammocuetes, 312; head of, 312; mouth of, 318; table of phylogeny of, 327

Chorion, 237; villi of, 237, 257

Choroid coat, Ammoccetes, 99; general account of, 487

Choroid fissure, of Vertebrate eye, 486, 493 ; of Ammocoetes, 498 ; comparative development of, 500; of Chick, 501; of Lizards, 501 ; of Elasmobranchii, 502 ; of Teleostei, 503 ; Amphibia, 503 ; Mammals, 503, 504

Choroid gland, 320

Choroid pigment, 489

Choroid plexus, of fourth ventricle, 425 ; of third ventricle, 432 ; of lateral ventricle, 442

Ciliated sack of Ascidia, 18; Pyrosoma, 26; Salpa, 31

Ciliary ganglion, 461

Ciliary muscle, 490

Ciliary processes, 488; comparative development of, 506

Clavicle, 600

Clitoris, development of, 727

Clinoid ridge, 569

Cloaca, 766

Coccygeo-mesenteric vein, 66 1

Cochlear canal, 519

Coecilia, development of, 143; pronephros of, 707; mesonephros of, 709; Mill lerian duct of, 710

Coelenterata, larvae of, 367 ; eyes of, 47 1 ; auditory organs of, 513; generative organs of, 741

Columella auris, 529; of Amphibia, 582 ; of Sauropsida, 588

Commissures, of spinal cord, 417; of brain, 431, 432, 439, 443

Coni vasculosi, 724

Conus arteriosus, of Fishes, 638; of Amphibia, 638

Coracoid bone, 599

Cornea, of Ammocretes, 99 ; general development of, 495 ; corpuscles of, 496 ; comparative development of, 499; of Mammals, 499

Coronoid bone, 595

Corpora geniculata interna, 428

Corpora quadrigemina, 428

Corpora striata, development of, 437

Corpus callosum, development of, 443

Corti, organ of, 522; structure of, 525; fibres of, 525 ; development of, 526

Cranial flexure, of Elasmobranchii, 58, 60; of Teleostei, 77; Petromyzon, 93, 94; of Amphibia, 131, 132; Chick, 174; Rabbit, 231; characters of, 321; significance of, 322

Cranial nerves, development of, 455; relation of to head cavities, 461 ; anterior roots of, 462 464; view on position of roots of, 466

Crocodilia, arterial system of, 649

Crura cerebri, 429

Crustacea, nervous system of, 41 1 ; eye of, 481; auditory organs of, 515; generative cells of, 745 ; generative ducts of,

75

Cupola, 524

Cutaneous muscles, 676

Cyathozooid, 25

Cyclostomata, auditory organs of, 517; olfactory organ of, 532; notochord and vertebral column of, 546, 549; abdominal pores of, 626 ; segmental duct of, 700 ; pronephros of, 700 ; mesonephros of, 700 ; generative ducts of, 733, 749 ; venous system of, 651 ; excretory organs of, 700

Cystignathus, oviposition of, 122

Dactylethra, branchial chamber of, 136;

branchise of, 136; tadpole of, 140 Decidua reflexa, of Rat, 242 ; of Insecti vora, 243; of Man, 245 Deiter's cells, 526 Dental papilla, 777 Dental capsule, 777 Dentary bone, 595 Dentine, 780 Descemet's membrane, 496

784

INDEX.

Diaphragm, 631 ; muscle of, 676

Dipnoi, nares of, 534; vertebral column of, 548; membrane bones of skull of, 592 ; heart of, 638 ; arterial system of, 645 ; excretory system of, 707 ; stomodseum of, 777

Diptera, eye of, 481

Discophora, excretory organs of, 687

Dog, placenta of, 248

Dohni, on relations of Cyclostomata, 84 ; on ancestor of Chordata, 311, 319

Doliolum, development of, 28

Ductus arteriosus, 649

Ductus Botalli, 648

Ductus Cuvieri, 654

Ductus venosus Arantii, 663

Dugong, heart of, 642

Dysticus, eye of, 481

Ear, see ' Auditory organ '

Echinodermata, secondary symmetry of larva of, 380; excretory organs of, 689 ; generative ducts of, 752

Echinorhinus, lateral line of, 539; vertebral column of, 548

Echiurus, excretory organs of, 686

Ectostosis, 543

Edentata, placenta of, 248, 250, 256

Eel, generative ducts of, 703

Egg-shell of Elasmobranchii, 40 ; Chick, 146

Elasmobranchii, development of, 40; viviparous, 40; general features of development of, 55 ; gastrulaof, 281 ; development of mesoblast of, 294 ; notochord of, 294 ; meaning of formation of mesoblast of, 295; restiform tracts of, 425 ; optic lobes of, 427 ; cerebellum of, 425 ; pineal gland of, 432 ; pituitary body of, 435 ; cerebrum of, 438 ; olfactory lobes of, 444 ; spinal nerves, 449 ; cranial nerves of, 457; sympathetic nervous system of, 466; nares of, 533; lateral line of, 539; vertebral column of, 549 ; ribs of, 560 ; parachordals of, 567 ; mandibular and hyoid arches of, 576 ; pectoral girdle of, 600 ; pelvic girdle of, 607; limbs of, 609; pericardial cavity of, 627; arterial system of, 644 ; venous system of, 65 1 ; muscle-plates of, 668 ; excretory organs of, 690 ; constitution of excretory organs in adult of, 697; spermatozoa of, 747 ; swimming-bladder of, 763 ; intestines of, 767 ; liver of, 769; postanal gut of, 772

Elrcoblast of Pyrosoma, 28; Salpa, 30

Elephant, placenta of, 249

Embolic formation of gastrula, 333

Enamel organ, 777

Endolymph of ear, 522

Endostosis, 543

Endostyle of Ascidia, 18, 759; Pyrosoma, 25; Salpa, 32

Epiblast, of Elasmobranchii, 47 ; Teleostei, 71, 75; Petromyzon, 86; Lcpid

osteus, 112; Amphibia, 122, 125; Chick, 149, 166; Lacerta, 203; Rabbit, 216, 219; origin of in Rabbit, 221 ; comparative account of development of, 300

Epibolic formation of gastrula, 334

Epichordal formation of vertebral column, 556

Epicrium glutinosum, 143

Epidermis, in Ccelenterata, 393; protective structures of, 394

Epididymis, 724

Epigastric vein, 653

Episkeletal muscles, 676

Episternum, 602

Epoophoron, 725

Ethmoid bone, 597

Ethmoid region of skull, 570

Ethmopalatine ligament of Elasmobranchs, 576

Euphausia, eye of, 483

Eustachian tube, of Amphibia, 135; Chick, 1 80; Rabbit, 232; general development of, 528

Excretory organs, general constitution of, 680; of Platyelminthes, 680; of Mollusca, 681; of Polyzoa, 682; of Brachiopoda, 683 ; of Choetopoda, 683 ; of Gephyrea, 686 ; of Discophora, 687 ; of Arthropoda, 688; of Nematoda, 689; of Echinodermata, 689 ; constitution of in Craniata, 689; of Elasmobranchii, 690; constitution of in adult Elasmobranch, 697; of Petromyzon, 700; of Myxine, 701 ; of Teleostei, 701 ; of Ganoidei, 704; of Dipnoi, 707; of Amphibia, 707; of Amniota, 713; comparison of Vertebrate and Invertebrate, 737

Excretory system, of Elasmobranchii, 49 ; Teleostei, 78; Petromyzon, 95, 98; Acipenser, 99; Amphibia, 133

Exoccipital bone, 595

Exoskeleton, dermal, 393 395 ; epidermal, 393396

External generative organs, 726

Extra-branchial skeleton, 572

Eye, of Ascidia, 16; Salpa, 31; Elasmobranchii, 56, 57, 58; Teleostei, 73; Petromyzon, 92, 98; Aves, i/o; Rabbit, 229; general development of, 470; evolution of, 470, 471; simple, 480; compound, 481 ; aconous, 482; pseudoconous, 482 ; of Invertebrata, 471; of Vertebrata, 483 ; comparative development of Vertebrate, 497 ; of Ammoccetes, 497 ; of Tunicata, 507 ; of Chordata, general views on, 508 ; accessory eyes of Fishes, 509; muscles of, 677

Eyelids, development of, 506

Falciform ligament, 757

Falx cerebri, 439

Fasciculi terctes, of Elasmobranchii. 426

Feathers, development of, 396

INDEX.

785

Fenestra rotunda and ovalis, 529

Fertilization, of Amphioxus, 2 ; of Urochorda, 9; Salpa, 29; Elasmobranchii, 46; of Teleostei, 68; Petromyzon, 84 ; Amphibia, 120; Chick, 145 ; Reptilia, 202 ; meaning of, 331

Fifth nerve, development of, 460

Fifth ventricle, 443

Fins, of Elasmobranchii, 62 ; Teleostei, 78; Petromyzon, 94, 95; Acipenser, 109; Lepidosteus, 118; relation of paired to unpaired, 611, 612 ; development of pelvic, 614; development of pectoral, 615; views on nature of paired fins, 616

Fissures of spinal cord, 417

Foetal development, 360 ; secondary variations in, 361

Foot, 618

Foramen of Munro, 430, 438

Foramen ovale, 642

Forebrain, of Elasmobranchii, 55, 59, 60; Petromyzon, 93 ; general development of, 428

Formative cells, of Chick, 154

Fornix, development of, 443

Fornix of Gottsche, 428

Fourth nerve, 464

Frontals, 592

Fronto-nasal process of Chick, 179

Gaertner's canals, 724

Gall-bladder, 770

Ganoidei, development of, 102; relations of, 118; nares of, 534; notochord of, 546 ; vertebral column of, 546, 553 ; ribs of, 561 ; pelvic girdle of, 606; arterial system of, 645 ; excretory organs of, 704; generative ducts of, 734

Gastropoda, eye of, 472

Gastrula, of Amphioxus, 2; of Ascidia, lo; Elasmobranchii, 43, 44 ; Petromyzon, 86; Acipenser, 103; Amphibia, 123; comparative development of, in Invertebrata, 275 ; comparison of Mammalian, 291 ; phylogenetic meaning of, 333 ; ontogeny of (general), 333 ; phylogeny of, 338 343 ; secondary types of, 34!

Geckos, vertebral column of, 557

Generative cells, development of, 74! ; origin of in Ccelenterata, 741 ; of Invertebrata, 743 ; of Vertebrata, 746

Generative ducts, of Teleostei, 704, 735 ; of Ganoids, 704; of Cyclostomata, 733; origin of, 733 ; of Lepidosteus, 735, 750 ; development and evolution of, 748 ; of Ccelenterata, 748 ; of Sagitta, 749 ; of Tunicata, 749 ; Cheetopoda, Gephyrea, etc., 749; of Mollusca, 751; of Discophora, 751 ; of Echinodermata,

75*

Generative system of Elasmobranchii, 51 Gephyrea, nervous system of, 412; excretory organs of, 686 ; generative cells of, 743 ; generative ducts of, 749

B. III.

Germinal disc, of Elasmobranchii, 40; Teleostei, 68 ; Chick, 147

Germinal epithelium, 746

Germinal layers, summary of organs <lrrived from, in Vertebrata, 304 ; historical account of views of, 332 ; homologies of in the Metazoa, 345

Germinal wall of Chick, 152, 159; structure and changes of, 160

Geryonia, auditory organ of, 5 r 5

Gill of Salpa, 31

Giraldes, organ of, 725

Glands, epidermic, development of, 397

Glomerulus, external, of Chick, 716

Glossopharyngeal nerve, development of,

45 6 > 457 Grey matter of spinal cord, 417; of brain,

423 Growth in length of Vertebrate embryo,

306 Guinea-pig, primitive streak of, 223;

notochord of, 226 ; placenta of, 242 ;

development of, 262 Gymnophiona, see ' Ccecilia '

Habenula perforata, 525

Hairs, development of, 396

Halichrerus, placenta of, 250

Hand, 619

Head, comparative account of, 313; segmentation of, 314

Head cavities, of Elasmobranchii, 50 ; Petromyzon, 90, 96; Amphibia, 127; general development of, 676

Head-fold of Chick, 157, 167

Head kidney, see ' Pronephros '

Heart, of Pyrosoma, 25; Elasmobranchii, 50, 58 ; Petromyzon, 94, 97 ; Acipenser, 106; Chick, 170 ; first appearance of in Rabbit, 230; general development of, 633 ; of Fishes, 635, 637 ; of Mammalia, 638; of Birds, 637, 639; meaning of development of, 637 ; of Amphibia, 638 ; of Amniota, 639 ; change of position of, 643

Hind-brain, Elasmobranchii, 55, 59, 60 ; Petromyzon, 93 ; general account of, 424

Hippocampus major, development of, 442

Hirudo, development of blood-vessels of, 633 ; excretory organs of, 688

Horse, placenta of, 253

Hyaloid membrane, 492

Hylodes, oviposition of, 1 21 ; metamorphosis of, -1 37

Hyobranchial cleft, 572

Hyoid arch, of Chick, 179; general account of, 572, 575 ; modifications of, e !73> 577 > f Elasmobranchii, 576; of Teleostei, 577 ; of Amphibia, 582 ; of Sauropsida, 588; of Mammalia,

589

Hyomandibular bar of Elasmobranchii, 576, 577 ; of Teleostei, 579 ; of Amphibia, 582

50

86

INDEX.

Hyomandibular cleft, of Fetromyzon, 91 ; Chick, 179 ; general account of, 572

Hyostylic skulls, 582

Hypoblast of Elasmobranchii, 5! ; Teleostei, 71, 75; Petromyzon, 86; Acipenser, 104; Lepidosteus, 113; Amphibia, 122, 129; Chick, 151, 167 ; Lacerta, 203; Rabbit, 215, 216, 219 ; origin of in Rabbit, 220

Hyposkeletal muscles, 675

Ilyrax, placenta of, 249

Incus, 529, 590

Infraclavicle, 600

Infundibulum of Petromyzon, 92 ; Chick, 175 ; general development of, 430

Insectivora, placenta of, 243

Insects, nervous system of, 410 ; eye of, 481; generative organs of, 745; generative ducts of, 751

Intercalated pieces of vertebral column,

55 1

Interclavicle, homologies of, 602

Intermediate cell-mass of Chick, 183

Intermuscular septa, 672

Interorbital septum, 570

Interrenal bodies, 665

Iris, 489 ; comparative development of,

506

Iris of Ammoccetes, 98 Island of Reil, 444

Jacobson's organ, 537 Jugal bone, 594

Kidney, see ' Metanephros '

Labia majora, development of, 727

Labial cartilages, 597

Labium tympanicum, 525 ; vestibulare,

5 2 5

Lacertilia, general development of, 202 ; nares of, 537 ; pectoral girdle of, 603 ; pelvic girdle of, 607 ; arterial system of, 649

Lacrymal bone, 593

Lacrymal duct, 506

Lacrymal glands, 506

Lremargus, vertebral column of, 548

Lagena, 524

Lamina spiralis, 524

Lamina terminalis, 438

Larva of Amphioxus, 2 ; of Ascidia, 1 5 it ; Teleostei, 81 ; Petromyzon, 89, 95; Lepidosteus, 117, 318; Amphibia, 134, 142; types of, in the Invertebrata, 363

Larvre, nature, origin, and affinities of, 360 386; secondary variations of less likely to be retained, 362 ; ancestral history more fully recorded in, 362 ; secondary variations in development of, 363 ; ontogenetic record of secondary variations in, 361; of freshwater and land animals, 362; types of, 36.2; phosphorescence of, 364; of Coelenterata,

367 ; table of, 365 ; of Invertebrata, 367 et seq.

Larynx, 766

Lateral line sense organs, 538 ; comparison of, with invertebrate, 538 ; development of, in Teleostei, 538 ; development of, in Elasmobranchii, 539

Lateral ventricle, 438 ; anterior cornu of, 440 ; descending cornu of, 440 ; choroicl plexus of, 443

Layers, formation of, in Elasmobrancliii, 41, 56 ; Teleostei, 71 ; Petromyzon, 85 ; Acipenser, 103 ; Lepidosteus, 1 1 1 ; Amphibia, 121; Chick, 150, 152; Lacerta, 202; Rabbit, 215 227; comparison of Mammalia with lower forms, 226, 289; comparison of formation of in Vertebrata, 275; origin and homologies of, in the Metazoa, 331

Leech, see ' Hirudo '

Lemuridre, placenta, 256

Lens, of Elasmobranchii, 57, 58 ; Petromyzon, 94, 99; Acipenser, 106 ; Lepidosteus, 115 ; Amphibia, 127 ; Chick, 177 ; of Vertebrate eyes, 485 ; general account of, 493 ; capsule of, 493 ; comparative development of, 499 ; of Amphibia, Teleostei, Lepidosteus, 499

Lepidosteus, development of, 1 1 1 ; larva of, 117; relations of, 119; spinal nerves of, 455; ribs of, 561 ; generative ducts of, 704, 735 ; swimming-bladder of,

763

Ligamentum pectinatum, 490

Ligamentum suspensorium, 557, 558

Ligamentum vesicse medium, 239

Limbs, of Elasmobranchii, 59 ; Teleostei, 80 ; first appearance of in Chick, 184 ; Rabbit, 232 ; muscles of, 673 ; of Fishes, 609; relation of, to unpaired fins of Fishes, 611, 612; of Amphibia, 61 8

Liver of Teleostei, 78 ; Petromyzon, 95, 96; Acipenser, no; Amphibia 130; general account of, 769

Lizard, development of, 202; general growth of embryo of, 208 ; Mullerian duct of, 721

Lizzia, eye of, 471

Lobi inferiores, 431

Lungs of Amphibia, 137 ; development of, 763 ; homology of, 766

Lymphatic system, 664

Malleus, 529, 591 ; views on, 591 Malpighian bodies, development of accessory in Elasmobranchs, 695 Mammalia, development of, 214; comparison of gastrula of, 291 ; cerebellum of, 427 ; infundibulum of, 431 ; pineal gland of, 434; pituitary body of, 436; cerebrum of, 439 ; spinal nerves of, 449 ; sympathetic of, 466; vertebral column of, 558; branchial arches of, 573, 574; mandibular and hyoid arches of, 589 ; pectoral girdle of, 604; pelvic girdle of,

INDEX.

787

608 ; heart of, 636 ; arterial system of, 647; venous system of, 661 ; muscleplates of, 671 ; mesonephros of, 714; testicular network of, 724 ; urinogenital sinus of, 727 ; spermatozoa of, 747 ; lungs of, 765 ; intestines of, 768 ; liver of> 769; postanal gut of, 774; stomodseum of, 775

Mammary gland, development of, 398 Man, placenta of, 244 ; general account of development of, 265 ; characters of embryo of, 270

Mandibular arch of Elasmobranchii, 62, 576; Petromyzon, 91 ; Acipenser, 106, 116; Chick, 179; general account of,

572, 575; modification of to form jaws,

573, 575; of Teleostei, 580; of Amphibia, 582; Sauropsida, 588; Mammalia, 589

Mandibular bar, evolution of, 311, 321

Manis, placenta of, 256

Marsupial bones, 608

Marsupialia, foetal membranes of, 240 ; cerebellum of, 426 ; corpus callosum of, ' 443 ; uterus of, 726

Maxilla, 594

Meatus auditorius externus, of Chick, 181; development of, 527

Meckelian cartilage, of Elasmobranchii, 576; of Teleostei, 581 ; of Amphibia, 584, 585; of Sauropsida, 588 ; of Mammalia, 590

Mediastinum anterior and posterior, 630

Medulla oblongata, of Chick, 176 ; general development of, 425

Medullary plate of Amphioxus, 4, 5 ; of Ascidia, n; Elasmobranchii, 44, 47, 55; Teleostei, 72; Petromyzon, 88; Acipenser, 104; Lepidosteus, 1 1 1 ; Amphibia, 126, 127, 131; Chick, 159; Lacerta, 204; Rabbit, 223, 227, 228; primitive bilobed character of, 303, 317

Medusae, auditory organs of, 513

Membrana capsulo-pupillaris, 494, 504,

507

Membrana elastica externa, 546

Membrana limitans of retina, 491

Membrana tectoria, 522, 525

Membrane bones, of Amphibia, 582 ; of Sauropsida, 588; of Mammalia, 590; of mandibular arch, 593 ; of pectoral girdle, 599, 602 ; origin of, 592 ; homologies of, 593

Membranous labyrinth, development of in Man, 519

Menobranchus, branchial arches of, 142

Mesenteron of Elasmobranchii, 43 ; Teleostei, 75 ; Petromyzon, 85 ; Acipenser, 104; Amphibia, 123, 124, 129; Chick, 167; general account of, 754

Mesentery, 626, 756

Mesoblast, of Amphioxus, 6 ; Ascidia, 17, 20; Pyrosoma, 24; Salpa, 30; Elasmobranchii, 44, 47; Teleostei, 75; Petromyzon, 86; Acipenser, 105; Lepi

dosteus, 113; Amphibia, 125, 128, 129; of Chick, 154, 167; double origin of in Chick, 154, 158, 159; origin of from lips of blastopore in Chick, 158; of area vasculosa of Chick, iOo; Lacerta, 203; origin of in Rabbit, 218, 223; of area vasculosa in Rabbit, 227; comparative account of formation of, 292 ; discussion of development of in Vertebrata, 297 ; meaning of development of in Amniota, 298; phylogenetic origin of, 346 ; summary of ontogeny of, 349 352 ; views on ontogeny of, 352 360

Mesoblastic somites, of Amphioxus, 6 ; Elasmobranchii, 48, 55 ; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 114; Amphibia, 129, 131; Chick, 161, 1 80; Rabbit, 228; development of in Chordata, 325; meaning of development of, 331; of head, 676

Mesogastrium, 758

Mesonephros, of Teleostei, 78, 702; Petromyzon, 95, 98, 700; Acipenser, 1 10, 705; Amphibia, 134, 708; Chick, 184, 714; general account of, 690 ; development of in Elasmobranchs, 691 ; of Cyclostomata, 700 ; Ganoidei, 705 ; sexual and non-sexual part of in Amphibia, 710; of Amniota, 713, 724; summary and general conclusions as to, 729; relation of to pronephros, 731

Mesopterygium, 616

Metagenesis of Ascidians, 34

Metamorphosis of Amphibia, 137, 140

Metanephros, 690; development of in Elasmobranchii, 697; of Amphibia, 712; of Amniota, 713; of Chick, 722; of Lacertilia, 723; phylogeny of, 736

Metapterygium, 616

Metapterygoid, of Elasmobranchii, 576; of Teleostei, 581

Metazoa, evolution of, 339, 342 ; ancestral form of, 333, 345

Mid-brain, of Elasmobranchii, 55, 58, 59; Petromyzon, 92; general account of development of, 427

Moina, generative organs of, 745

Molgula, development of, 22

Mollusca, nervous system of, 414 ; eyes of, 472; auditory organs of, 515; excretory organs of, 68 1

Monotremata, foetal membranes of, 240 ; cerebellum of, 426; corpus callosum of, 443 ; cerebrum of, 443 ; urinogenital sinus of, 726

Mormyrus, generative ducts of, 704

Mouth, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 27; Salpa, 31; Elasmobranchii, 57, 60, 61, 62; Petromyzon, 92, 94, 95, 99; Acipenser, 107; Lepidosteus, 118; Amphibia, 129, 132, "134; Rabbit, 231 ; origin of, 317

Mouth, suctorial, of Petromyzon, 99; Acipenser, 107; Lepidosteus, 116, 317; Amphibia, 133, 141, 317

88

INDEX.

Mullerian duct, 690; of Elasmobranchs, 693 ; of Ganoids, 704 ; of Amphibia, 710; of Aves, 717,720; opening of into cloaca, 727; origin of, 733; summary of development of, 733; relation of to pronephros, 733

Muscle-plates, of Amphioxus, 6; Elasmobranchii, 49, 668 ; Teleostei, 670 ; Petromyzon, 94; Chick, 183, 670; general development of, 669 ; of Amphibia, 670; Aves, 670; of Mammalia, 671; origin of muscles from, 672

Muscles, of Ascidia, II, 17; development of from muscle-plates, 672; of limbs, 673 ; of head, 676 ; of branchial arches, 678; of eye, 678

Muscular fibres, epithelial origin of, 667

Muscular system, development of, 667; of Chordata, 668

Mustelus, placenta of, 66

Myoepithelial cells, 667

Mysis, auditory organ of, 517

Myxine, ovum of, loo; olfactory organ of, 533 ; portal sinus of, 652 ; excretory system of, 701

Nails, development of, 397

Nares, of Acipenser, 108; of Ichthyopsida, 534; development of in Chick, 535; development of in Lacertilia, 537; development of in Amphibia, 537

Nasal bones, 592

Nasal pits, Acipenser, 108; Chick, 176; general development of, 531

Nematoda, excretory organs of, 689 ; generative organs of, 745 ; generative ducts of, 752

Nemertines, nervous system of, 311 ; excretory organs of, 68 1

Nerve cord, origin of ventral, 378

Nerves, spinal, 449 ; cranial, 455 466

Nervous system, central, general account of development of in Vertebrata, 415 ; conclusions as to, 445; sympathetic, 466

Nervous system, of Amphioxus, 4; Ascidia, 15, 16; Molgula, 22; Pyrosoma, 24, 25; Salpa, 30, 31; Elasmobranchii, 44; Teleostei, 77 ; Petromyzon, 89, 93; Acipenser, 105; Amphibia, 126; comparative account of formation of central, 301; of Sagitta, 349; origin of in Ccelenterata, 349; of pneoral lobe, 377, 380; evolution of, 400405; development of in Invertebrates, 406; of Arthropoda, 408; of Gephyrea, 412; Mollusca, 414

Neural canal, of Ascidia, 10; Teleostei, 72; Petromyzon, 88; Acipenser, 105; Lepidosteus, 114; Amphibia, 126, 131 ; Chick, 1 66, 171 ; Lacerta, 208; closure of in Frog and Amphioxus, 279; closure of in Elasmobranchii, 284; phylogcuctic origin of, 316

Neural crest, 449, 456, 457

Neurenteric canal, of Amphioxus, 4, 5 ; Ascidia, lo; Elasmobranchii, 54; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 113; Aves, 162; Lacerta, 203, 206; general account of, 323; meaning of, 3 2 3

Newt, ovum of, 120; development of, I2 55 general growth of, 141

Notidanus, vertebral column of, 548; branchial arches of, 572

Notochord of Amphioxus, 6; Ascidia, II, 17; Elasmobranchii, 51; Teleostei, 74; Petromyzon, 86, 94; Acipenser, 104; Lepidosteus, 113; Amphibia, 128, 129; Chick, 157; canal of, in Chick, 163; Lacerta, 204, 205; Guinea-pig, 226; comparative account of formation of, 292, 325; sheath of, 545; later histological changes in, 546; cartilaginous sheath of, 547; in head, 566; absence of in region of trabeculas, 567

Notodelphys, brood-pouch of, 121 ; branchiae of, 140

Nototrema, brood-pouch of, 121

Nucleus pulposus, 559

Oceania, eye of, 471

Occipital bone, 595

CEsophagus, solid, of Elasmobranchii, 61, 759; of Teleostei, 78

Olfactory capsules, 571

Olfactory lobes, development of, 444

Olfactory nerves, Ammoccetes, 99; general development of, 464

Olfactory organ, of aquatic forms, 531; Insects and Crustacea, 531; of Tunicata, 532 ; of Amphioxus, 532 ; of Vertebrata, 533; Petromyzon, 533; of Myxine, 533

Olfactory sacks, of Elasmobranchii, 60; Teleostei, 73; Petromyzon, 92, 97; Acipenser, 106, 108; Lepidosteus, 116; Chick, 176

Oligochreta, excretory organs of, 683

Olivary bodies, 426

Omentum, lesser and greater, 757

Onchidium, eye of, 473

Opercular bones, 593

Operculum, of Teleostei, 77; Acipenser, 107; Lepidosteus, 117, 118; Amphibia,

r 3.5.

Ophidia, development of, 210; arterial system of, 649 ; venous system of, 656

Optic chiasma, 430, 493

Optic cup, retinal part of, 488 ; ciliary portion of, 489

Optic lobes, 428

Optic nerve, development of, 492 ; comparative development of, 500

Optic thalami, development of, 431

Optic vesicle, of Elasmobranchii, 57 59; Teleostei, 74, 499 ; Petromyzon, 89, 92 ; Acipenser, 106; Lepidosteus, 115; Chick, 170; Rabbit, 229; general development of, 429 ; formation of secon

INDKX.

7*9

dary, 487 ; obliteration of cavity of, 488 ; comparative development of, 499; of Lepidosteus and Teleostei, 499. See also ' Eye '

Ora serrata, 488

Orbitosphenoid region of skull, 570

Organs, classification of, 391 ; derivation of from germinal layers, 392

Orycteropus, placenta of, 249

Otic process of Axolotl, 583; of Frog, 585 et seq.

Otoliths, 512

Oviposition, of Amphioxus, i ; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 84; Amphibia, 121; Reptilia, 202

Ovum, of Amphioxus, i; Pyrosoma, 23; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 83 ; Myxine, loo; Acipenser, 102; Lepidosteus, in; Amphibia, 120; Chick, 146; Reptilia, 202 ; Mammalia, 214; of Porifera, 741; migration of in Ccelenterata, 742; Vertebrata, 746

Palatine bone, of Teleostei, 580; origin of, 594

Pancreas, Acipenser, no; general development of, 770

Pancreatic caeca, of Teleostei, etc. 768

Papillae, oral, of Acipenser, 108; Lepidosteus, n6

Parachordals, 565, 566

Parasphenoid bone, 594

Parepididymis, 725

Parietal bones, 592

Paroophorori, 725

Parovarium, 725

Pectoral girdle, 599 ; of Elasmobranchs, 600; of Teleostei, 600; of Amphibia and Amniota, 60 1 ; comparison of with pelvic, 608

Pecten, eye of, 479

Pecten, of Ammoccetes, 498; of Chick, 501 ; Lizard, 501 ; Elasmobranchs, 501

Pedicle, of Axolotl, 484 ; of Frog, 485

Pelobates, branchial apertures of, 136; vertebral column of, 556

Pelodytes, branchial chamber of, 135

Pelvic girdle, 606; of Fishes, 606; Amphibia and Amniota, 607 ; of Lacertilia, 607 ; of Mammalia, 608 ; comparison with pectoral, 608

Penis, development of, 727

Peribranchial cavity, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 24

Pericardial cavity, of Pyrosoma, 26 ; Elasmobranchii, 49 ; Petromyzon, 94; general account of, 626; of Fishes, 627 ; of Amphibia, Sauropsida and Mammalia, 628

Perichordal formation of vertebral column, 5^6

Perilymph of ear, 523 Periotic capsules, ossifications in, 595, 596

Peripatus, nervous system of, 409 ; eye of 480 ; excretory organs of, 688

Peritoneal membrane, 626

Petromyzon, development of, 83; affinities of, 83, 84; general development of, 87; hatching of, 89; comparison of gastrula of, 280; branchial skeleton of, 312, 572; cerebellum of, 425; pineal gland of, 434 ; pituitary body of, 436 ; cerebrum of, 439; auditory organ of, 517; olfactory organ of, 533; comparison of oral skeleton of with Tadpole, 586; pericardial cavity of, 627; abdominal pores of, 626 ; venous system of, 651 ; excretory organs of, 700; segmental duct of, 700; pronephros of, 700; mesonephros of, 700 ; thyroid body of, 760; postanalgut of, 774; stomodx-um

of, 775

Phosphorescence of larvae, 364

Phylogeny, of the Chordata, 327; of the Metazoa, 384

Pig, placenta of, 251; mandibular and hyoid arches of, 589

Pineal gland, of Petromyzon, 93 ; Chick, 175; general development of, 432; nature of, 432, 434

Pipa, brood-pouch of, 121 ; metamorphosis of, 139; yolk-sack of, 140; vertebral column of, 556

Pituitary body, of Rabbit, 231 ; general development of, 435 ; meaning of, 436 ; Placenta, of Salpa, 29; Elasmobranchii, 66; of Mammalia, 232; villi of, 235 ; deciduate and non-deciduate, 239; comparative account of, 239 259 ; characters of primitive type of, 240; zonary, 248; non-deciduate, 250; histology of, 257; evolution of, 259

Placoid scales, 395

Planorbis, excretory organs of, 68 1

Planula, structure of, 367

Pleural cavities, 631

Pleuronectidae, development of, 80

Pneumatoccela, characters of, 327

Polygordius, excretory organs of, 684

Polyophthalmus, eye of, 479

Polypedates, brood-pouch of, 121

Polyzoa, excretory organs of, 682 ; generative cells of, 745 ; generative ducts

of, 751

Pons Varolii, 426, 427

Pori abdominales, Ammoccetes, 99

Porifera, ancestral form of, 345 ; development of generative cells of, 74!

Portal vein, 653

Postanal gut of Elasmobranchii, 58, 59, 60; Teleostei, 75; Chick, 169; general account of, 323, 772

Prsemaxilla, 594

Praeopercular bone, 593

Prrcoral lobe, ganglion of, 377, 380

Prefrontals, 597

Presphenoid region of skull, 570

Primitive groove of Chick, 1 55

790

INDEX.

Primitive streak, of Chick, 152, 161; meaning of, 153; origin of mesoblast form in Chick, 154; continuity of hypoblast with epiblast at anterior end of, in Chick, 156; comparison of with blastopore, 165 ; fate of, in Chick, 165 ; of Lacerta, 203; of Rabbit, 221; of Guinea-pig, 223 ; fusion of layers at, in Rabbit, 224; comparison of with blastopore of lower forms, 226, 287 ; of Mammalia, 290

Processus falciformis of Ammoccetes, 498 ; of Elasmobranch, 502 ; of Teleostei , 503 Proctodseum, 778

Pronephros, of Teleostei, 78, 701 ; Petromyzon, 95, 99, 700; Acipenser, 106, no; Amphibia, 134, 707; general account of, 689 ; of Cyclostomata, 700 ; of Myxine, 701 ; Ganoidei, 705 ; of Amniota, 714; of Chick, 718; summary of and general conclusions as to, 728; relation of, to mesonephros, 731 ; cause of atrophy of, 729 Prootic, 596, 597 Propterygium, 616 Proteus, branchial arches of, 142 Protochordata, characters of, 327 Protoganoidei, characters of, 328 Protognathostomata, characters of, 328 Protopentadactyloidei, characters of, 329 Protovertebrata, characters of, 328 Pseudis, Tadpole of, 139; vertebral

column of, 556

Pseud ophryne, yolk-sack of, 140; Tadpole of, 140 Pterygoid bone, of Teleostei, 581; origin

of, 597

Pterygoquadrate bar, of Elasmobranchii, 576; of Teleostei, 581; Axolotl, 584; F r g, 584; ofSauropsida, 588; of Mammalia, 589

Pulmonary artery, origin of, 645 ; of Amphibia, 645 ; of Amniota, 649

Pulmonary vein, 655

Pupil, 489

Pyrosoma, development of, 23

Quadrate bone of Teleostei, 581 ; of Axolotl, 584; Frog, 585; Sauropsida, 588

Quadratojugal bone, 594

Rabbit, development of, 214; general growth of embryo of, 227 ; placenta of, 248

Radiate symmetry, passage from to bilateral symmetry, 373 376

Raja, caudal vertebras of, 553

Rat, placenta of, 242

Recessus labyrinthi, 519

Reissner's membrane, 524

Reptilia, development of, 202; viviparous, 202; cerebellum of, 426; infundibulum of, 431; pituitary body of, 436; cerebrum of, 439; vertebral column of,

556; arterial system of, 648; venous system of, 656; mesonephros of, 713; testicular network of, 723; spermatozoa of, 747

Restiform tracts of Elasmobranchii and Teleostei, 425

Retina, histogenesis of, 490

Retinulse, 482

Rhabdom, 482

Rhinoderma, brood-pouch of, 121; metamorphosis of, 1 39

Ribs, development of, 560

Roseniniiller's organ, 725

Rotifera, excretory organs of, 680

Round ligament of liver, 663

Ruminantia, placenta of, 253

Sacci vasculosi, 437

Sacculus hemisphericus, 519; of Mammals, 519, 520

Sagitta. See ' Chaetognatha'

Salpa, sexual development of, 29; asexual development of, 33

Salamandra, larva of, 142; vertebral column of, 553; limbs of, 619; mesonephros of, 708; Miillerian duct of, 710

Salmonidse, hypoblast of, 71; generative ducts of, 704

Sauropsida, gastrula of, 286; meaning of primitive streak of, 288; blastopore of, 289 ; mandibular and hyoid arches of, 588 ; pectoral girdle of, 60 1

Scala, vestibuli, 522; tympani, 523; media, 522

Scales, general development of, 396 ; development of placoid scales, 395

Scapula, 599

Sclerotic, 488

Scrotum, development of, 727

Scyllium, caudal vertebrse of, 553; mandibular and hyoid arches of, 578; pectoral girdle of, 600; limbs of, 610; pelvic fin of, 614; pectoral fin of, 615

Segmental duct, 690 ; development of in Elasmobranchs, 690; of Cyclostomata, 700; of Teleostei, 701; of Ganoidei, 704, 705 ; of Amphibia, 707 ; of Amniota, 713

Segmental organs, 682

Segmental tubes, 690 ; development of in Elasmobranchs, 691 ; rudimentary anterior in Elasmobranchs, 693 ; development of secondary, 731

Segmentation cavity, of Elasmobranchii, 42 44; Teleostei, 69, 85, 86; Amphibia, 122, 125

Segmentation, meaning of, 331

Segmentation of ovum, in Amphioxus, 2 ; Ascidia, 9 ; Molgula, 22 ; Pyrosoma, 23; Salpa, 30; Elasmobranchii, 40; Telostei, 69; Petromyzon, 84; Acipenser, IOT, Lcpidosteus, in; Amphibia, 122, 124; Newt, 125; Chick, 146; Lizard, 202: Rabbit, 214

INDEX.

791

Semicircular canals, 519

Sense organs, comparative account of development of, 304

Septum lucidum, 443

Serous membrane, Lacerta, 209; of Rabbit, 237

Seventh nerve, development of, 459

Shell-gland of Crustacea, 689

Shield, embryonic, of Chick, 151 ; of Lacerta, 202

SimiadiK, placenta of, 247

Sinus rhomboidalis, of Chick, 162

Sinus venosus, 637

Sirenia, placenta of, 255

Sixth nerve, 463

Skate, mandibular and hyoid arches of,

577

Skeleton, elements of found in Vertebrata, 542

Skull, general development of, 564 ; historical account of, 564 ; development of cartilaginous, 566; cartilaginous walls of, 570; composition of primitive cartilaginous cranium, 565

Somatopleure, of Chick, 170

Spelerpes, branchial arches of, 142

Spermatozoa, of Porifera, 741; of Vertebrata, 746

Sphenoid bone, 595

Sphenodon, hyoid arch of, 588

Spinal cord, general account of, 415; white matter of, 415; central canal of, 417, 418; commissures of, 417; grey matter of, 417; fissures of, 418

Spinal nerves, posterior roots of, 449; anterior roots of, 453

Spiracle, of Elasmobranchii, 62 ; Acipenser, 105; Amphibia, 136

Spiral valve. See 'Valve'

Spleen, 664

Splenial bone, 595

Squamosal bone, 593

Stapes, 529; of Mammal, 590

Sternum, development of, 562

Stolon of Doliolum, 29 ; Salpa, 33

Stomodaeum, 774

Stria vascularis, 524

Styloid process, 591

Sub-intestinal vein, 65 1 ; meaning of,

651

Syngnathus, brood-pouch of, 68 Subnotochordal rod, of Elasmobranchii,

54; Petromyzon, 94; Acipenser, no;

Lepidosteus, 115; general account of,

754; comparison of with siphon of

Chsetopods, 756

Subzonal membrane, 237; villi of, 236 Sulcus of Munro, 432 Supraclavicle, 600 Suprarenal bodies, 664 Supra-temporal bone, 593 Swimming bladder, see Air bladder Sylvian aqueduct, 428 Sylvian fissure, 444 Sympathetic ganglia, development of, 467

Tadpole, 134, 139, 140; phylogenetic meaning of, 137; metamorphosis of, 137; m can ing of suctorial mouth of, 585

Tail of Teleostei, 80; Acipenser, 109; Lepidosteus, 109; Amphibia, 132

Tarsus, development of, 620

Teeth, horny provisional, of Amphibia, 136; general development of, 776; origin of, 777

Teleostei, development of, 68; viviparous, 68; comparison of formation of layers in, 286; restiform tracts of, 425 ; mid-brain of, 425 ; infundibulum of, 431 ; cerebrum of, 439; nares of, 534; lateral line of, 538; notochord and membrana elastica of, 549 ; vertebral column of, 553; ribs of, 561; hyoid and mandibular arches of, 579; pectoral girdle of, 601 : pelvic girdle of, 606; limbs of, 618; heart of, 637; arterial system of, 645; muscle-plates of, 670; excretory organs of, 701 ; generative ducts of, 704, 735, 749; swimming bladder of, 763 ; postanal gut of,

Teredo, nervous system of, 414

Test of Ascidia, 14; Salpa, 31

Testicular network, of Elasmobranchs, 697 ; of Amphibia, 712 ; Reptilia, 723 ; of Mammals, 724

Testis of Vertebrata, 746

Testis, connection of with Wolffian body, in Elasmobranchii, 697; in Amphibia, 710; in Amniota, 723; origin of, 735

Thalamencephalon of Chick, 175; general development of, 430

Third nerve, development of, 461

Thymus gland, 762

Thyroid gland, Petromyzon, 92 ; general account of, 759; nature of, 760; development of in Vertebrata, 761

Tooth. See 1 Teeth'

Tori semicirculares, 428

Tornaria, 372

Trabeculas, 565, 567; nature of, 568

Trachea, 766

Trematoda, excretory organs of, 68 1

Triton alpestris, sexual larva of, 143

Triton, development of limbs of, 619} urinogenital organs of, 7 12

Truncus arteriosus, 638; of Amphibia, 638; of Birds, 639

Turiicata, development of mesoblast of, 293; test of, 394; eye of, 507; auditory organ of, 530; olfactory organ of, 532; generative duct of, 749 ; intestine of, 767; postanal gut of, 771; stomodseum of, 775

Turbellaria, excretory organs of, 68 1

Tympanic annulus of *'rog, 587

Tympanic cavity, of Amphibia, 135; Chick, 1 80; Rabbit, 232; general development of, 528; of Mammals, 591

Tympanic membrane, of Chick, 180; general development of, 528

792

INDEX.

Tympanohyal, 591

Umbilical canal of Elasmobranchii, 54,

57, 58, 59

Umbilical cord, 238; vessels of, 239

Ungulata, placenta of, 250

Urachus, 239, 726

Ureters, of Elasmobranchii, 696; development of, 723

Urethra, 727

Urinary bladder of Amphibia, "Jii; of Amniota, 726

Urinogenital organs, see Excretory organs

Urinogenital sinus of Petromyzon, 700; of Sauropsida, 726; of Mammalia, 727

Urochorda, development of, 9

Uterus, development of, 726; of Marsupials, 726

Uterus masculinus, 726

Utriculus, 519

Uvea of iris, 489

Vagus nerve, development of, 456, 457; intestinal branch of, 458; branch of to lateral line, 459

Valve, spiral, of Petromyzon, 97; Acipenser, no; general account of, 767

Valves, semilunar, 641; auriculo-ventricular, 642

Vasa efferentia, of Elasmobranchs, 697 ; of Amphibia, 711; general origin of, 724

Vascular system, of Amphioxus, 8; Petromyzon, 97; Lepidosteus, 116; general development of, 632

Vas deferens, of Elasmobranchii, 697 ; of Amniota, 723

Vein, sub-intestinal of Petromyzon, 97 ; Acipenser, no; Lepidosteus, 116

Velum of Petromyzon, 9 1

Vena cava inferior, development of, 655

Venous system of Petromyzon, 97; general development of, 651; of Fishes, 651 ; of Amphibia and Amniota, 655 ; of Reptilia, 656; of Ophidia, 656; of Aves, 658; of Mammalia, 661

Ventricle, fourth, of Chick, 176; history of, 424

Ventricle, lateral, 438, 440; fifth, 443

Ventricle, third, of Chick, 175

Vertebral bodies, of Chick, 183

Vertebral column, development of, 545, 549; epichordal and perichordal development of in Amphibia, 556

Vespertilionidse, early development of, 217

Vieussens, valve of, 426

Villi, placental, of zona radiata, 235 ; subzonal membrane, 235; chorion, 237;

Man, 246; comparative account of, 2 575 of young human ovum, 265, 269

Visceral arches, Amphioxus, 7 ; Elasmobranchii, 57 60; Teleostei, 77; Acipenser, 1 06; Lepidosteus, 116; Amphibia, 133; Chick, 177; Rabbit, 231; prseoral, 570; relation of to head cavities, 572; disappearance of posterior, 573; dental plates of in Teleostei, 574

Visual organs, evolution of, 470

Vitelline arteries of Chick, 195

Vitelline veins of Chick, 195

Vitreous humour, of Ammoccetes, 98 ; general development of, 494; blood* vessels of in Mammals, 503 ; mesoblastic ingrowth in Mammals, 503

Vomer, 594

White matter, of spinal cord, 415; of brain, 423

Wolffian body, see ' Mesonephros '

Wolffian duct, first appearance of in Chick, 183; general account of, 690; of Elasmobranchs, 693 ; of Ganoids, 704; of Amphibia, 710; of Amniota, 713; atrophy of in Amniota, 724

Wolffian ridge, 185

Yolk blastopore, of Elasmobranchii, 64

Yolk, folding off of embryo from, in Elasmobranchii, 55; in Teleostei, 76; Acipenser, 106; Chick, 168, 170

Yolk nuclei, of Elasmobranchii, 41, 53; Teleostei, 69, 75

Yolk, of Elasmobranchii, 40; Teleostei, 68; Petromyzon, 96; Acipenser, 109; Amphibia, 122, 129; Chick, 146; influence of on formation of layers, 278; influence of on early development,

341, 342

Yolk-sack, Amphibia, 131, 140, 141; enclosure of, 123

.Yolk-sack, development of in Rabbit, 227; of Mammalia reduced, 227; circulation of in Rabbit, 233 ; enclosure of in Sauropsida, 289

Yolk-sack, enclosure of, Petromyzon, 86

Yolk-sack, Lepidosteus, 118

Yolk-sack of Chick, enclosure of, 160; stalk of, 174; general account of, 193; circulation of, 195 ; later history of, 198

Yolk-sack of Elasmobranchii, enclosure of, 62, 283; circulation of, 64

Yolk-sack of Lacerta, 209 ; circulation of, 209

Yolk-sack, Teleostei, 75, 81; enclosure of, 75 ; circulation of, 81

Zona radiata, villi of, 237 Zonula of Zinn, 495

BIBLIOGRAPHY.

CEPHALOPODA.

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UROCHORDA.

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B. Hi. a

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Ill

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a 2

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(84) Ph. Owsjannikoff. On the development of Petromyzon fiuviatihs (Russian).

(85) Anton Schneider. Beitrdge z. vergleich. Anat. a. Entwick. d. Wirbelthiere. Quarto. Berlin, 1879.

(86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte Preisschrift. Haarlem, 1856.

(87) W. B. Scott. " Vorlaufige Mittheilung iib. d. Entwicklungsgeschichte d. Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. ill. Jahrg. 1880.

GANOIDEI. A cipenseridce.

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

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick. d. Store." Vorlauf. Mittheilung. Melanges Biologizes tires du Bulletin d. VAcad. Imp. St Petersbowg, Vol. VII. 1870.

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(91) W. Salensky. " Zur Embryologie d. Ganoiden (Acipenser)." Zoologischer Anzeiger, Vol. I., Nos. n, 12, 13.

Lepidosteidce.

(92) Al. Agassiz. "The development of Lepidosteus." Proc. Amer. Acad. of Arts and Sciences, Vol. xm. 1878.

AMPHIBIA.

(93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate brun." Mc/noires coitronncs, etc. de I 1 Acad. roy. de Belgique, 1868.

(94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens." /!iill,-tin dc V Acad. roy. de Belgique, 1875.

(95) Ch. van Bambeke. " Nouvelles recherches sur 1'embryologie des Batraciens." Archives de Biologic, Vol. I. 1880.

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(97) B. Benecke. "Ueber die Entwicklung des Erdsalamanders." Zoolo. isch er An zeiger, 1880.

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AVES.

(117) K. E. vonBaer. " Ueb. Entwickhmgsgeschichte d. Thiere." Konigsberg, 18281837.

(118) F. M. Balfour. "The development and growth of the layers of the Blastoderm," and "On the disappearance of the Primitive Groove in the Embryo Chick." Quart. J. of Micros. Science, Vol. xin. 1873.

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(121) J. Disse. " Die Entwicklung des mittleren Keimblattes im Htirmerei. Archiv fur mikr. Anat., Vol. xv. 1878.

(122) J. Disse. "Die Entstehung d. Blutes u. d. ersten Gefasse im Hiihnerei.' Archiv f. mikr. Anat., Vol. xvi. 1879.

(123) Fr. Durante. "Sulla struttura della macula germinativa delle uova di Gallina." Ricerche nel Laboratorio di Anatomia della R. Universita di Roma.

(124) E. Dursy. Der Primitivstreif des Hiihnchens. 1867.

(125) M. Duval. "Etude sur la ligne primitive de 1'embryon de Poulet. Annales des Sciences Naturelles, Vol. vn. 1879.

(126) M. Foster and F. M. Balfour. Elements of Embryology. Part I. London, 1874.

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(129) V. Hensen. " Embryol. Mitth." Archiv f. mikr. Anat., Vol. in. 1867.

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(131) W. His. Unsere Kbrperform tmd das physiol. Problem ihrer Entstehung. Leipzig, 1875.

(132) W. His. "Der Keimwall des Hiihnereies u. d. Entstehung d. parablastischen Zellen." Zeit.f. Anat.u. Entwicklungsgeschichte. Bd. I. 1876.

(133) W. His. " Neue Untersuchungen iib. die Bildung des Hiihnerembryo I." Archiv f. Anat. u. Phys. 1877.

(134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entwick. d. ers. Blutgefiisse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. Akad., Vol. LXIII. 1871.

(135) A. K6 Hiker. Entwicklungsgeschichte d. Menschen u. d. hbheren 7'hii'rc. Leipzig, 1879.

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(137) C. Kupffer and B. Benecke. " Photogramme z. Ontogenie d. Vogel." Nov. Act. d. k. Leop.-Carol.-Deutschen Akad. d. Naturforscher, Vol. XLI. 1879.

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(139) C. H. Pander. Beitrage z. Entwick. d. Hiinchens im Eie. Wiirzburg, 1817.

(140) A. Rauber. " Ueber die Etnbryonalanlage des Hiihnchens." Centralblatt fur d. medic. Wissenschaften. 1874 75.

(141) A. Rauber. Ueber die Stellung des Hiihnchens im Entwickhingsplan. 1876.

(142) A. Rauber. " Primitivrinne und Urmund. Beitrage zur Entwicklungsgeschichte des Hiihnchens." Morphol. Jahrbuch, B. II. 1876.

(143) A. Rauber. Primitivslreifen und Neurula der Wirbelthiere in normalcr und pathologischer Beziehung. 1877.

(144) R. Remak. Untersuch. iib. d. Entwicklung d. Wirbelthiere. Berlin, 185055.

(145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen Keimblatt. Sitz. Wien. Akad., Vol. LVII. 1860.

(146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat., Vol. vii. 1871.

(147) S. L. Schenk. Lehrbuch d. vergleich. Embryol. d. Wirbelthiere. Wien, 1874.

(148) S. Strieker. " Mittheil. iib. d. selbststiindigen Bewegungen embryonaler Zellen." Sitz. Wien. Akad., Vol. XLIX. 1864.

(149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sitzungsber., Vol. LIV. 1866.

(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug. Diss. Berlin, 1875.

(151) W. Waldeyer. "Ueber die Keimblatter und den Primitivstreifen bei der Entwicklung des Hiihnerembryo." Zeitschrift fiir ratioudle Medicin. 1869.

(152) C. F. Wolff. Theoria generationis. Halse, 1759.

(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriitcten Hiinchen. Halle, 1812.

REPTILIA.

(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Keptilien. Konigsberg, 1878.

BIBLIOGRAPHY, vii

(155) C. Kupffer. "Die Entstehung d. Allantois u. <1. Gastrula d. Wirbclthiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.

Lacertilia.

(156) F. M. Balfour. " On the early Development of the Lacertilia, together with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(157) Emmert u. Hochstetter. " Untersuchung lib. d. Entwick. d. Eidechsen in ihren Eiern." Reil's Archiv, Vol. X. 1811.

(158) M. Lereboullet. "Developpement de la Truite, du Lc/ard et du Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn. 1862.

(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia. Phil. Trans., Vol. 170, p. 2. 1879.

(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d. Gesell. z. Be/or, d. gesam. Naturwiss. Marburg. July 23, 1880.

Ophidia.

(161) H. Dutrochet. " Recherches s. 1. en veloppes du foetus." Mem. d. Soc. Mcd. if Emulation, Paris, Vol. vm. 1.816.

(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans. , Vol. 169, Part II. 1878.

(163) H. Rathke. EntTvick. d. Natter. Konigsberg, 1839.

Chelonia.

(164) L. Agassiz. Contributions to the Natural History of the United Slates, Vol. u. 1857. Embryology of the Turtle.

(165) W. K. Parker. "On the development of the skull and nerves in the green Turtle." Proc. of the Roy. Soc., Vol. xxvin. 1879. Vide also Nature, April 14, 1879, and Challenger Reports, Vol. I. 1880.

(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.

Crocodilia.

(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.

MAMMALIA.

(168) K. E. von Baer. Ueb. Entwicklungsgcschichte d. Jhiere. Konigsberg,

(169) Barry. "Researches on Embryology." First Series. Philosophical Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid. 1840.

(170) Ed. van Beneden. La maturation de Foeuf, la fecondation et les premieres phases du developpement embryonaire d. Mammiferes. Bruxelles, 1875.

(171) Ed. van Beneden. " Recherches sur 1'embryologie des Mammiferes. Archives de Biologic, Vol. I. 1880.

(172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc. de 1'oeuf chez les Cheiropteres." Archives de Biologie, Vol. I. 1880.

(173) Th. L. W. Bischoff. Entivicklungsgeschichte d. Siiugethiere 11. des Menschcn. Leipzig, 1842.

(174) Th. L. W. Bischoff. Entivicklungsgeschichte des Kanmcheneies. Braunschweig, 1842.

(175) Th. L. W. Bischoff. Entwicklungsgeschuhte des Hundeeies.

schweig, 1845.

(176) Th. L. W. Bischoff. Entivicklungsgesclnchte des Meerschivcinchens.

Giessen. 1852.

viii BIBLIOGRAPHY.

(177) Th. L. W. Bischoff. Entivicklungsgeschichte des Rehcs. Giesscn, 1854.

(178) Th. L. W. Bischoff. " Neue Beobachtungen z. Entwicklungsgesch. des Meerschweinchens." Abh. d. bayr. Akad., Cl. n. Vol. X. 1866.

(179) Th. L. W. Bischoff. Historisch-kritische B enter kungen z. d. naicstcn Alittheilungen iil>. d. erste Entwick. d. Siitigethiereier. Miinchen, 1877.

(180) M. Coste. Embryogenie comparee. Paris, 1837.

(181) E. Haeckel. Anthropogenie, Entwicklungsgeschichte des Menschen. Lci])zig, 1874.

(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens u. Meerschweinchens." Zeit.f. Anat. u. Entwick., Vol. I. 1876.

(183) A. Kolliker. Entivicklungsgeschichte d. Menschen u. d. hb'hcren Thiere. Leipzig, 1879.

(184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoologist her Anseiger, Nos. 61, 62, Vol. in. 1880.

(185) N. Lieberkiihn. Ueber d. Keimblatter d. Siiugethiere. Doctor- Jubelfeier d. Herrn H. Nasse. Marburg, 1879.

(186) N. Lieberkiihn. "Z. Lehre von d. Keimblattern d. Saugethiere." Sitz. d. Gesell. z. Beford. d. gesam. Natunviss. Marburg, No. 3. 1880.

(187) Rauber. "Die erste Entwicklung d. Kaninchens." Sitzungsber. d. naturfor. Gesell. z. Leipzig. 1875.

(188) C. B. Reichert. " Entwicklung des Meerschweinchens." Abh. der. Berl. Akad. 1862.

(189) E. A. S chafer. " Description of a Mammalian ovum in an early condition of development." Proc. Roy. Soc., No. 168. 1876.

(190) E. A. Schafer. "A contribution to the history of development of the guinea-pig." Journal of Anal, and Phys. , Vol. x. and xi. 1876 and 1877.

Fcetal Membranes and Placenta of Mammalia.

(191) John Anderson. Anatomical and Zoological Researches in Western Yunnan. London, 1878.

(192) K. E. von Baer. Untersuchungen iiber die Gef&ssverbindung zwischen Mutter und Fruc/tf, 1828.

(193) C. G. Cams. Tabulae anatomiam comparali-vam illustrantes. 1831, 1840.

(194) H. C. Chapman. "The placenta and generative apparatus of the Elephant." Journ. Acad. Nat. Sc., Philadelphia. Vol. viii. 1880.

(195) C. Creighton. " On the formation of the placenta in the guinea-pig." Journal of Anat. and Phys., Vol. XII. 1878.

(196) Ecker. Icones Physiologicae. 1852-1859.

(197) G. B. Ercolani. 7'he utricular glands of the uterus, etc., translated from the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations of memoirs published in the Mem. deW Accad. d. Scienze d. Bologna, and additional matter written specially for the translation.

(198) G. B. Ercolani. Nuove ricerche sulla placenta nei pesci cartilaginosi e nei mammiferi. Bologna, 1 880.

(199) Eschricht. De organis quae respirationi et mttritioni fcetus Mammaliutn inservinnt. Hafniae, 1837.

(200) A. H. Gar rod and W. Turner. "The gravid uterus and placenta of Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878.

(201) P. Hart ing. Het ei en de placenta van Halicore Dugong. Inaug. diss. Utrecht. " On the ovum and placenta of the Dugong." Abstract by Prof. Turner. Journal of Anat. and Phys., Vol. xin.

(202) Th. H. Huxley. The Elements of Comparative Anatomy. London, 1864.

(203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Verh. der Wiirzb. phys.-med. Gesellschaft, Bd. x.

(204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Virginiana)." Amer. Phil. Soc. Trans., Vol. x. 1853.

(205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences Nat., Ser. 3, Vol. I. 1844.

BIBLIOGRAPHY.

IX

(206) Alf. Milne-Edwards. "Kecherches sur la famille dcs Chcvrutains.' 1 Ann. dcs Sciences Nat., Series V., Vol. II. 1864.

(207) Alf. Milne-Edwards. " Observations sur quelqucs points <le I'Kmbryologie des Lemuriens, etc." Ann. Sci. Nat., Ser. V., Vol. xv. 1872.

(208) Alf. Milne-Edwards. " Sur la conformation du placenta chcz le Tainandua." Ann. des Sci. Nat., xv. 1872.

(209) Alf. Milne-Edwards. " Kecherches s. 1. enveloppes fcetales du Tatou a neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vill. 1878.

(210) R. Owen. "On the generation of Marsupial animals, with a description of the impregnated uterus of the Kangaroo." Phil. Trans., 1834.

(211) R. Owen. "Description of the membranes of the uterine foetus of the Kangaroo." Mag. Nat. Hist., Vol. I. 1837.

(212) R. Owen. "On the existence of an Allantois in a foetal Kangaroo (Macropus major)." Zool. Soc. Proc., v. 1837.

(213) R. Owen. "Description of the foetal membranes and placenta of the Elephant." Phil. Trans., 1857.

(214) R.Owen. On the Anatomy of Vertebrates, Vol. III. London, 1868.

(215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions of the Zoological Society, Vol. V. 1866.

(216) W. Turner. "Observations on the structure of the human placenta." Journal of Anat. and Phys., Vol. vn. 1868.

(217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc. Edinb,, Vol. xxvi. 1872.

(218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffrnanni)." Trans, of R. Society of Edinburgh, Vol. xxvn. 1875.

(219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)." Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.

(220) W.Turner. "On the placentation of the Cape Ant-eater (Orycteropus capensis)." Journal of Anat. and Phys., Vol. X. 1876.

(221) W. Turner. Lectures on the Anatomy of the Placenta. First Series. Edinburgh, 1876.

(222) W. Turner. "Some general observations on the placenta, with special reference to the theory of Evolution." Journal of Anat. and Phys., Vol. XI. 1877.

(223) W.Turner. " On the placentation of the Lemurs." Phil. Trans., Vol. 166, p. 2. 1877.

(224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878.

(225) W. Turner. "The cotyledonary and diffused placenta of the Mexican deer (Cervus Americanus). " Journal of Anat. and Phys., Vol. xm. 1879.

Human Embryo.

(226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies." Archiv f. Gynaekologie, Bd. xm. 1878.

(227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaekologie, Bd. xn. 1877.

(228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der Graviditat." Wiener medicinische Wochenschrift, 1877.

(229) M. Coste. Histoire generale et particuliere du developpement des corps organises, 1847-59.

(230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859.

(231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d. menschlichen Embryos." Archiv f. Anat. u. Phys., 1877.

(232) W. His. Anatomie menschlicher Etnbryonen, Part I. Embryonen d. ersten Monats. Leipzig, 1880.

(233) J. Kollmann. " Die menschlichen Eier von 6 MM. Grosse." Archiv f.

Anat. und Phys., 1879.

(234) W. Krause. Phys., 1875.

(235) W. Krause. /. wiss. Zool., Vol. xxxv.

Ueber d. Allantois d. Menschen." Archiv f. Anat. und

' Ueber zwei friihzeitige menschliche Embryonen." 1880.

Zeit.

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(236) L. Loewe. "Im Sachen cler Eihaute jiingster menschlicher Eicr. " Archiv fiir Gynaekologie, Bd. xiv. 1879.

(237) C. B. Reichert. " Beschreibung einer friihzeitigen menschlichcn Frucht im blaschenformigen Bildungszustande (sackformiger Keim von Baer) nebst vergleichenden Untersuchungen iiber die blaschenformigen Friichte der Saugethiere und des Menschen. " Abhandlungcn der konigl. Akad. d, Wiss, zu Berlin, 1873.

(238) Allen Thomson. "Contributions to the history of the structure of the human ovum and embryo before the third week after conception ; with a description of some early ova." Edinburgh Med. Siirg.Journal, Vol. LI I. 1839.

COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS IN THE VERTEBRATA.

(239) F. M. Balfour. "A comparison of the early stages in the development of Vertebrates." Quart. J. of Micr. Science, Vol. xv. 1875.

(240) F. M. Balfour. "A monograph on the development of Elasmobranch Fishes." London, 1878.

(241) F. M. Balfour. " On the early development of the Lacertilia together with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anal. it. Entwick., Vol. II. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.

(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad. des Sciences St Petersbourg, Ser. vii. Tom. XI. 1867.

(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lanceolatus." Archiv f. mikr. Anal., Vol. XIII. 1877.

(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." Zool. Anzeiger, Vol. II. 1879, PP- 5 2 ' 593' 61?.

(247) R. Remak. Untersuchungen iib. d. Entiuicklung d. Wirbelthiere, 1850 1858.

(248) A. Rauber. Primitivstreifen ti. Neurula d. Wirbelthiere, Leipzig, 1877.

PHYLOGENY OF THE CHORDATA.

(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, London, 1878.

(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.

(251) E. Haeckel. Schb'pfungsgeschichte. Leipzig. Vide also Translation. The History of Creation. King and Co. , London. 1876.

(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Antliropogeny. Kegan Paul and Co., London, 1878.

(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus." Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. xi. 1867, and Archiv f. ?nikr. Anat., Vol. XIII. 1877.

(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.

(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. u. 1875.

(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876 1877.

GENERAL WORKS ON EMBRYOLOGY.

(257) Allen Thomson. British Association Address, 1877.

(258) A. Agassiz. "Embryology of the Ctenophoroe." Mem. Amcr. Acad. of Arts and Sciences, Vol. X. 1874.

(259) K. E. von Baer. Ueb. Entivicklnngsgeschichle d. Thiere. Konigsberg, 18281837.

BIBLIOGRAPHY.

XI

(260) F. M. Balfour. "A Comparison of the Early Stages in the Development of Vertebrates." Qttart. Journ. of Micr. Set., Vol. XV. 1875.

(261) 1874.

C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastnca-theorie. Wieii,

(262) C. Claus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.

(263) A. Dohrn. Der Ursprung d. Wirbdlhiere u. d. Princip des Functionswechsds. Leipzig, 1875.

(264) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also Translation. Elements of Comparative Anatomy. Macmillan Co. 1878.

(265) A. Gotte. Ent^vicklungsgeschichte d. Unke. Leipzig, 1874.

(266) E. Haeckel. Studien z. Gastrcca-theorie, Jena, 1877; anc ' a ' so Jenaische Zeitschrift, Vols. vm. and IX. 1874-5.

(267) E. Haeckel. Schdpfungsgeschichte. Leipzig. Vide also Translation, The History of Creation. King & Co., London, 1878.

(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Atithropogeny. Kegan Paul & Co., London, 1878.

(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden." Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.

(270) O. and R. Hertwig. " Die Actinien." Jenaische Zeitschrift, Vols. xiil. and XIV. 1879.

(271) O. and R. Hertwig. Die Cctlomtheorie. Jena, 1881.

(272) O. Hertwig. Die Chatognathen. Jena, 1880.

(273) R. Hertwig. Ueb. d. Ban d. Ctenophoren. Jena, 1880.

(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill, 1877.

(274*) T. H. Huxley. "On the Classification of the Animal Kingdom." Quart. J. of Micr. Science, Vol. XV. 1875.

(275) N. Kleinenberg. Hydra, eine anatomisch-entivicklungsgeschichte Untersnchung. Leipzig, 1872.

(276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hbh. Thiere. Leipzig, 1879.

(277) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Acad. Petersbourg, Series vii. Vol. xvi. 1871.

(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist.

1873 (279) E. R. Lankester. " Notes on Embryology and Classification." Quart.

Jotirn. of Alter. Set., Vol. xvn. 1877.

(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme." Zeit. f. wiss. Zool., Vol. xxiv. 1874.

(281) E. Metschnikoff. " Spongiologische Studien." Zeit. f. wiss. Zool., Vol. xxxn. 1879.

(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines of Comparative Embryology. Holt and Co., New York, 1876.

(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol. x. 1876.

(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschneke (Planorbis)." Morph. Jahrbuch, Vol. v. 1879.

(285) H. Rathke. Abhandhmgen z. Bildung und Enlwicklungsgesch.d. Menschen u. d. Thiere. Leipzig, 1833.

(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig, 1829.

(287) R. Remak. Untersuch. ilb. d. Entwick. d. Wirbelthiere. Berlin, 1855.

(288) Salensky. " Bemerkungen lib. Haeckels Gastrsea-theorie." Archiv /. Naturgeschichte, 1874.

(289) E. Schafer. "Some Teachings of Development." Quart. Jotint. of Micr. Science, Vol. xx. 1880.

(290) C. Semper. " Die Verwandtschaftbeziehungen d. gegliederten Thiere." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876-7.

Xll BIBLIOGRAPHY.

GENERAL WORKS DEALING WITH THE DEVELOPMENT OF THE ORGANS OF THE CHORDATA.

(291) K. E. von Baer. Ueber Enlwicklungsgeschichte d. Thiere. Konigsberg, ! 828 1837.

(292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878.

(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Siiugdhiere u. d. Menschen. Leipzig, 1842.

(294) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also English translation, Elements of Comp. Anatomy. London, 1878.

(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I. London, 1874.

(296) Alex. Gotte. Entwickhmgsgeschichte d. Unke. Leipzig, 1875.

(297) W. His. Untersuch. ilb. d. erste Anlage d. Wirbelthierleibes. Leipzig, 1868.

(298) A. K 6 Hiker. Entwickhmgsgeschichte d. Menschen u. der hoheren Thiere. Leipzig, 1879.

(299) H. Rathke. Abhandlungen u. Bildung und Entwickhingsgeschichle d. Menschen u. d. Thiere. Leipzig, 1838.

(300) H. Rathke. Entwicklungs. d. Natter. Konigsberg, 1839.

(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.

(302) R. Remak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere. Berlin, 18501855.

(303) S. L. Schenk. Lehrbuch d. vergleich. Embryologie d. Wirbelthiere. Wien, 1874.

. EPIDERMIS AND ITS DERIVATIVES. General.

(304) T. H. Huxley. " Tegumentary organs." Todd's Cyclopedia of Anat. and Physiol.

(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv /. mikr. Anat. Vol. XV. 1876. Pft&also Kolliker (No. 298).

Scales of the Pisces.

(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zahne d. Selachier." Jenaische Zeitschrift, Vol. vill. 1874.

(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrbuch, Vol. u. 1876. (Siluroiden u. Acipenseridae.)

(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbuch, Vol. v. 1879.

Feathers.

(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.

(310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit.f. wiss. Zool., Vol. xxx. 1878.

Sweat-glands.

(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes Rendus, Dec. 29, 1879.

BIBLIOGRAPHY. xiii

Mammary glands.

(312) C. Creighton. "On the development of the Mamma and the Mammary function." Jour, of Anat. and Phys. , Vol. xi. 1877.

(313) C. Gegenbaur. " Bemerkungen lib. d. Milchdriisen-Papillen d. Saugethiere." Jenaische Zeit.. Vol. VII. 1873.

(314) M. Huss. " Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wiederkauern." Jenaische Zeit., Vol. vil. 1873.

(315) C. Langer. " Ueber d. Bau u. d. Entwicklung d. Milchdriisen." Denk. d. k. Akad. Wiss. Wien, Vol. in. 1851.

THE NERVOUS SYSTEM. Evolution of the Nervous System.

(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the British Association." 1880.

(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen, Discomedusen." Denk. d. math.-natiirwiss. Classe d. k. Akad. Wiss. Wien, Vol. xxxvin. 1877.

(318) Th. Eimer. Zoologische Studien a. Capri. I. Ueber Beroe ovatus. Ein Beitrag z. Anat. d. Rippenquallen. Leipzig, 1873.

(319) V. Hensen. " Zur Entwicklung d. Nervensystems. " Virchow's Archiv, Vol. xxx. 1864.

(320) O. and R. Hertwig. Das Nerven system u. d. Sinnesorgane d. Medusen. Leipzig, 1878.

(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriicksichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xiii. 1879.

(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift, Vol. xiv. 1880.

(323) A. W. Hubrecht. "The Peripheral Nervous System in Palseo- and Schizonemertini, one of the layers of the body-wall." Quart, y. of Micr. Science, Vol. xx. 1880.

(324) N. Kleinenberg. Hydra, eine anatomisch-entwickhmgsgeschichthche Untersuchung. Leipzig, 1872.

(325) A. Kowalevsky. " Embryologische Studien an Wtirmern u. Arthropoden." Mem. Acad. Petersboiirg, Series vil., Vol. XVI. 1871.

(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita." Phil. Trans. 1878.

Nervous System of the Invertebrata.

(327) F. M. Balfour. "Notes on the development of the Araneina." Quart. J. of Micr. Science, Vol. xx. 1880.

(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren.' Jenaische Zeitschrift, Vol. XI. 1877.

(329) N. Kleinenberg. "The development of the Earthworm, Lumbncus Trapezoides." Quart. J. of Micr. Science, Vol. xix. 1879.

(330) A. Kowalevsky. "Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, Series vin., Vol. xvi. 1871.

(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Flusskrebses." Zeit.f. wiss. Zool, Vol. xxix. 1877.

Central Nervous System of the Vertebrata.

(332) C. J. Carus. Versuch einer Darstellung d. Nervensystems, etc. Leipzig,

(333) J. L. Clark. " Researches on the development of the spinal cord in Man, Mammalia and Birds." Phil. Trans., 1862.

xiv BIBLIOGRAPHY.

(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. " Centralblatt f. d. med. \Vissenschaften, 1 868. Nr. 8.

(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen und der hb'heren Wirbelthiere. Tiibingen, 1869.

(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v. Ecker und Lindenschmidt. Vol. ill. 1868.

(337) E. Ehlers. " Die Epiphyse am Gehirn d. Plagiostomen." Zeit.f.wiss. Zool. Vol. xxx., suppl. 1878.

(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des Menschen. Auf Grtind entwicklungsgeschichtlicher Untersuchungen. Leipzig, 1876.

(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoisfs Archiv, Bd. xxx. 1864.

(340) L. Lowe. " Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Saugethiere u. d. Menschen." Berlin, 1880.

(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nervensystems d. Wirbelthiere." Mitthcil. a. d. embryol. Instit. Wien, Vol. u. 1880.

(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ." Quart. J. of Micr. Science, Vol. xix. 1879.

(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.

(344) W. Miiller. " Ueber Entwicklung und Bau der Hypophysis und des Processus infundibuli cerebri. " Jenaische Zeitschrift. Bd. vi. 1871.

(345) H. Rahl- Ruck hard. "Die gegenseitigen Verhaltnisse d. Chorda, Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d. einzelnen Theile d. Fischgehirns." Morphol. Jahrbuch, Vol. vi. 1880.

(346) H. Rathke. " Ueber die Entstehung der glandula pituitaria. " Mullens Archiv f. Anat. und Physiol. , Bd. V. 1838.

(347) C. B. Reich ert. Der Bau des menschlichen Gehirns. Leipzig, 1859 u 1861.

(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns." Zcitschrift f. wiss. Zoologie, 1862. Bd. xi.

(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Sitz. d. Jenaischen Gesell.f. Med. u. Natttnviss. Jan. 23, 1880.

(350) Fried. Tiedemann. Anatomie und Bildungsgeschichte des Gehirns im Foetus des Menschen. Niirnberg, 1816.

Peripheral Nervous System of the Vertebrata.

(351) F. M. Balfour. "On the development of the spinal nerves in Elasmobranch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A monograph on the development of Elasmobranch Fishes. London, 1878, pp. 191216.

(352) W. His. " Ueb. d. Anfiinge d. peripherischen Nervensystems." Archiv f. Anat. u. Physiol., 1879.

(353) A. M. Marshall. " On the early stages of development of the nerves in Birds." Jottrnal of Anat. and Fkys.,Vo\. XI. 1877.

(354) A. M. Marshall. "The development of the cranial nerves in the Chick." Quart, y. of Micr. Science, Vol. xvni. 1878.

(355) A. M. Marshall. "The morphology of the vertebrate olfactory organ." Quart, y. of Micr. Science, Vol. xix. 1879.

(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmobranchs." Quart, y. of Micr. Science, Vol. xxi. 1881.

(357) C. Schwalbe. "Das Ganglion oculomotorii. " Jenaische Zeitschrift, Vol. xni. 1879.

Sympathetic Nervous System.

(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878, p. 173.

(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung d. Ganglien d. Sympatheticus." Mittheil. a. d. cmbryologischen Instit. Wien. Heft III. 1879.

BIBLIOGRAPHY. XV

THE EYE.

Eye of the Mollusca.

(362) N. Bobretzky. " Observations on the development of the Cephalopoda " (Russian). Nachrichtcn d. kaiserlichen Gesell. d. Frennde der Natuna iss. Anthropolog. Ethnogr. bei d. Universitdt Moskau.

(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit. f. wiss. Zool., Bd. xxiv. 1874.

(364) V. Hensen. "Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss. Zool., Vol. xv. 1865.

(365) E. R. Lankester. " Observations on the development of the Cephalopoda." Quart. J. of Micr. Science, Vol. xv. 1875.

(366) C. Semper. Ueber Sehorganevon Typus d. Wirbelthicraugen. Wiesbaden, 1877.

Eye of the Arthropoda.

(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.

(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden. Palinurus und Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.

(369) E. Claparede. "Morphologic d. zusammengesetzten Auges bei den Arthropoden." Zeit. f. wiss. Zool., Bd. X. 1860.

(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden. Gottingen, 1879.

The Vertebrate Eye.

(371) J.Arnold. Beitrage zur Entwicklungsgeschichle des A uges. Heidelberg, 1874.

(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wiirzliurger naturwissenschaftliche Zeitschrift, Bd. 8.

(373) L. Kessler. Zur Ent^vicklung d. Auges d. Wirbclthiere. Leipzig, 1877.

(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.

(375) N. Lieberkiihn. " Beitrage z. Anat. d. embryonalen Auges." Archiv f. Anat. und Phys., 1879.

(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.

(377) V. Mihalkowics. "Untersuchungen iiber den Kamm des Vogelauges." Archiv f. mikr. Anat., Vol. IX. 1873.

(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wirbelthiere." Festgabe Carl Ludwig. Leipzig, 1874.

(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische." Wiener Sitzungsberichte, Bd. LV. 1867.

Accessory organs of the Vertebrate Eye.

(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien." Morphologisches Jahrbuch, Bd. II. 1876.

(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. V. 1879.

Eye of the T2tnicata,

(382) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.

(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1872.

xvi BIBLIOGRAPHY.

AUDITORY ORGANS. Auditory organs of tlie Invertebrata.

(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeil.f. wiss. Zool., Vol. xui. 1863.

(385) O. and R. Her twig. Das Nervensystem u. d. Sinnesorgane d. Medusen. Leipzig, 1878.

Auditory organs of the Vertebrata.

(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d. kaiserl. Leap. Carol. Akad. d. Wissenschaft., Vol. xxxv.

(387) C. Hasse. Dievergleich. Morphologieu. Histologied. hciutigen Gehororgane d. Wirbelthiere. Leipzig, 1873.

(388) V. Hensen. "Zur Morphologie d. Schnecke." Zeit. f, wiss. ZooI.,Vo\.

XIII. 1863.

(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim bebrliteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. VI.

(390) Reissner. De Auris internee formatione. Inaug. Diss. Dorpat, 1851.

Accessory parts of Vertebrate Ear.

(391) David Hunt. "A comparative sketch of the development of the ear and eye in the Pig. " Transactions of the International Otological Congress, 1 876.

(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres." Morphol. Jahrbiich, Vol. ill. 1877.

(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trommelfelles." Mittheil. a. d. embryol. Instit. Wien, Heft I. 1877.

OLFACTORY ORGAN.

(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.

(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen." Festschrift f. Rienecker, 1877.

(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." Quart. Journ. of Micr. Science, Vol. xix., 1879.

SENSE-ORGANS OF THE LATERAL LINE.

(397) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, pp. 141 146. London, 1878.

(398) H. Eisig. "Die Segmentalorgane d. Capitelliden." Mitlhcil. a. d. zool. Station zu Neapel, Vol. I. 1879.

(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(400) Fr. Ley dig. Lehrbuch d. Histologie des Menschen u. d. Thiere. Hamm.

T857 (401) Fr. Ley dig. Nene Beitrdge z. anat. Kenntniss d. Haiitdecke u. IJautsinnesorgane d. Fische. Halle, 1879.

(402) F. E. Schulze. "Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.

(403) C. Semper. "Das Urogenitalsystem d. Selachier." Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. II.

(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische." Archiv f. mikr. Anat., Vol. xvil. and xvni. 1879 and 1880.

ORIGIN OF THE SKELETON.

(405) C. Gegenbaur. "Ueb. primare u. secundare Knochenliildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium." Jciiaischc Zeitschrifl, Vol. in. 1867.

BIBLIOGRAPHY. xvii

(406) O. Hertwig. "Ueber Bau u. Entwicklung cl. Placoidschuppcn u. d. Ziihne d. Selachicr." Jetiaische Zeitschrift, Vol. vm. 1874.

(407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bcdeutung f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supplementheft, 1874.

(408) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrlmch, Vol. u. 1876. (Siluroiden u. Acipenseriden.)

(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. I'olypterus)." Morph. Jahrbnch, Vol. v. 1879.

(410) A. Kolliker. "AllgemeineBetrachtungenub. die Entstehungd. knocliernen Schadels d. Wirbelthiere. " Berichle v. d. konigl. zoot. Anstalt z. \Viirzlwrg, 1849.

(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bichir." Zeit.f. wiss. Zool., Vol. V. 1858.

(412) H. Muller. "Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc." Zeit. f. wiss. Zool., Vol. IX. 1859.

(413) Williamson. "On the structure and development of the Scales and Bones of Fishes." Phil. Trans., 1851.

(414) Vrolik. " Studien iib. d. Verknocherung u. die Knochen d. Schadels d. Teleostier." Niederldndisches Archiv f. Zoologie, Vol. i.

NOTOCHORD AND VERTEBRAL COLUMN.

(415) Cartier. " Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeitschrift fur wiss. Zool., Bd. xxv. Suppl. 1875.

(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wirbelsaule der Amphibien und Reptilien. Leipzig, 1862.

(417) C. Gegenbaur. "Ueber die Entwickelung der Wirbelsaule des Lepidosteus mit vergleichend anatomischen Bemerkungen." Jenaisckc Zeitschrift, Bd. ill. 1863.

(418) C. Gegenbaur. "Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische Zeitschrift, Vol. v. 1870.

(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d. Wirbelthiere." II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol. xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879 (Teleostier).

(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der Wirbelsaule u. s. w." Hasse, Anatomische Studiett, 1872.

(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.

(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical, medicin. Gesellschaft in Wiirzburg, Bd. X.

(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier insbesondere iiber die Wirbel der Lamnoidei." Abhandhmgen der senkenbergischen naturforschenden Gesellschaft in Frankfurt, Bd. V.

(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.

(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische und Reptilien. Berlin, 1853.

(426) Aug. Muller. "Beobachtungen zur vergleichenden Anatomic der Wirbelsaule." Miiller's Archiv. 1853.

(427) J. Muller. " Vergleichende Anatomic der Myxinoiden u. der Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungcn der koniglichen Akademie der Wissenschaften zu Berlin. 1834.

(428) W. Muller. "Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.

(429) A. Schneider. Beitrage z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

B. III. *

xviii BIBLIOGRAPHY.

RIBS AND STERNUM.

(430) C. Claus. " Beitrage z. vergleich. Osteol. d. Vertcbraten. I. Rippen u. unteres Bogensystem." Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(431) A. E. Fick. "Zur Entwicklungsgeschichte d. Rippen und Querfortsritze." Archiv f. Anat. und Physiol. 1879.

(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.

(433) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergiirtel." Archiv f. mikr. Anat., Vol. xiv. 1877.

(434) C. Hasse u. G. Born. " Bcmerkungen lib. d. Morphologic d. Rippen." Zoologischer Anzeiger, 1879.

(435) C.K.Hoffmann. " Beitrage z. vergl. Anat. d. Wirbelthiere." Niederliind. Archiv Zool., Vol. iv. 1878.

(436) W. K. Parker. " A monograph on the structure and development of the shoulder-girdle and sternum." Ray Soc. 1867.

(437) H. Rathke. Ueb. d. Ban u. d. Enlivicklung d. Brustbeins d. Sanricr.

1853 (438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen." Morphol. Jahrlmch., Vol. VI. 1880.

THE SKULL.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a leur differents ages." Paris, Mem. savans tirang. 1835, and An. Sci. Nat. Vol. I. 1834.

(440) C. Gegenbaur. UntersucJmngen z. vergleich. Anat. d. Wirbelthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehbrorgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr, Anat., Vol. xi. 1874, suppl.

(443) T. H. Huxley. "On the theory of the vertebrate skull." Proc. Royal Soc., Vol. ix. 1858.

f444) T.H.Huxley. The Elements of Comparative Anatomy . London, 1869.

(445 (446 (447

T. H. Huxley. "On the Malleus and Incus." Proc. Zool. Soc.,

T. H. Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."

Journ. of Anat. and Phys., Vol. X. 1876.

(448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the Common Fowl (Gallus Domesticus). " Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the Pig (Susscrofa)." Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the Batrachia." Part II. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the Common Snake (Tropidonotus natrix)." Phil. Trans. , 1878.

(456) W. K. Parker. "On the structure and development of the skull in Sharks and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(1.17) W. K. Parker. "On the structure and development of the skull in the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879.

BIBLIOGRAPHY,

(458) W. K. Parker. "The development of the Green Turtle." The Zoolo-v of the Voyage of H.M.S. Challenger. Vol. I. pt. v.

(459) W. K. Parker. "The structure and development of the skull in the Batrachia." 1't. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Bettany. The Morphology of the Skull. London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1830.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Mailer's Archiv, 1837.

(462) W. Salensky. " Beitrage z. Entwick. d. knorpeligen Gehorknochelchen." Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull; Gotte (No. 296).

THE PECTORAL GIRDLE.

(463) Bruch. "Ueber die Entwicklung der Clavicula und die Farbe des Blutes." Zeit.f. wiss. Zool., IV. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a leurs differents ages." Memoires des savants etrang. Academic royale des sciences de Finstitut de France, Vol. VI. 1835.

(465) C. Gegenbaur. Unterstichungen zur vergleichenden Anatomic der Wirbelthiere, i Heft. Schultergilrtel der Wirbelthiere. Brustflosse der Fische. Leipzig, 1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel. " Archiv f. mikr. Anat. Vol. XIV. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederldndisches Archiv f. Zool. , Vol. V. 1879.

(468) W. K. Parker. " A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.

(469) H. Rathke. Ueber die Entwicklung der Schildkroten. Braunschweig, 1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der Satirier, 1853.

(471) A. Sab a tier. Comparaison des ceintures et des menibres anteneurs et posterieurs d. la Serie d. Vertebrcs. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. lib. d. Entwick. d. Schultergiirtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

THE PELVIC GIRDLE.

(473) A. Bunge. Untersuch. z. Entwick. d. Beckengilrtels d. Amphibien, Reptilien u. Vdgel. Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc." Proc. of Roy. Soc., Vol. xxvin. 1879.

(476) A. S aba tier. Comparaison des ceintures et des membres anterieurs ct postb-ieurs dans la Serie d. Vertebres. Montpellier, 1880.

SKELETON OF THE LIMBS.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuchungen z. vergleich. Anat. d. Wirbelthiere. Leipzig, 18645. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d. Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeilschrift, Vol. V. 1870.

XX BIBLIOGRAPHY.

(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol. vn. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere." Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entwick. u. Regeneration d. Gliedmaassenskelets d. Molche. Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthiere." Zeit.f. wiss. Zool., Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi d. Menschen." Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u. Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Unterstich. iib. d. Entwick. d. Schnltergiirtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history of the Vertebrate limbs." Trans, oftke Connecticut Acad., Vol. III. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut Acad., Vol. IV. 1877.

PLEURAL AND PERICARDIAL CAVITIES.

(491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 1'cesophage." Journal de FAnatomie et de la Physiologic, Vol. xiv. 1878.

VASCULAR SYSTEM. The Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." Morphol. Jahrbuch, Vol. 11. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f. mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX. 1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), and Balfour (292).

The Arterial System.

(496) H. Rathke. "Ueb. d. Entwick. d. Arterien w. bei d. Saugethiere von d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(41)7) PI. Rathke. " Untersuchungen iib. d. Aortenwurzeln d. Saurier." Denkschriften d. k. Akad. Wien, Vol. xiil. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

The Venous System.

(498) J.Marshall. "On the development of the great anterior veins." Phil. Trans., 1859.

BIHLIOGRAI'IIY. XXJ

(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge thieren." Meckel 's Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbclthiere." Bericht. iib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke (Nos. 299, 300, and 301).

THE SPLEEN.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. "Ueb. d. Entwick. d. Milz." Silz. d. Wien. Akad. Wiss., Vol. LVI. 1867.

THE SUPRARENAL BODIES.

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilian." Arbeit, a. d. zool.-zoot. Institut Wilrzburg, Vol. v. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." Archiv f. mikr. Anat., Vol. vni. 1872.

(505) Fr. Leydig. Untersuch. ilb. Fische u. Reptilien. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

THE MUSCULAR SYSTEM OF THE VERTEBRATA.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." J our n. of Anat. and Phys., Vol. vi. 1872.

(508) J. Miiller. "Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." Quart. J. of Micr. Science, Vol. XXI. 1881.

(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." Sitz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc.

EXCRETORY ORGANS.

INVER TEBRA TA .

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Slat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Irematc Cestoides." Archives de Biologie, Vol. I. 1880.

(514) B. Hatschek. "Studien iib. Entwick. d. Annehden. Arbeit, a. d. zool. Instil. Wien, Vol. I. 1878. .

(515) B. Hatschek. "Ueber Entwick. von Echmrus, etc. Arbeit, a.

zool. Instit. Wien, Vol. ill. 1880.

VERTEBRATA.

General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." Journal of Anat. and Phys., Vol. X. 1876.

XXJi BIBLIOGRAPHY.

(517) Max. Fiirbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meckel. Zur Morphol. d. Harn- u. Geschlechtswerkz.d. Wirbelthiere, etc. Halle, 1848.

(519) Job. Mtiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Ratbke. "Beobachtungen u. Betrachtungen ii. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825.

(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. ubrigen Wirbelthiere." Arb. a. d. zool.-zoot. Insiit. Wiirzburg, Vol. u.

1875 (522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.

ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anal., Vol. xi. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. M uller. " Untersuchungen ii. d. Eingeweide d. Fische. " Abh. d. k. Ak. Wiss. Berlin, 1845.

(525) W. Muller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Muller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschrift, Vol. ix. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. II. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Enlwicklung d. Teleostierniere. Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologisclie Untcrsiiclniii^cn ii. die mdnnlichcn Geschlec/its- tmd Harmverkzeuge d. nackten Amphibien. Dorpat, 1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Ley dig. Analomie d. Amphibien u. Keptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Histologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wiirzburg. Vol. in. 1876.

(538) Von Wittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit. f. wiss. Zool., Vol. iv.

Vide also Gotte (No. 296).

1 The papers of Fiirbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.

BIBLIOGRAPHY. xxiii

Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of ahead-kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. XIX. 1878.

(540) Banks. On the Wolffian bodies of the foetus and their remains in the adult. Edinburgh, 1864.

(541) Th. Bornhaupt. UntersucJnmgen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. IV. 1877.

(543) J. Dansky u. J. Kostenitsch. " Ueb. d. Entwick. d. Keimblatter u. d. Wolffschen Ganges im Htihnerei." Me"m. Acad. Imp. Petersbourg, vn. Series, Vol. xxvn. 1880.

(544) Th. Egli. Beitrdge zur Anat. tmd Entiuick. d. Geschlechtsorgane. Inaug. Diss. Zurich, 1876.

(545) E. Gasser. Beitrdge zur Entwickhmgsgeschichte d. Allantois, der MiUler' schen Giinge u. des Afters. Frankfurt, 1874.

(546) E. Gasser. " Beob. iib. d. Entstehung d. WolfFschen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Htihnerembryonen." Sitz. d. Cesell. zur Beforderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchung liber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.

(550) A. Sedgwick. "On the development of the structure known as the glomerulus of the head -kidney in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. XIV. 1879.

(553) E. H. Weber. Zusdtze z, Lehre von Bane u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298).

GENERATIVE ORGANS.

(554) G. Balbiani. Lemons s. la generation des Vertebres. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart. J. of Micr. Science, Vol. XVIII.

(556) E. van Beneden. "De la distinction originelledutecticuleet del'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrhim." Zeit. f. wiss. Zool., Vol. xxxv. 1 88 r.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche. " Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach clem Ursprung d. Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

ALIMENTARY CANAL AND ITS APPENDAGES.

(561) B. Afanassiew. " Ueber Bau u. Entwicklung d. Thymus d. Saugeth." Archiv f. mikr. Anat. Bd. XIV. 1877.

XXIV BIBLIOGRAPHY.

(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffhung hei Hiihneremhryonen." Archiv f. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beitrage zur Entwicklungsgeschichte 'd. Darmkanah im Hithnchcn. 1867.

(565) W. Miiller. " Ueber die Entwickelung der Schilddriise." ycnaische Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaischc Zeitschrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische UntersucJnmgcn. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. lib. d. Entivick. d. Glandula Thymus, Glandula thyroidea, u. Glandula carotica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad. Petrop. 1766.

(571) A. Wblfler. Ueb. d. Entwick. it. d. Ban d. Schilddriise. Berlin, 1880. Vide also Kolliker (298), Qotte (296), His (232 and 297), Foster and Balfour (2!)5),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of Micros. Science, Vol. III. 1855.

(573) R. Owen. Odontography. London, 1840 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative. London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros. Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker 's Histology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.

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