Book - Vertebrate Embryology (1913) 7

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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures
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Chapter VII The Early Stages in the Development of the Embryo

A. In the Anamnia

The common frog affords a very good type of the development of the embryo from a small-yolked egg.

We left the frog's egg at the moment when the rotation was complete, the blastopore had been reduced to a small circle, and the material for the thi-ee germinal layers laid down and brought into position.

The circular blastopore soon becomes laterally compressed and so reduced to a narrow vertical slit, the yolk-plug being at the same time withdrawn into the interior. The opposite sides Avill in a little while meet together and fuse, so dividing the blastopore into a dorsal and a ventral portion. The dorsal portion will become the neurenteric canal. The ventral portion will close but reopen later as the proctodaeum or anus (Fig. 95).

Meanwhile, the rudiment of the nervous system has appeared in the form of a raised area of thickened ectoderm upon the dorsal side of the egg. This area is triangular or rather pearshaped, being broad in front, narrow behind (Fig. 95, a). It is known as the medullary plate. From the broad anterior end the brain will be developed, from the narrow posterior end the spinal cord. The edges of the medullary plate fade away on each side of the slit-shaped blastopore (Fig. 95, b).

There presently appears in the middle Hne a groove running the whole length of the medullary plate, the medullary groove. In the brain region this is wide and divided by transverse ridges into three depressions, the rudiments of the fore-, mid-, and hind-brains, in the region of the spinal cord narrow. At the same time the edges of the medullary plate begin to rise up on each side as the medullary ridges or folds. These folds are each divided longitudinally into two, an outer and an inner fold. The latter will for in the wall of the medullary tube. The outer fold is especially wide and prominent in front, where it is divided by a slight transverse furrow into two areas, the sense-plate and the gill-plate (Fig. 95, c). The posterior part of the outer medullary fold is narrow.

The outer medullary fold is due to the presence below the surface of the neural crest, a ridge of ectodermal cells from which the ganglia of the spinal nerves and of some of the cranial nerves are derived. The sense-plate contains the material for the fifth and seventh cranial nerve ganglia, the giU-plate that for the ninth and tenth, while the posterior narrow portion gives rise to the spinal ganglia.

The inner medullary folds now approach one another in the middle line, meet and fuse (Fig. 95, d). The groove is thus converted into a closed canal, the medullary tube ; from the wide anterior portion of this the ventricles of the brain will be formed, from the narrow posterior portion the canal of the spinal cord. At the extreme hind end, as we have already seen, the medullary folds pass into the sides of the blastopore. When they meet in this region they naturally cover over the dorsal half of the latter. The enteron therefore no longer opens to the exterior by means of the blastopore, but into the hinder end of the medullary tube. In this way the dorsal part of the blastopore is converted into a passage of communication between the nervous system and the gut : it has become the neurenteric canal (Fig. 95, f).

The ventral division of the blastopore is not covered by the medullary folds. It closes, but will reopen as the proctodaeum or anus.

The embryo now begins to elongate (Fig. 95) and a constriction appears behind the gill-plate separating the head from the trunk. At the anterior end and rather on the ventral side a depression is now seen - the stomodaeum or mouth invagination - and a little way behind this a V-shaped groove with prominent lips, the apex of the V pointing backwards. This is the cement gland or sucker. At the hinder end the proctodaeum is now visible in the place where the ventral division of the blastopore closed (Fig. 95, Q).

The body is at this time ciHated ; by this means the embryo turns over and over inside the jelly.

The tail appears as an outgrowth of the posterior end above the proctodaeum (Fig. 95). Apparently single, the tail is in reaUty double, as it is due to the fusion in the middle line of two separate tail buds or caudal swellings. These two tail buds arise one on each side of the blastopore, and the lateral compression of the latter is in reality the approximation of the two buds. The double (bilateral) origin of the tail is clearly to be seen in those cases where, as by the application of some external agent (heat, salt solution, and so on), the blastopore is prevented from closing, the tail buds are unable to meet, and consequently the tadpole has two tails.

At the anterior end the olfactory pit is seen on each side. The front end of the head is obliquely truncated. The V-shaped sucker is now divided into two. The gill-plate has become subdivided by transverse furrows into three bars, the first, second, and third branchial arches. The hyoid and mandibular arches lie in the region of the sense-plate (Fig. 95, h).

The trunk is laterally compressed dorsally, but ventrally swollen out by the yolk in the floor of the gut. Later the tail grows longer and is provided with a ventral and a dorsal fin, the latter being continued into the trunk region. On the dorsal side of the body and in the tail the myotomes or muscle segments become clearly visible.

The embryo elongates still more (Fig. 95, i), the external gills are developed as branched filaments on the three branchial arches in regular order from before backwards, the eye and the divisions of the brain cause prominent swellings at the side of the head, while another lateral swelling - behind the gills - marks the position of the pronephros or larval kidney. The caudal fin becomes wider.

Soon after this the embryo hatches out of the jelly as the larva or tadpole. The mouth now opens and the tadpole, fastening on to the jelly by its suckers, begins to feed on it. The suckers, however, are transitory organs and soon disappear. The mouth becomes transversely elongated and provided with rows of horny teeth. The external gills are soon covered by the operculum, a membranous fold growing back from the hyoid arch, which becomes fused with the body behind, leaving only one aperture, the spiracle, by which the water taken in at the mouth and passed out by the gill-slits can escape. This is on the left side. The external gills atrophy and are replaced by internal gills. Further details of the tadpole's structure and its metamorphosis into the frog do not, however, concern us here, and may be passed over.

We turn to the changes that have been taking place internally, and begin with the organs derived from the ectoderm. These are the epidermis and the nervous system and the sense-organs : the stomodaeum and proctodaeum may be considered with the alimentary canal.

The ectoderm is composed of two layers, an outer pigmented epidermal layer of columnar cells, and an inner nervous layer of polyhedral cells. Both layers are present in the medullary plate, but while the epidermal layer remains thin, the nervous layer is very considerably thickened, being composed here of six or more layers of cubical or columnar cells. The whole medullary plate is seen in section (Fig. 96, a) to be divisible into five tracts, a median, two internal lateral, and two external lateral. In the median tract the ectoderm is thin, in the lateral tracts it is thickened.

As the medullary folds rise up (Fig. 96, b), meet and fuse (Fig. 97), it is seen that the thin median tract becomes the floor of the medullary tube, the thick inner lateral tracts (which are the inner medullary folds) the thick side walls of the medullary tube, while the outer lateral tracts (the outer medullary folds) are carried up in the angle of the folds on either side as wedgeshaped masses of cells, the neural crests. When the folds have finally closed the outer layer of ectoderm is detached from the thin roof of the medullary tube, while the neural crests remam adherent to the latter. The neural crests are longitudinal ridges. Later they become transversely divided into segments in accordance with the segmentation of the mesoderm (see below). In the region of the spinal cord these segments become the gangUa and give rise to the dorsal roots of the spinal nerves. In the region of the brain they give rise to the roots and ganglia of some of the cranial nerves, namely, the fifth, seventh, eighth, ninth, and tenth.

Fig. 96. - Transverse sections of the embryo of the frog at two succeeding stages, A and b. a 1, b 1, Sections transverse to the trunk. A 2, b 2, Sections transverse to the head and therefore cutting the blastopore (b.p.) behind, m.p., medullary plate; in./., medullary fold; m.g., medullary groove; n.c, neural crest ; n., notochord ; m., mesoderm ;, dorsal mesoderm ; v.m.., ventral mesoderm ; ec, ectoderm ; g., gut. The mesoderm in this and the following figures is shaded.

The ventral roots of the spinal nerves are not formed from the neural crest, but by outgrowth of cells of the spinal cord. The third, foiu-th, and sixth cranial nerves (all purely motor) are formed in the same way as ventral spinal-nerve roots. The first cranial nerve arises by outgrowth of the front end of the brain, while the fibres of the second grow back from cells in the retina. The retina is, however, itself a derivative of the brain, as we are now to see.

The brain, as pointed out already, is divided into fore-, mid-, and hind-brains (Fig. 98).

By what is known as the primary cranial flexure the forebrain is in the embryo bent ventrally upon the mid-brain, so that the latter is brought to the front end of the body (Fig. 98). The brain, however, soon straightens out again.

Fig, 97. - Transverse sections of frog embryos showing the further development of the nervous system and mesoderm, m.t, medullary tube ; nc neural crest; n., notochord ; s.n., sub-notochordal rod; m.v.p., vertebral plate, m.l.p., lateral plate of the mesoderm ; my., myotom ; scl, sclerotom ; c, coelom (splanchnocoel) ; so., somatopleure ; sjjL, splanchnopleure ; prn., pronephric ridge ; v.v., vitelline vein ; g., gut ; I., liver.

The hind-brain gives rise to the medulla oblongata and cerebellum, its cavity becomes the fourth ventricle ; the mid-brain, whose cavity becomes the iter, gives rise to the optic lobes and crura cerebri, while the fore-brain becomes divided into the prosencephalon and thalamencephalon. The first comprises the two cerebral hemispheres, which are lateral outgrowths of the fore-brain, the rest of which then becomes the thalamencephalon ; its cavity, the third ventricle, is produced into a hollow dorsal outgrowth, the pineal body, a hollow ventral outgrowth, the infunclibulum, and two ventro-lateral outgrowths in front of the infundibulum, the optic vesicles.

The optic vesicles (Fig. 99, a) are the rudiments of the retinae of the eyes. Each hollow outgrowth communicates at first by a wide aperture with the lumen of the fore-brain, but the aperture soon becomes narrowed to a tube, the optic stalk. The optic vesicle then becomes converted into the optic cup, by the pushing in of the outer and thicker wall (Fig. 99, b). The cavity of the vesicle is thus obliterated. It is essential for the due comprehension of the embryology and anatomy of the eye to observe that this inpushing is not confined to the outer surface of the optic vesicle, but is extended along its ventral surface as well. There is therefore an opening to the optic cup on its lower as well as on its outer side, and when the latter is closed by the lens the former remains as a narrow slit through which mesodermal structures- blood-vessels and the cells which secrete the vitreous humour- pass into the cup. This slit is the choroid fissure. The wall of the cup, from the mode of its formation, is composed of two layers : the outer, which is thin, consisting of one sheet of cells, becomes the pigment layer of the retina, while; the inner, which soon comes to consist of several sheets of cells, becomes the retina itself, except at the edge of the cup (Fig. 100). Here it remains thin, and together with the outer layer, with which it is continuous, gives rise to the ciliaryprocesses and iris.

Fig. 98. - Median longitudinal section of a trog embryo, when the medullary tube has closed and the proctodaeum {pr.) has opened, f.b., fore-brain ; i., infundibulum; m.b., mid-brain; h.b., hiud-brain; sp.c, spinal cord; n.e.c, neurenteric canal ; n., notochord ; p.b., pituitary body ; sL, stomodaeum ; pc, pericardium ; ht., heart ; v.m., ventral mesoderm ; I., liver.

Fig. 99.- Sections illustrating the formation of the eye (a, b), ear, and heai-t (c, D) in the frog, f.h., fore-brain ; h.h., hind-brain ; o.v. optic vesicle ; o.c, optic cup ; o.sL, optic stalk ; le., lens ; p.h., pituitary body ; St., stomodaeu'm ; s., suckers ; n., notochord ; a., aorta ; a.v., auditory vesicle; lit., heart endothelium; U.m., muscular wall of the heart; 'pc, pericardium.

The lens is derived from the superficial ectoderm by invagination of the deep or nervous layer opposite the mouth of the cup. The invaginatcd cells become detached as a hollow vesicle, which is then fitted into the mouth of the optic cup. The cells of the outer wall of the lens vesicle remain cubical, and are the lens epithelium, but those of the inner wall become elongated into the lens fibres. The cavity of the vesicle is thus obliterated. The sclerotic, choroid, vitreous body and cornea are all mesodermal structures ; the first and second are formed from cells applied to the outer wall of the optic cup, the third from cells which migrate in by the choroid fissure, the fourth from cells which pass between the superficial ectoderm and the lens. The pigment soon disappears from this superficial ectoderm.

The muscles of the eyeball - ^recti and obliques - are derived from the mesodermal head somites, to which we shall refer later.

Fig. 100. - Section, transverse to the body, of the eye at a later stage. The section therefore passes down the length of the choroid fissure,, optic stalk; o.c.l, outer layer of optic cup (pigment layer of retina); o.c. 2, inner layer of optic cup (retina) ; U., lens ; co., cornea ; i;.6., vitreous body mesoderm ; h.v., blood-vessel ;, mesoderm which will form the sclerotic and the choroid.

The optic nerve-fibres are outgrowths of nerve-cells situated in the retina : the fibres pass out by the choroid fissure and back to the brain along the optic stalk. The stalk, therefore, merely serves as a guiding path to the fibres. The passage of the fibres through the choroid fissure explains the apparent perforation of the back of the eyeball by the optic nerve.

The olfactory pit (Fig. 104) arises by simple invagination of both layers of the ectoderm : it later becomes deepened to form the olfactory sac, the aperture being the nostril. The auditory vesicle- which wiU develop into the labyrinth or internal ear- is formed, like the lens of the eye, by invagination of the nervous layer of the ectoderm (Fig. 99, c, d). Its connexion with the ectoderm is soon severed, the ductus endolymphaticus being the remains of the communicating passage. The organs derived from the mesoderm are the muscular and skeletal systems, the connective tissue, the blood and vascular system, the coelom and urogenital organs.

We have seen how the mesoderm is laid down in the form of two sheets of tissde lying between the ectoderm and the endoderm, separated dorsally in the middle line by the notochord (and when it is formed, by the medullary tube as well), but continuous with one another below the gut (Fig. 96, a1, b1). The first diflEerentiation that occurs in the mesoderm is the separation on each side of a smaU dorsal portion- the vertebral plate - ^from a large ventral portion- the lateral plate (Fig. 97, a). The vertebral plates of the two sides are separated, but the lateral plates are continuous below. The vertebral plate soon becomes segmented transversely into a number of protovertebral or mesodermal somites, which give rise to the skeletal tissue and the muscles of the trunk and Umbs. This segmentation begins at the front end and extends backwards, there being therefore for some time at the hind end and eventually in the tail a strip of unsegmented mesoderm (Fig. 101). The first mesodermal somite is found behind the auditory vesicle. In front of this the mesoderm is not compact, but composed of scattered cells, and no traces of segments are found. This is also true of aU higher Vertebrates, but there is good reason for behevmg that virtual if not actual somites are present in this region, smce m Elasmobranch fishes and in Cyclostomes head somites are clearly visible The number of these somites is three, one in front of the mandibular visceral arch, one at the level of the mandibular arch, and one at the level of the hyoid arch. From these somites the recti and oblique muscles of the eyeball are formed.

The part of the head in which the mesoderm is thus cut up into somites comparable with the somites of the trunk is the posterior part, including the mid- and hind-brains and the anterior extremity of the notochord. It is of the greatest interest to observe that the cranial nerves which anse from these two regions of the brain are also derived from segmental nerves, comparable, though not in every detail, with the segmental nerves of the trunk. We have already seen that in the trunk the neural crest becomes segmented, in confot-mity with the segmentation of the mesoderm, into a number of pieces, out of which the gangUa and dorsal roots of the spinal nerves are developed, while the ventral roots arise separately from the spinal cord. The neural crest is continued into the posterior region of the head and is divided into segments, the first of which lies between the first and second head somites and gives rise to the ramus ophthalmicus of the fifth nerve, the second between the second and third head somites and gives rise to the main branch of the fifth nerve, the third between the third head somite and first trunk somite and gives rise to the seventh and eighth nerves, the fourth between the first and second trunk somites and gives rise to the ninth nerve, while the fifth between the second and third trunk somites gives rise to the tenth nerve ; the next segment of the neural crest becomes the ganglion and dorsal roots of the first spinal nerve.

It is clear, therefore, that the fifth, the seventh with the eighth, the ninth, and the tenth cranial nerves are developed in the same way as the dorsal roots of spinal nerves, and represent the dorsal roots of the nerves corresponding to the head somites and anterior trunk somites. The corresponding ventral roots - of the first, second, and third head somites - become the motor nerves innervating the eye-muscles, namely, the third, fourth, and sixth. These three are therefore the ventral roots of the same somites to which the fifth (two divisions) and the seventh with the eighth belong. The ventral roots corresponding to the ninth and tenth disappear (in the lamprey).

Fig. 101. - Horizontal section of the hind end of a frog embryo ; m.s., mesodermal somites ; m., posterior imsegmented mesoderm ; m.t., medullary tube; w., notochord; ew., endoderm ; ft.y., hind gut ; ec., ectoderm.

Hence the part of the head containing the anterior extremity of the notochord and the mid- and hind-brains consists in reality of a number of trunk segments with their corresponding dorsal and ventral nerve roots, fused together and telescoped on to the back of a more anterior region which comprises the fore-brain with its sensory nerves, the olfactory and optic. The relations of these dorsal nerve roots, ventral nerve roots, and somites are shown in the accompanying table.

Segments of neural crest

Nerves derived from them







V ramus ophthalmicus

V main branch




dorsal root of

Ist spinal

Mesodermal Somites

Ventral roots

Nerves derived from them

Head Somites









Trunk Somites











root of

1st spinal


To return to the mesoderm. The somites remain for some little time connected to the lateral plate, each by a Httle neck of tissue, the intermediate cell-mass. These necks of tissue are, Uke the somites, metamerically segmented. They are a morphologicaUy distinct part of the mesoderm of great importance, since from them is derived the whole of the system of kidneytubules and ducts. For this reason they are termed the nephrotoms. The lateral plate mesoderm remains unsegmented and unpaired ; it is continuous below the gut from the right side to the left, and also from the anterior end to the posterior.

The coelomic cavity soon appears, as a narrow space in the mesoderm (Fig. 97, b). It is not only found in the lateral plate, but extends into the intermediate cell-mass and somite. Three distinct divisions of the coelom may therefore be recognized - the muscle coelom or myocoel of the somite, the nephrocoel, the canal of the nephrotom or intermediate cell-mass, and the splanchnocoel or gut-coelom in the lateral plate. The third of these extends ventrally below the gut and from the front to the hind end of the body. The three divisions of the coelom communicate freely with one another as long as the intermediate section of a dog-fish embryo, through the front end of the body, m t., medullary tube ; n.c, neural crest ; n., notochord ; my.c, myocoel; scZ., sclerotom ; nt.c, intermediate canal or nephrocoel; prn., pronephric tubule growing out from the somatopleure of the nephrocoel; s^,i.c., splanchnocoel; so., somatopleure; spl., splanchnopleuref en., endoderm of gut (here open to yolk-sac). ;

cell-masses remain in connexion with the somites on the one hand and the lateral plate on the other. In the frog the relations of these parts are not so clear, as the intermediate cell-masses soon become detached from the somites and merged in the lateral plates, and the myocoel is smaller. They are, however easily made out in, for example, an Elasmobranch embryo' (Fig. 102).

From the ventral inner end of the somite the skeletal cells are produced, by simple emigration, or by outgrowth of a partially hollow mass, suggesting an evagination. These several segmental groups of cells produced from the segmental somites are known as sclerotoms. The cells pass in and up, round the notochord and the medullary tube to form later the centra and neural and haemal arches of the vertebrae (Figs. 97 B, 102).

The remainder of the somite, now termed a myotom, gives rise to the muscles of the back, limbs, and body-wall. The myocoel disappears, the cells of the inner wall become differentiated as muscle fibres, while those of the outer waU form a connective tissue cutis.

Fig. 103.- Transverse section of an advanced frog embryo, m.t., meduUary tube ; n., notochord ; s.n., sub-notochordal rod ; ?ny., myotom ; a., aorta ; p.c.v., posterior cardinal vein ; prn., pronephric tubule ; pm.J., pronephric funnel ; gl., glomus ; c, coelom ; so., somatopleure ; spl., splanchnopleure ; g., gut ; I., liver ; v.v., vitelline vem ; ec, ectoderm.

From the nephrocoel the kidney is derived. Kidney tubules arise by outgrowth- hollow or solid- of the outer wall or somatopleure of the nephrocoel. In all cases the pronephros is the first part of the kidney system to appear (Fig. 97, b). In the frog the pronephros consists of three well-developed tubules, produced by outgrowth of the somatopleure of the nephrotom (but after that has become merged in the lateral plate) : each tubule opens by a ciliated funnel into the coelom, and at its other extremity into a longitudinal duct which in turn discharges its contents into the cloaca. The duct is split off from the somatopleure, from before backwards, its anterior end being ab origine in continuity with the outgrowths which give rise to the tubules. The pronephric tubules coil amongst the capillaries of the posterior cardinal vein. The glomus is a bunch of blood-vessels hanging into the coelom at the root of the mesentery of the gut opposite the mouths of the pronephric tubules : its blood-supply is from the aorta. The pronephros is the larval kidney. The origin of the germ-cells has been described in a previous chapter.

Fig. 104. - Horizontal section through the head of an embryo of the same age as the last, ol., olfactory pit ; /.&., fore-brain ; i., infundibulum ;, optic stalk ; a., aorta ; 1, 2, 3, 4, 5, the five gill-slits (hyomandibular and four branchial) ; e.g., external gills on the first and second branchial arches. Those on the third are not yet formed. The mandibular arch Hes in front of gill-slit 1, the hyoid in front of gill- slit 2. ph., pharynx ; p.c.v., posterior cardinal vein ; pm.f., pronephric funnel ; prn.t., pronephric tubule.

The coelom of the lateral plate or splanchnocoel becomes the body-cavity of the adult. Its anterior end contains the heart, and is subsequently shut off as the pericardium from the posterior end or peritoneal cavity in which lie the gut, liver, kidneys, and gonads. When the lungs are developed and pushed into this cavity it is pleuro-peritoneal. The epithelium lining it is of course the peritoneum, and reflected over the gut and other viscera. The mesentery of the gut is therefore a double layer of peritoneum. The coelom naturally divides the mesoderm into an outer and an inner layer. The former, next the body wall, is the somatopleure ; the latter, near the gut, the splanchnopleure (Fig. 97, b). From the splanchnopleure come the muscles of the alimentary canal and of the heart.

The endothelial lining of the heart is derived from some scattered cells detached from the floor of the fore-gut (Fig. 99, c). As we have already seen, the ventral mesoderm is separated off from the yolk-cells which lie in the floor of the archenteron. The separation of these heart-cells is to be regarded as a later phase of the same process. The scattered cells soon unite to form a tube, the heart, which quickly pushes the splanchnopleure of the gut in front of it and so projects into the coelom, now the pericardium (Fig. 99, d). The splanchnopleure svirrounding the endothelial tube becomes the muscular wall of the heart. For a short time the heart is attached to the ventral body-wall by a ventral mesocardium, but as soon as the two sides of the pericardium coalesce this disappears. At its anterior end the heart gives off the arterial arches : of these there are at first three, passing to the external gills on the first, second, and third branchial arches : later a fourth is added. These are the four afferent branchial arteries : from the gill capillaries the blood passes by the four efferent arteries into the two dorsal aortae, which are united only some way back into a single median aorta. In the frog there are no arterial arches in the mandibular and hyoid arches.

Posteriorly the heart receives the two ductus Cuvieri, bringing back the blood from the cardinal veins in the body- wall, and the two vitelline veins bringing blood from the liver and gut. These last are formed from the ventral surface of the gut in the same way as the endothelial cells of the heart, and are to be looked upon as retarded ventral mesoderm. Blood corpuscles are derived from the same source and immediately fall into the veins. Other blood-vessels, aortae, cardinal veins, and so on, are produced by the union of scattered mesoderm cells, that is, wandering cells detached from the general mesoderm. Other cells detached in the same way form the connective tissue.

The notochord, which, we have seen, takes its origin at the same time and in the same way as the dorsal mesoderm, quickly assumes its characteristic histological features. It becomes cylindrical, the cells flat and discoidal, placed at right angles to the length of the notochord, and highly vacuolated. The notochord is surrounded by a delicate cuticular sheath, the chordal sheath or membrana elastica interna. Round it the vertebral column is laid down by the skeletal cells of the sclerotoms. Anteriorly the notochord terminates at the pituitary body behind the fore-brain.

The alimentary canal is derived from the endoderm, with the exception of the short stomodaeum and proctodaeum.

From the dorsal side of the stomodaeum the pituitary body grows, as a cord of ectodermal cells, up towards the infundibulum to which it becomes attached. By the formation of the floor of the skull it is shut off from the mouth (Figs. 98, 99 a).

From the endodermal lining of the alimentary canal come the thyroid, gill-sUts, thymus, lungs, liver, pancreas, and bladder. The gill-slits are endodermal outgrowths met by slight ectodermal ingrowths. There are five in the tadpole, the hyomandibular and four branchial (Fig. 104). The hyomandibular is never open. The remaining become perforated and functional.

The first or hyomandibular persists as the Eustachian passage.

The gill-arches alternate with the gill-slits. The mandibular lies in front of the first, then come in order the hyoid and four branchial arches. The arterial arches (of which in the frog there are only four pairs) run in the branchial arches.

The thyroid is a median ventral diverticulum of the pharynx opposite the second gill-sHt. The thymus is formed from dorsal epithelial remains of the gill-slits. The carotid gland and the epithelial corpuscles are ventral epithelial remains. The parathyroid is a rudimentary sixth giU-sUt. Lungs, liver, pancreas and bladder arise as ventral diverticula of the gut.

The mass of yolk-cells in the floor of the gut of the embryo becomes slowly absorbed. While present it forms what might almost be termed a yolk-sac.

In the forms with large -yolked eggs, Myxinoids, Elasmobranchs, and Teleosts, there is a well-developed yolk-sac. The embryo is developed from the median and posterior area of the blastoderm before that has spread over the yolk, and is folded oflE from it. The extra-embryonic blastoderm then encloses the yolk, the sac so formed being attached by a narrow (Elasmobranchs) or wide (Teleostei) yolk-stalk to the body (see above, Figs. 71, 72). The ectoderm of the embryo is thus continuous with that of the yolk-sac, the endoderm passes out on to the yolk, and a layer of mesoderm extends in between these two.

The general development of the embryo itself is quite similar to that in small-yolked forms, and only one or two points need to be mentioned. The tail is paired in origin, being formed by the coalescence of the two caudal swelUngs. The tail grows back freely above the surface of the yolk. On its upper surface is the hinder end of the medullary tube, on its lower side the tail gut or post-anal gut. This is formed by the bending down of the sides of the archenteric roof - after separation of the notochord and mesoderm - ^until they meet and fuse ventrally. At the hind end - the dorsal lip of the blastopore - all three germ-layers unite in a common cell-mass, and behind this point the medullary tube and post-anal gut are in communication by a neurenteric canal.

The heart is formed, after the fore-gut has been folded ofE from the yolk-sac, in the same way as in the frog, as a single median tube. The nervous system usually arises by medullary folds, but in Teleostei, Petromyzon, Lepidosteus, and Lepidosiren there is a solid wedge-shaped ingrowth of ectoderm along the middle dorsal line (see above, Figs. 65 c, 75 a). In this the cavity of the medullary tube appears later on. The rudiments of optic vesicles, auditory vesicles, and olfactory pits are similarly sohd at first.

The yolk-sac is provided with an area vasculosa of blood vessels by which the food material is conveyed to the body of the embryo. In Elasmobranchs there are in this area at first two venous rings, one peripheral - the sinus terminalis- and connected with the subintestinal vein of the embryo at the root of the tail ; the other central, pouring its blood directly into the heart. The latter becomes converted into an arterial ring by being connected directly to the aorta, disconnected from the heart. The arterial ring then becomes broken into two arteries, which finally fuse by their bases into a single median stem. As the blastoderm encloses the yolk the marginal sinus terminalis is correspondingly reduced (Pig. 104*).

Fig. 104. - Development of the area vasculosa in the Elasmobranch Torpedo. (After Riickert.)

A, Early stage. There are two venous rings ; the external one (sinus terminalis) at the edge of the blastoderm opens into the sub-intestmal veins of the embryo behind ; the internal ring opens into the heart in front.

B, Later stage when the internal ring has become arterial, bemg now connected to the aorta.

C, Last stage, when the arterial ring has become modified to form the anterior median artery (stippled) while the sinus terminalis is reduced to a small ring as the blastoderm encloses the yolk.

B. In the Amniota

In all Amniota the embryo is developed from the central area of the blastoderm only, the remainder being extra-embryonic.

This central embryonic area is at first 'spread out flat like the rest of the blastoderm - but as the embryo is developed it becomes gradually folded and constricted off from the surrounding blastoderm, as in the large-yolked Fishes. Here, however, the process is complicated by the development of the amnion, a sac enclosing the body of the embryo. The development of the chick may be taken as tjrpical.

As we have already seen, the blastoderm of the chick at the beginning of incubation consists of an upper and a lower layer. The upper layer is a columnar epithehum ; the lower layer is a sheet of scattered cells. At its edge the blastoderm rests upon the yolk, with which its marginal cells are continuous : in the yolk immediately surrounding the blastoderm are numerous nuclei, without cell-divisions. This nucleated ring is the syncytium or germinal wall.

Between the upper and lower layers is the segmentation cavity, and between the lower layer and the yolk the subgerminal cavity (continuous with and a part of the segmentation cavity).

After incubation has been in progress for a short time the lower layer cells unite to form a definite membrane or lower layer. The segmentation cavity is now separated from the subgerminal cavity. Marginally the lower layer is continuous with the germinal wall and with the upper layer. The blastoderm grows over the yolk by nuclear and cell division in this marginal zone, and several layers of cells are formed. At the surface is a layer continuous on the one hand with the upper layer of the blastoderm, on the other by its extreme marginal cells with the yolk. Underneath this are several layers of cells continuous with the lower layer of the blastoderm : the lowermost cells and the marginal cells are still continuous with the nucleated yolk or syncytium.

The subgerminal cavity below the central area of the blastoderm gives to it a transparent appearance ; this area is hence known as the area pellucida ; but the marginal zone, resting directly on the yolk, is opaque, and termed the area opaca. The extension of the subgerminal cavity is less rapid than the cell-division going on in the marginal zone, hence the area opaca increases more quickly than the area pellucida and soon forms a broad zone round about it (Fig. 106, a). The area pellucida meanwhile becomes pear-shaped, the broad end being anterior, and soon the first sign of the embryo appears (about the twelfth hour of incubation) in the form of the primitive streak, a dark median line in the posterior part of the area, down the axis of which runs the primitive groove (Pig. 105).

The area pellucida - and therefore later the embryo - ^is always oriented in a definite way with regard to the egg-shell, in which the ovum is so placed that the long axis of the pear-shaped area pellucida lies transversely to the long axis of the shell, while the broad anterior end is away from the observer when the blunt end of the shell is on his left.

As we already know, the primitive groove is an elongated laterally compressed blastopore, and the primitive streak the mesoderm produced at its sides and hinder end, the notochord being given off in front. (To the notochord the term ' head process of the primitive streak ' has been applied.)

The sheets of mesoderm grow forwards on the right and left, flanking the median notochord ; at the anterior end - ^in front of where the head of the embryo will be - ^they diverge somewhat, leaving between them a space in which the ectoderm rests directly upon the endoderm. This space is known as the proamnion. In front of the proamnion the mesoderm -sheets (at a later stage) meet and fuse, and eventually the proamnion is invaded by mesoderm and so disappears. Meanwhile a third area - the area vasculosa - has begun to appear between the pellucida and the opaca (Fig. 106).

The vascular area is first seen in the form of blotches of tissue along the inner edge of the area opaca, behind and at the sides of, but not in front of, the area pelliicida. These blotches are the blood-islands. They are formed from the masses of cells in the area opaca which lie between the upper layer above and the nucleated yolk below, between the lower layer on the central side and the nucleated yolk again on the peripheral side (Fig. 107).

Fig. 105.- Area pellucida of the hen's egg. a, After 12 hours', b, After 18 iiours mcubation, as seen by transmitted light, fr.g., primitive eroove ; n.cli., notochord;, pro-ainnion.

Fig. 106- a, Blastoderm of the hen s egg after 20 hours' incubation. 7h.f., head-fold of the embryo ; a.p., area pellucida; a.v., area vasculosa; a.o., area opaca.

B, Embryo, area pellucida, and area vasculosa of the same blastoderm, m./, medullary folds. Other abbreviations as before. Both as seen by transmitted light.

Fig. 107.- a. Edge of the blastoderm of the unincubated hen's egg. U.I., upper layer ; l.l., the lower layer cells in several sheets, the lowermost of which is continuous with the yolk.

B, Edge of the blastoderm after 15 hours' incubation, u.l., upper layer ; lower layer continued into the mass of cells lying on the yolk (area opaca). The lowermost of these cells are continuous with the yoUc. In the yolk are also nuclei with no cell-boundaries, the yolk-syncytium (y.syX This extends a httle way below the subgermmal cavity {s.g.c).

The cells become closely packed in groups, which are the bloodislands : from them the blood-vessels and blood-corpuscles of the area vasculosa are derived (Fig. 108). In each group the outermost cells become arranged in a thin flat epithelium - which becomes the endotheUum of a capillary vessel - while the rounded cells inside are the corpuscles. By the secretion of fluid cavities appear in between these cells, and the cavities run together to form the lumen of the capillary, inside which the corpuscles float freely. By the anastomosis of the blood-islands with one another the network of vessels of the area vasculosa is formed. The vessels soon come into connexion with others formed in the area pellucida.

Fig. 108. - Formation of a blood-vessel (b) from a blood-island (a). blood-island ; h.v., blood-vessel ; ec, ectoderm ; so., somatopleure ; c."' coelom ; s-p., splanchnopleure ;, yolk-syncytium ; y.s.ep., yolk-sac epithelium.

Fib 109 - Chick with area pelKicida and area vasculosa after 24 hours' iacubatlon: fore-gat ; ..I viteim.e vein ; Ts Sen by m.l.V; mesoderm of lateral plate. Other letters as before. As se.n oy transmitted light.

There is an evident similarity between the mode of formation of these vessels and corpuscles and the origin of the bloodcorpuscles and endothehum of the vitelhne veins in the frog. As these vessels and their corpuscles are derived in the frog from the large yolk-laden cells of the gut, so here the blood-islands arise from the thickened margin of the lower layer or endoderm : and just as in the former case, so in the latter we may consider this to be a retarded development of mesodermal structures from the yolk-cells.

In the meantime the outline of the body of the embryo has begun to appear in the form of the medullary plate (Fig. 109). This lies in front of the primitive groove. The notochord and mesoderm extend beneath it.

Down the middle of the plate the medullary groove is soon formed, bordered on each side by the mediillary folds, which diverge behind and then pass inwards into the sides of the primitive groove. In front the groove is wide, and divides later on into the three regions of fore-, mid-, and hind-brains ; behind it is narrow, the spinal cord. By the fusion of the folds the groove is converted into the closed medullary canal.

The head of the embryo now begins to be hfted up and folded off from the blastoderm. This is known as the head-fold of the embryo (Figs. 109, 111). By an exactly similar process lateral folds and a tail-fold are formed, and so the whole body of the embryo is graduaUy constricted off from the blastoderm. We shaU see later that the gut of the embryo, which is at the same time and in the same way folded off from the yolk-sac, remains connected to the latter by the yolk-stalk, but that the body-wall of the embryo is united with the amnion.

Before the embryo has been folded off from the blastoderm there is no ventral side to its body : the ventral side can only be made by the folding off, during which process parts which lie in front, at the sides of, and behind the embryonic area are bent underneath it.

The head of the embryo is immediately over the mesodermfree area, or proamnion. In front of this there soon rises up a fold of the extra-embryonic blastoderm. This is the head-fold of the amnion. It grows back, as a sort of hood, over the head and body of the embryo : presently it is met by side-folds and a tail-fold, and eventually all the folds meet over the back of the embryo in the posterior region, and the amnion becomes closed.

The mesoderm undergoes the same differentiation that we have made ourselves familiar with in the frog (Fig. 110). It becomes divided on each side into a vertebral plate next the notochord and medullary tube, and a lateral plate which extends outwards into the extra-embryonic region of the area pellucida and finally into the area vasculosa. The vertebral plate is segmented into somites. The separation and segmentation of the vertebral plate take place in regular order from before backwards, so that at the hinder end there is a strip of mesoderm still unsegmented and still imited with the lateral plate, and passing back to the primitive groove (Fig. 109).

Fig. 110. - Chick. Differentiation of the mesoderm, a, Posterior; b, Anterior section of the blastoderm at 24 hours, m.g., medullary groove ; n., notochord ; m., mesoderm ; m.l.p., lateral plate ; m.v.p., vertebral plate ; en., endoderm.

The first somite is at the side of the hind-bram- immediately behind the auditory vesicle, but, as abeady stated, there is every reason to beheve that virtual if not actual somites exist in the mesoderm in front of this.

Each somite remains connected to the lateral plate for some time by an intermediate cell-mass or nephrotom. From the nephrotoms are produced the kidney tubules (Figs. 112, 118). The anterior tubules or pronephros are rudimentary in the chick (and in all Amniota), but the segmental duct is formed from their union. The mesonephric or Wolffian tubules are, however, well developed, and function as the embryonic kidney. The adult kidney or metanephros is formed from more posterior tubules.

The coelom comprises the myocoel of the mesodermal somite, the nephrocoel of the nephrotom, and the splanchnocoel of the lateral plate (Fig. 110). The first soon disappears. The somite is differentiated into sclerotom and myotom. The second persists as the cavity of the capsule of the kidney tubule. The third also persists. In the embryonic region it becomes subdivided into the pericardium in front and the pleuro-peritoneal cavity behind (in Mammalia the pleural cavities become of course separated by the diaphragm from the peritoneal cavity). In the extra-embryonic region it extends out to the edge of the mesoderm. Here, therefore, the somatopleure lies against the extra-embryonic ectoderm, the splanchnopleure against the (endodermal) epithelium of the yolk-sac. In the area vasculosa, therefore, the splanchnopleure lies over the top of the blood islands, and the capillaries, when they are formed, are between the splanchnopleure and the yolk-sac (Fig. 108).

Fig. 111. - Diagram of a longitudinal section of a chick of about 30 hours, to show the folding off of the head of the embryo from the blastoderm, the folding off of the fore-gut from the yolk-sac, and the position of the heart, h.f., head-fold of embryo; f.b., fore-brain; m.b., mid- brain ; h.b., hind-brain ; sp.c, spinal cord ; p.s., primitive streak ; n., notochord ; gjj., endoderm ; f.g., fore-gut ; a.i.p., anterior intestinal portal ; st., stomodaeum ; ht., heart ; p.c, pericardium ; pr., proamnion. 1-5, The planes of the sections shown in Fig. 112.

Fio. 113.- Chick of 30 homs as seen by reflected light on a dark background, a from above, b from below, f.b., fore-brain with optic vesicles ; m.b., mid-brain ; h.b., hind-brain ; a.i.p., anterior intestinal portal ; j}.s., primitive streak ; b.i., blood-islands ; ht., heart. Other letters as before.

There is thus an extra-embryonic as well as an intra-embryomc coelom. The former is coextensive with the mesoderm, is found, therefore, behind and at the sides of the embryo, but not in the proamnion in front.

A network of blood-vessels soon appears in the area pellucida, continuous with those of the area vasculosa. It seems that these arise, not by encroachment of the capillaries of the latter region upon the former, but in situ. They are formed from the splanchnopleure and come into connexion with the others. The blood-vessels of the embryo, the two dorsal aortae and the cardinal veins, are also formed in situ from loose connective tissue, mesodermal cells which come together to form tubes, the vessels. The heart of the chick (and of all Amniota) is not, however, formed in the body of the embryo after that has been folded off from the blastoderm (as is the case in Fishes), but from the union of two veins which lie on the right and left, in the area pellucida, apparently outside the body of the embryo.

These are the vitelUne veins, into which flows the blood from the capillaries of the area vasculosa. By the actual process of folding off the head and with it the fore-gut from the blastoderm, the two veins are made to lie side by side underneath the foregut (Fig. Ill), where they coalesce to form a single median tube, the heart (Figs. 112, 113). The heart lies in a cavity, the pericardium, which is simply the anterior portion of the coelom of the lateral plate. As the head is folded off, somatopleure and splanchnopleure are naturally folded off with it, and with them the coelom. In the pericardium the heart is suspended by a mesocardium to the ventral side of the gut. The vitelline veins merely form the endothelium of the heart. Its muscular coat comes from the splanchnopleure covering it.

Though thus derived from two separate veins the cavity of the heart soon becomes single. The double origin has nothing whatever to do with the subsequent division into systemic and pulmonary portions. It is due simply to the fact that the veins are there before the gut is folded off from the yolk-sac. In Fishes the reverse is the case, and the heart is a single tube from the beginning.

The heart continues to receive the two vitelllne veins at its hinder end. These come from the anterior region of the area vasculosa, which extends just as far as the mesoderm, that is, up to the edge of the proamnion on each side.

The two vitelline veins reach the heart by travelHng along the anterior edge of the opening of the fore-gut into the yolksac, or, to put it in another way, along the line which marks the posterior limit of the head-fold of the endoderm. This opening is the anterior intestinal portal.

At its anterior end the heart gives rise to the aortic arches which pass round the sides of the throat between the gill-slits. These take the blood into the two dorsal aortae, whence it escapes, by the vitelline arteries, on to the area vasculosa again. We shall study the distribution of the blood-vessels in the yolksac later on.

Just as the fore-gut so is the hind-gut folded off from the yolk-sac, the opening being the posterior intestinal portal. The middle region remains for some time widely open to the cavity of the yolk-sac below, the communication being the yolk-stalk, but as development proceeds this becomes reduced to a narrow tube.

The changes we have so far described - closure of the medullary tube, differentiation of the mesoderm, formation of the coelom, folding off of the head and fore-gut, and development of the heart - have all taken place before the thirtieth hour of incubation. Other events now occur (Figs. 114, 115). The optic vesicles become apparent as lateral outgrowths of the fore-brain, the auditory vesicles are visible as two shallow pits Ijang one on each side of the hind-brain, and between the thirtieth and thirty-sixth hours the heart begins to be bent to the right-hand side. The part of the heart that is so bent is the' ventricle, and Avill become divided into the two ventricles of the adult heart. In front of the ventricular region is the truncus arteriosus ; this remains in the middle line and gives off the aortic arches. Behind the ventricle is the auricle, also in the middle line, and behind that the sinus venosus receiving the two vitelline veins.

Fig. 114. - Chick of 36 hours as seen, by transmitted light. A, from above, b, from below. The heart is bent out to the right. The head-fold of the amnion {am.) lias begun to grow over the head, o.v., optic vesicle ; a.v., auditory vesicle ; v.a., vitelline artery. Other abbreviations as before.

Fig. 115. - Cliick of 44 hours' incubation with area pellucida and area vasculosa, seen by reflected light. At x the mesoderm sheets have met in front of the pro-amnion. au., auditory vesicle ; a.t;., area vasculosa. Other letters as before. The head is beginning to turn to the right.

Fig 116.- Chick of 60 hours, A, from above by transmitted light, b, from below'bv reflected light. The head is turned to the right and lies with its left Tide on the yolk-sac. The fore-brain is bent down on the mid-brain crania flexure) Ld the hind-brain slightly bent on the body (cervical Kre) Three gill-slits are present (1, 2, 3) and three aortic arches fman^bular hyoid. and first branchial). The optic cup and lens are formed the'auditory vesicle has sunk into the head. The head-fold of the amnion has groin over the head and partly over the trunk. The hind-2ut is bei^inning to be folded ofi from the yolk-sac.

Z%oid arch ; h.g., hind-gut ; , tail-fold of embryo.

The head of the chick now begins to be turned to the right, so that it lies with its left side upon the blastoderm (48 hours). At the same time the fore-brain is beginning to be bent down below the mid-brain so that the latter comes to lie at the anterior end of the body. This is Imown as the primary cranial flexure. Later, by the cervical flexure, the hind-brain becomes bent down upon the body.

At the sides of the fore-brain may be seen the two optic cups - formed by pushing in the outer wall of the optic vesicles - and opposite the mouth of each optic cup the lens is being invaginated from the superficial ectoderm. The head-fold of the amnion has grown a little way back over the head.

By the middle of the third day (Fig. 116) all the four gill-slits are formed, and there are three pairs of aortic arches conveying the blood into the dorsal aortae. The blood which is distributed to the body of the embryo makes its way back to the heart by the anterior and posterior cardinal veins, and the ductus Cuvieri.

The cerebral hemispheres are beginning to .be protruded from the front of the fore-Dxain. The lens invagination has closed. The auditory vesicle has sunk into the head and lost its connexion with the exterior. The amnion has grown back over the head and front part of the trunk. By the end of the third day the aminion is closed and the allantois is visible outside the body of the erdbryo at the posterior end.

The foundations of the various systems of organs are now well estabhshed. It will be clear that they arise in essentially the same manner as in the frog. The nervous system - medullary tube and neural crest - and the sense-organs do not differ in any important particular. It may be pointed out, however, that the lens (Fig. 117) and the auditory vesicle (Fig. 112, 5) are, in the chick, invaginations of the whole thickness of the ectoderm, and not merely of an inner layer. The pituitary body is a hollow, not a solid upgrowth from the stomodaeum. The differentiations of the primary mesoderm are the same in the two cases. Attention has already been directed to the mode of formation of the heart. From the endoderm the same set of structures arises, the gut and its outgrowths or derivatives, thyroid, gill-slits, thymus, lungs, liver, pancreas, and bladder or allantois. We shall have to refer to the last-mentioned again, as it is one of the foetal membranes or appendages. Though the chick - like other Amniote embryo - possesses gill-slits, formed in the same way as in the frog by outgrowths of the pharynx wall, gills are never present. The number of these gill-slits is four, namely, the hyomandibular and three branchial. The first three are perforated and remain open up to the fourth (first and second) or fifth (third) day. The fourth slit is never open. The hyomandibular cleft remains to form the tympanic cavity and Eustachian passage. The arterial aortic arches bear the same relation to these gill-clefts as in the lower water-breathing forms, but are not divided by the gill-capillaries into afferent and efferent portions, and so pass uninterruptedly from the ventral aorta (truncus arteriosus) to the dorsal aortae.

Fig. 117. - Development of the eye in the chick. A, Section including the choroid fissure (transverse to the head) ; B, Section horizontal to the head ; c. Section parallel to the sagittal plane of the head and so transverse to the choroid fissure,, optic stalk ; o.c.l, outer layer of optic cup ; 0.C.2, inner layer of optic cup ; le., lens ; ch.f., choroid fissure ; f.b., forebrain.

By the middle of the third day three aortic arches are formed. A fourth is added at the end of the third day, and later two more, thus making six in all. The first of these is the mandibular, the second the hyoid, the remaining the four branchial aortic arches, being named from the gill-arches in which they run.

The two dorsal aortae unite posteriorly into one (Fig. 118).

The foetal membranes. The real difference between the early development of the Amniota and the Anamnia is due to the presence in the former of certain wrappings and appendages, known as the foetal membranes.

The foetal membranes are the amnion, the false amnion or chorion, the yolk-sac, and the allantois.

Fig. 118. - Section through the hind end of the chick on the third day when the amnion is closing, ec, ectoderm of false anmion with so., its somatopleure ; ec'. and so'., ectoderm and somatopleure of true amnion ; am.c, amniotic cavity ; c, extra-embryonic, c'., intra- embryonic coelom ; u., umbilicus; m.L, medullary tube; d.r., dorsal root of spinal nerve; 7ny., myotom ; s.d., segmental duct (Wolffian duct after degeneration of pronephros) ; cv., cardinal vein ; a., aorta ; v.a. vitelline artery ; spl., splanchnopleure ; en., endoderm ; g., gut.

The yolk-sac is the layer of endoderm with its covering of vascular splanchnopleure which encloses the yolk. It is connected to the gut, which has been folded off from it by a hollow stalk, the yolk-stalk (Fig. 120). In the splanchnopleure are the vessels of the area vasculosa.

The origin of these has already been seen. The arrangement of the main vessels undergoes considerable modification during the first few days of incubation (Fig. 119).

At about the thirtieth hour the two vitelline veins (anterior viteUine veins) come from the anterior end of the area vasculosa.passing along the inner edge of the mesoderm which borders the proamnion ; there they are seen to arise from an annular vessel - the sinus terminalis - ^which runs round the edge of the area vasculosa. Into this venous ring the blood leaks from the capillaries of the area, to which it is brought by a pair of vitelline arteries which are given off from the aortae. These vitelline arteries are only just being differentiated out of the network in the area (Fig. 119, a).

A little later (44 hours) the vitelline arteries and their branches are well developed. The two (anterior) viteUine veins are beginning to unite in front, just where they spring from the sinus terminalis. At the same time traces of two new lateral viteUine veins can be seen (Fig. 119, b).

On the third day the anterior vitelline veins are united in front, and the right-hand one is being reduced in diameter. The lateral viteUine veins are further advanced and receive their blood from a central part of the network, which is now no longer arterial but venous. This venous network is placed at the sides of and behind the embryo. Veins- called intermediate- open from it into the sinus terminaUs. The viteUine arteries- which are still better developed- pass right through the venous network before breaking up into capiUaries in the marginal part of the area vasculosa (Fig. 119, c).

On the fourth day the right anterior viteUine vein has nearly disappeared, while the new lateral vitelUne veins are conspicuous, receiving their blood from the central part of the area vasculosa. From this central part the blood passes also in the other direction by the intermediate veins into the sinus terminaUs. There is also a single posterior vitelline vein which, arises from the sinus terminaUs : it lies on the left-hand side.

The arteries go through the central venous area as before to reach the marginal part. They course alongside the mam venous trunks, lying always on the ventral side of the latter

(Fig. 119, D).

Finally (on the tenth day) the anterior and posterior vitelline veins, the intermediate veins, and the sinus terminaUs aU disappear, and only the lateral veins are left.

The lining epithelium of the yolk-sac-which is made of large vacuolated columnar cells - is produced internally into septa (Fig. 121), which are perforated by stomata.

The septa are suppUed with blood-vessels from the area vasculosa.

As the blastoderm grows over the yolk the latter becomes more and more completely enclosed by the yolk-sac, the edges of which finally almost meet.

With the growth of the blastoderm the mesoderm and the extra embryonic coelom have also been advancing, so that the yolk-sac with its covering of vascular splanchnopleure becomes more and more detached from the somatopleure, until only a very small connexion is left between the two (Fig. 120, 4).

On the nineteenth day the yolk-sac, togethet with the adherent albumen sac (see below) is drawn into the body-cavity through the umbilicus, where it remains visible for some time as an appendage of the ahmentary canal.

The amnion arises (Figs. 118 and 120, 1-3) by the formation of a fold of the extra embryonic ectoderm, together with the somatopleure which is appUed to it. The yolk-sac and its splanchnopleure have no share in the process.

There are four parts to the amnion fold, the head fold, which arises first and is much larger than the others, the tail fold, and the two lateral folds.

The extra-embryonic coelom is continued into the folds, each of which consists of two layers. The folds grow up over the back of the embryo and meet and fuse towards the posterior end of the body. When the fusion is complete the outer layer of the folds is separated from the inner by the coelom. Each is composed of a sheet of ectoderm and a sheet of somatopleure. The outer layer, now detached from the body of the embryo, is continuous with the two upper layers, ectoderm and somatopleure, of the extra-embryonic blastoderm. It is known as the false amnion or chorion or serosa. The inner layer, on the other hand, also composed of ectoderm and somatopleure, is continuous with the two corresponding layers of the body-wall of the embryo. It is known as the true amnion. The embryo has meanwhile been folded off, but a large aperture is necessarily left on its ventral side. This aperture is the umbilicus or navel. The amnion, therefore, now forms a completely closed sac inside which the body of the embryo is placed : the sac being inserted into the edges of an aperture, the umbilicus, which is left on the ventral side of the body. Through this aperture the intra embryonic is in free communication with the extra-embryonic coelom ; through it pass out the stalks of the yolk-sac and the allantois. The amriiotic cavity is filled with a fluid, the liquor amnii. The function of the amnion is to act as a water-bath and protect the embryo against shocks.

It has just been said that the false amnion becomes completely separated from the true. This is not quite accurate, since a small double strand of somatopleure is left at the point of closure, the sero-amniotic connexion (Figs. 120, 4, and 121). On the eleventh day this connexion becomes perforated and some albumen makes its way from outside into the amniotic cavity.

The false amnion continues to grow round the yolk with the rest of the extra-embryonic blastoderm, of which it is, of course, the outer layer. At its edge it is continuous, as heretofore, with the wall of the yolk-sac, the somatopleure of the false amnion mth the splanchnopleure of the yolk-sac, the ectoderm of the former with the epithelium of the latter (Fig. 120). With the final enclosure of the yolk (Fig. 121) the false amnion practically becomes a closed sac, inside which lies the embryo in its amnion with its yolk-sac and its allantois.

The allantois is a median ventral diverticulum of the hind gut (Fig. 120, 3). It is covered by a layer of splanchnopleure. It grows out and through the umbilicus into the extra-embryonic coelom, where it expands into a large sac occupying aU the available space between the amnion and yolk-sac on the inside, and the false amnion on the outside (Fig. 120, 4). The splanchnopleure covering it is vascular, and by means of its blood-vessels, the umbilical arteries and veins, the allantois is enabled to function as a respiratory organ. It is appHed closely to the inside of the porous shell ; and here oxygen is taken up and carbon dioxide given oflE by the blood in its capillaries.

At the narrow end of the shell the allantois pushes out the false amnion in the form of a circular fold which encloses the albumen. This is the albumen sac (Fig. 121). The albumen loses a great deal of water by evaporation during incubation. What remains of the albumen sac passes along with the yolk-sac through the umbilicus into the body cavity of the embryo.

Fig. 120. - Diagrams shoMdag the formation of the amnion, false amnion, yolk-sac, and allantois in the chick.

1, Transverse section. The lateral folds of the amnion are rising up ; the gut is not yet folded oS from the yolk-sac.

2, Transverse section. The aimiion is closed and the gut is folded off from the yolk-sac. The section passes down the yolk-stalk.

3, Longitudinal section, when the amnion is about to close and, the allantois is beginning to grow out.

4, Longitudinal section of a later stage, when the allantois has extended into the extra-embryonic coelom, and the yolk has nearly been enclosed at the vegetative pole.

In all the diagrams the ectoderm is represented by a thin continuous Ime, the mesoderm by a thick line swollen at intervals, the endoderm by a thick broken line, while the yolk is shaded. Lam., lateral amnion fold ;, head amnion fold;, tail amnion fold; am., true amnion ;, talse amnion ; am.c, amniotic cavity ; s.a.c, sero-amniotic connexion ; M., umbilicus; c, extra- embryonic coelom; all., aUantois : y., yolk in yolk-sac. '

At the time of hatching the amnion is broken and, with the allantois and false amnion, shrivels up. Morphologically, the allantois is an extra-embryonic bladder. It is, like the bladder of Amphibia, a median ventral diverticulum of the hind gut. Its veins, the umbilical veins, take the' same peculiar course as is taken by the anterior abdominal vein of the Amphibia, which receives the blood from the bladder, namely, in the ventral bodywaU, and thence into the capillary system of the liver. In the Reptiles the umbiHcal veins of the embryo remain as the anterior abdominal vein or veins of the adult. Lastly, the stalk of the aUantois persists as the bladder in the Reptiles and Mammals. In Birds the bladder is absent.

Fig 121 - Diagram of the final arrangement of the foetal membranes in the chick. (After Duval and LiUie.) sh., shell;, air-chamber; all, allantois ;, stalk of aUantois ; a.s., albumen sac ; x., pomt ot closure of yolk-sac ; am.c, amniotic cavity ; s.a c., sero-ammotic connexion ; c, extra-embryonic coelom ; y., yolk m yolk-sac.

The same foetal membranes are present in all Ammota. in the Ditrematous Mammalia they merit particular attention. The variety of their behaviour is manifold ; the amnion has many modes of formation, the yolk-sac, though innocent of yolk, is always present to point to the descent of the small-yolked MammaHan ovum from some large-yolked type, while the allantois vascularizes a placenta developed from the trophoblast, or ectoderm of the false amnion. To the study of these questions we may now proceed.


F. M. Balfour. Comparative Embryology, vol. ii, London, 1885. T. H. Bryce. Embryology, vol. i of Quain's Anatomy, London, 1908. M. Duval. Atlas d'Embryologie. Paris, 1887.

0. Hektwig. Handbuch der Entwicklungslehre der Wirbeltiere. Jena, 1906.

0. Hertwig. Die Elemente der Entwicklungslehre des Menschen und der Wirbeltiere. Jena, 1910.

N. K. KoLTZOFF. Entwickelungsgeschichte des Kopfes von Pelromyzon planeri. Bull. Soc. Imp. Nat. Moscou, 1901.

F. R. LiLLiE. The development of the chick. New York, 1908.

A. IVIiLNES Marshall. Vertebrate Embryology. London, 1893.

C. S. MiNOT. A Laboratory Text-book of Embryology. Philadelphia, 1910.

T. H. Morgan. The development of the frog's egg. New York, 1897. J. RiiCKERT. Die Entwickelung von Blut und Gefassen der Selachier, in 0. Hertwig's Handbuch der Entwicklungslehre der Wirbeltiere. Jena, 1906.

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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures

Cite this page: Hill, M.A. (2021, May 15) Embryology Book - Vertebrate Embryology (1913) 7. Retrieved from

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