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growth of foetal vessels into the foetal mesodermal cores. The secondary villi, therefore, consist of a mesodermal core covered by a layer of cellular trophoblast and a layer of plasmodium, the latter lying outside the former. Still later the

secondary villi send out numerous branches into the blood [blast spaces, and thus increase greatly in complexity (Figs. 75, 76, 77).

As development progresses still further a part of the chorion is converted into the foetal portion of an organ called the placenta, and thus the chorion is divided into placental and non-placental regions. Upon the placental part the villi continue to increase, but they disappear entirely from the nonplacental part, which is then called the chorion læve (Fig.

Anterior end of neural fold
Plasmodial trophoblast
Cellular trophoblast
Amnion cavity

[graphic]

Chorion

Cavity of entodermal vesicle

FIG. 70.-SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 77).

{Mesoderm in Cellular trophobla

Amnion cavity
Extra-embryonic cœlom

Mesoderm of amnion

Ectoderm of amnion

Mesoderm covering

entoderm

The Amnion, the BodyStalk (Allantoic Stalk), and the Umbilical Cord.-The amnion is formed from that portion of the wall of the larger of the two inner vesicles of the zygote, the ecto-mesodermal vesicle (p. 22), which does not take part in the formation of the embryo. It consists of ectoderm cells covered. externally by a layer of extra-embryonic mesoderm, and it is continuous with the margin of the embryonic area (Figs. 70, 71).

The cavity of the ectomesodermal vesicle, enclosed the cavity of the amnion; it

between the amnion and the embryonic area, is is filled with fluid, which raises the amnion in the form of a cupola over the embryonic region (Fig. 70).

The Body-Stalk (Allantoic Stalk).-It has been noted already that the mesoderm of the median part of the posterior or caudal portion of the amnion becomes

Entoderm

Cavity of entodermal vesicle

Plasmodial trophoblast

Mesoderm

Ectoderm

of amnion

Plasmodial trophoblast Neural groove

Notochord

Plasmodial trophoblast Cellular trophoblast-ndrer

Neurenteric canal

FIG. 71.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG
LINE B (Fig. 31).

Plasmodial
trophoblast

Cellular
trophoblast

Plasmodial.
trophoblast
Cellular
trophoblast
Efferent vessel
of villus

Afferent vessel of villus

Fused mesoderm

of chorion and amnion Ectoderm of amnion

FIG. 72.-SCHEMA OF THREE STAGES IN THE FORMATION OF A CHORIONIC VILLUS.

Fused mesoderm of Famnion and chorion Ectoderm of amnion

thickened. In the thickened strand lies the allantoic diverticulum of the entodermal vesicle (Fig. 70), whilst through it, on either side of the allantoic diverticulum, pass the umbilical arteries and veins, by means of which blood is conveyed between the embryo and the chorion.

This segment of the wall of the amnion vesicle was termed by His the body-stalk. It takes no direct part in the formation of the embryo, and as it

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contains the rudimentary allantoic diverticulum and represents the much more highly developed allantois of other forms, it would, perhaps, be better to term it the allantoic stalk. For the present purpose it is important to note that the bloodvessels which pass through the body-stalk enter or leave the body through the umbilical orifice, which is, at first, a relatively large aperture (Fig. 50).

As the embryonic area is folded into the form of the embryo the amnion. increases in extent, filling more and more of the extra-embryonic coelom, and the embryo rises into the interior of its cavity. In other words, the walls of the amnion bulge ventrally round the cranial and caudal extremities and the lateral borders. of the embryo (Figs. 75, 76, 77). As the distension of the amnion still continues, the ventral bulging, round the margin of the umbilical orifice, becomes more pronounced, the yolk-sac is forced farther and farther away from the embryo, the vitello-intestinal duct is elongated, and it is surrounded by a hollow tube. The cavity of the tube is an elongated part of the extra-embryonic coelom, and its walls are formed by the amnion (Figs. 57,62,63).

Plasmodial trophoblast

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Cellular trophoblast

Afferent vessel

The caudal wall of the tube necessarily consists of the elongated body-stalk (allantoic stalk).

As the distension of the amnion still continues, the walls of the tube are forced against the vitello-intestinal duct, and FIG. 73.-SCHEMA OF A TRANSVERSE SECTION OF A SECONDARY CHORIONIC VILLUS. A loop of the the amniotic mesoderm fuses with the afferent vessel has been cut at two points. mesoderm of the vitello-intestinal duct. When the fusion is completed, a solid cord, the umbilical cord, is formed (Figs. 77, 78, 80). It consists of an external covering of amniotic ectoderm, and a core of mesoderm in which lie the two umbilical arteries of the body-stalk, a single umbilical vein formed by the fusion of the two primitive veins, and the remains of the vitello-intestinal duct and the vitelline vessels. The proximal end of the umbilical cord is connected with the embryo; the distal end is attached to the chorion, and in its neighbourhood lies the now relatively small vesicular yolk-sac (Fig. 62).

of villus

Mesoderm of villus

Efferent vessel of villus

As the amnion grows still larger, all that part of its outer surface which does not take part in the formation of the umbilical cord is ultimately pressed into contact with the inner surface of the chorion, with which it fuses, and the cavity of the extra-embryonic part of the cœlom is obliterated (Fig. 78).

The outer wall of the zygote now consists of the fused chorion and amnion, and it contains in its interior the amniotic cavity and the embryo, which is attached to the chorion by the umbilical cord.

When it is first formed the umbilical cord is comparatively short, but, as the amniotic cavity increases, the cord elongates, until it attains a length of from 18 to 20 inches, a condition which allows the embryo to float freely in the fluid in the amniotic cavity, whilst its nutrition is provided for by the flow and return of blood, through the umbilical cord, to and from the placenta, where interchanges take place between the maternal and the foetal blood.

The Yolk-Sac or Umbilical Vesicle. When the embryonic area is folded into the form of the embryo, the entodermal vesicle is differentiated into three parts: (1) a part enclosed in the embryo, where it forms the primitive entodermal alimentary canal; (2) a part which lies external to the embryo in the extraembryonic cœlom-this is the yolk-sac or umbilical vesicle; (3) the third portion is the vitello-intestinal duct, which connects the primitive alimentary canal and the yolk-sac together (Figs. 40, 62).

The walls and the cavity of the yolk sac are, therefore, continuous with the walls of the primitive alimentary canal, and the structural features of the two are identical, each consisting of an internal layer of entodermal cells and an external layer of splanchnic mesoderm.

Free communication between the yolk-sac and the primitive alimentary canal

appears to exist in the human subject till the embryo is three weeks old and about 2.5 mm. long. During the fourth week the vitello-intestinal duct is elongated into a relatively long narrow tube, which is lodged in the umbilical cord and the yolk-sac, which has become a relatively small vesicle, is placed between the outer surface of the amnion and the inner surface of the chorion, in the region of the placenta (Fig. 62). During the latter part of the fourth or the early part of the fifth week, when the embryo has attained a length of about 5 mm., the vitellointestinal duct separates from the intestine and commences to undergo atrophy, but remnants of it may be found in the umbilical cord up to the third month.

The yolk-sac itself persists until birth, when it is, relatively, a very minute object which lies either between the amnion and the placenta or between the amnion and the chorion læve.

At a very early period, before the paraxial mesoderm has commenced to divide into mesodermal somites, a number of arteries, the primitive vitelline arteries, are distributed to the yolk-sac from the primitive arterial trunks of the embryo, the primitive aortæ, and the blood is returned from the yolk-sac to the embryo by a pair of vitelline veins (Fig. 81).

After a time the arteries are reduced to a single pair, and after the two primitive dorsal aortæ have fused into a single trunk, the pair of vitelline arteries also becomes converted into a single trunk, which passes through the umbilical orifice along the vitello-intestinal duct to the yolk-sac (Fig. 83).

The vitelline veins also pass through the umbilical orifice on their way to the heart of the embryo, and they become connected together, in the interior of the body of the embryo, by transverse anastomoses, which are described in the account of the development of the vascular system.

After the umbilical cord is formed, the extra-embryonic parts of the vitelline veins disappear, and can no longer be traced in the cord. The same fate overtakes the extra-embryonic and a portion of the intra-embryonic part of the vitelline artery, and the remainder of the artery persists as the superior mesenteric.

THE PLACENTA.

The placenta is an organ developed for the purpose of providing first the embryo and later the foetus with food and oxygen, and for removing the effete products produced by the metabolic processes which take place in the growing organism. It is formed partly from the zygote and partly from the mucous membrane of the uterus of the mother.

In the placenta the blood-vessels of the embryo of the earlier stages and the foetus of the later stages and the blood of the mother are brought into close relationship with one another, so that free interchanges may readily take place between the two blood streams; and the modifications and transformations of the uterine mucous membrane and the chorion of the zygote, by which this intimate relationship is attained, constitute the phenomena of the development of the placenta.

The details of the development of the human zygote for the first ten or twelve days after the fertilisation of the ovum are not known, but the knowledge of what happens in other mammals justifies the belief that during that time the zygote is formed, in the ovarian, or the middle part of the uterine tube, by the union of a spermatozoon with the mature ovum. During the first ten to fourteen days after its formation it passes along the uterine tube, towards the uterus, whilst, at the same time, it undergoes the divisions which convert it into a morula.

The Formation of the Placenta.-Before the zygote reaches the uterus the mucous membrane which lines the cavity of that organ undergoes changes, in preparation for its reception and retention, and when the changes are completed the modified mucous membrane is known as the uterine decidua.

The changes which take place are, for the most part, hypertrophic in character; the vascularity of the mucous membrane is increased, mainly by the dilatation of its capillaries; the tubular glands of the membrane are elongated, they become

tortuous, and dilatations form in their walls a short distance from their outer closed At the same time the interglandular tissue increases in amount, and as a result of the various processes the decidua is thicker, softer, more spongy, and more vascular than the mucous membrane from which it was evolved.

Partly on account of the dilatation of the deep part of the glands and partly on account of differences in texture of the internal as contrasted with the external part of the decidua, the membrane may be looked upon as consisting of three layers. (1) An internal layer, next the cavity, the stratum compactum. (2) An intermediate layer, the stratum spongiosum, formed largely by the dilated parts of the glands. (3) An external layer, the unchanged layer, in which lie the comparatively unaltered outer ends of the glands.

When the zygote, in the morula stage, reaches the uterus, from the tenth to the fourteenth day, it acts as a parasite, it eats its way through the epithelium on the surface of the decidua, and implants itself in the stratum compactum.

The zygote may penetrate the decidua at any point of the wall of the uterine cavity, but it usually enters at some point of the dorsal or the ventral wall. The entrance generally takes place between the mouths of adjacent glands, which are pushed aside, and the zygote becomes at once surrounded by the interglandular tissue of the stratum compactum of the decidua. The aperture through which it passes may be closed by a fibrinous plug or its margins may converge rapidly and fuse together.

The portion of the decidua in which the zygote is embedded is thicker than the other parts of the membrane, and it is separated by the zygote into an internal part, the decidua capsularis, and an external part, the decidua basalis. The junction of the decidua capsularis with the decidua

basalis is the decidua marginalis, and the remainder of the decidua, by far the larger portion, is the decidua vera.

As soon as the zygote becomes embedded in the decidua its trophoblast undergoes rapid proliferation. The superficial part of the growing trophoblast becomes converted into a mass of nucleated protoplasm, the plasmodial or syncytial layer, but the inner part remains more or less distinctly cellular.

Uterine

tube

Decidua vera
(compact
layer)

Decidua basalis

Trophoblast

[graphic]
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Inner mass of cells
Unchanged layer
Dilated part of gland
Inner part of
gland

FIG. 74. SCHEMA OF A FRONTAL SECTION OF THE UTERUS, showing the various parts of the decidua and a zygote embedded in the decidua.

The plasmodial portion of the trophoblast invades and destroys the surrounding maternal tissue, and at the same time spaces appear in its substance. As the plasmodium destroys the walls of the dilated maternal blood-vessels, channels are made through which the maternal blood flows into the spaces in the plasmodium, and thus maternal blood begins to circulate in the trophoblast of the zygote.

In the meantime the extra-embryonic cœlom has appeared in the primary mesoderm of the zygote, and the outer layer of the mesoderm has associated itself with the trophoblast to form the chorion.

The spaces in the plasmodium enlarge rapidly after the maternal blood

processes being shut off from the margin of the ledge by the maxillary processes (Fig. 65).

After the ledge is completed the dorsal ends of the olfactory pits are separated from the stomatodæum by a thin membrane, but this soon disappears, and the pits open again into the stomatodæal space, through apertures which are called the primitive choanæ.

After the formation of the primitive choanæ a ledge grows from the medial surface of each inaxillary process towards the median plane, caudal to the choanæ. These ledges, the palatine processes, meet and fuse during the third month of foetal life, the fusion commencing ventrally and being completed dorsally in the region of the uvula. As the ledges meet and fuse, the stomatodæum is separated into a cranial and a caudal portion. The cranial part is the nasal cavity; it is soon divided into two lateral halves by a septum which passes caudally from the base of the cranium. The caudal portion of the stomatodæum blends with the ventral part of the primitive pharynx and it forms the vestibule of the mouth and its derivatives, and the gums and teeth.

The details of the process by which the primitive lips are separated into the permanent lips, and the gums are defined, are described in the section dealing with the digestive system.

Anterior nasal orifice

[graphic]

Globular process.

Maxillary

process

Hypophyseal depression

FIG. 66. PORTION OF THE HEAD OF A HUMAN

EMBRYO ABOUT 2 MONTHS OLD (His). The lips are separated from the gums, and the line of the common dental germ is visible in the latter. The palatine processes are growing inwards from the maxillary processes.

The Derivative of the Proctodæum. -The proctodæum is a surface depression which owes its origin to the elevation of the surface round the margin of the anal membrane. It forms the lowest portion of the pars analis recti of the adult.

Urino-genital System. The formation of the internal parts of the urino-genital system from the intermediate cell tract, the urino-genital chamber, and the differentiation of the external genitals in the region of the cloacal membrane are described in the account of the urino-genital system.

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The development of the auditory organ is so intimately associated with the development of the pharyngeal portion of the primitive gut that a short consideration of the chief phenomena may with advantage be introduced here; but for the details of the development of the internal, middle, and external portions of the ear the student must refer to the account of the development given in association with the description of the auditory organ.

THE INTERNAL EAR, THE TYMPANUM AND AUDITORY TUBE, AND THE EXTERNAL EAR.

In the human subject, as in other mammals, the auditory organ consists of the internal ear or labyrinth, the middle ear or tympanum, with which is associated the auditory tube (O.T. Eustachian); and the external ear, which consists of the external acoustic meatus with the auricle at its lateral end.

The internal ear itself consists of two parts-the cochlea, which is the true organ of hearing, and the vestibule and the three semicircular canals connected with it, which are associated with the recognition of alterations in the position of the head, and, therefore, with the recognition and maintenance of equilibrium.

The whole of the internal ear is lined with ectodermal epithelium, the auditory epithelium, which is derived from the surface of the head of the embryo. It is recognisable in embryos of about 2.6 mm. (Fig. 67) as a thickened and slightly depressed plate of ectodermal cells which lies on the surface of the head, in the region of the hind-brain, dorsal to the second branchial cleft. As development

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