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growth of fætal vessels into the fetal 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 numerAnterior end of neural fold Plasmodial trophoblast

ous branches into the blood Cellular trophoblast Amnion cavity

[blast spaces, and thus increase greatly - Mesoderm lining of tropho- in complexity (Figs. 75, 76, 77).

Mesoderm of amnion As development progresses - Ectoderm of amnion still further a part of the chorion

Allantoic diverticulum of entoderm vesicle

is converted into the foetal Body stalk mesoderm portion of an organ called the Extra-embryonic cælom placenta, and thus the chorion Entodern

is divided into placental and Mesoderm covering of

non-placental regions. Upon entoderm vesicle the placental part the villi con

tinue to increase, but they disNeurenteric canal

appear entirely from the non

placental part, which is then Cavity of entodermal vesicle

called the chorion læve (Fig. Fig. 70.—SCHEMA OF SAGITTAL. SECTION OF ZYGOTE ALONG LINE A. 77). Plasmodial trophoblast Neural groove

The Amnion, the BodyChorion

Cellular trophoblast | Mesoderm lining of trophoblast

Stalk (Allantoic Stalk), and

the Umbilical Cord. - The Amnion cavity Extra-embryonic celom

amnion is formed from that Mesoderm of amnion

portion of the wall of the larger Ectoderm of amnion

of the two inner vesicles of the Mesoderm covering

zygote, the ecto-mesodermal entoderm

vesicle (p. 22), which does Entoderm

not take part in the formation Cavity of entodermal

of the embryo. It consists of vesicle

ectoderm cells covered. externally by a layer of extra-em

bryonic mesoderm, and it is Notochord

continuous with the margin of the embryonic area (Figs.

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

The cavity of the ecto

mesodermal vesicle, enclosed between the amnion and the embryonic area, is the cavity of the amnion; it 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 ampion becomes

Plasmodial

trophoblast Plasmodial.

Plasmodial trophoblast

Cellular

trophoblast trophoblast Plasmodial

trophoblast trophoblast

Efferent vessel Afferent vessel of villus Cellular trophoblastMesoderm

Fused mesoderm Ectoderm of chorion

Fused mesoderm of of amnion and amnion

Famnion and chorion Ectoderm

Ectoderm of amnion

[graphic]
[graphic]
[graphic]

Cellular

of villus

of amnion

Fig. 72.-SCHEMA OF THREE STAGES IN THE FORMATION OF A CHORIONIC Villus.

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

Afferent vessel of villus

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

Plasmodial trophoblast and farther away from the embryo, the vitello-intestinal duct is elongated, and

Afferent vessel

Cellular of villus

trophoblast 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).

of villus 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. the amniotic mesoderm fuses with the

A loop of 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).

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 celom 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 fatal 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 cælom—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

[graphic]

Upon

growth of foetal vessels into the fatal 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 numerAnterior end of neural fold Plasmodial trophoblast

ous branches into the blood Cellular trophoblast Amnion cavity

(blast spaces, and thus increase greatly Mesoderm lining of tropho- in complexity (Figs. 75, 76, 77). Mesoderm of amnion

As development progresses
Ectoderm of amnion still further a part of the chorion
Allantoic diverticulum
of entoderm vesicle

is converted into the fetal
Body stalk mesoderm portion of an organ called the
Extra-embryonic cælom placenta, and thus the chorion
Entoderin

is divided into placental and

non-placental regions. Mesoderm covering of entoderm vesicle the placental part the villi con

tinue to increase, but they disNeurenteric canal

appear entirely from the non

placental part, which is then Cavity of entodermal vesicle

called the chorion læve (Fig. Fig. 70.—SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 77). Plasmodial trophoblast Neural groove

The Amnion, the BodyChorion

Cellular trophoblast
Mesoderm lining of trophoblast

Stalk (Allantoic Stalk), and

the Umbilical Cord. - The Amnion cavity Extra-embryonic cælom

amnion is formed from that Mesoderm of amnion

portion of the wall of the larger Ectoderm of amnion

of the two inner vesicles of the Mesoderm covering

zygote, the ecto-mesodermal entoderm

vesicle (p. 22), which does Entoderm

not take part in the formation Cavity of entodermal

of the embryo. It consists of vesicle

ectoderm cells covered. externally by a layer of extra-em

bryonic mesoderm, and it is Notochord

continuous with the margin of the embryonic area (Figs.

70, 71). Fig. 71.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG

The cavity of the ectoLINE B (Fig. 31).

mesodermal vesicle, enclosed between the amnion and the embryonic area, is the cavity of the amnion; it 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

Plasmodial

trophoblast Plasmodial.

Plasmodial trophoblast

Cellular

trophoblast trophoblast

Cellular Plasmodial

trophoblast trophoblast

Efferent vessel

Afferent vessel of villus Cellular trophoblastMesoderm

Fused mesoderm Ectoderm of chorion

Fused mesoderm of of amnion and amnion

Faninion and chorion
Ectoderm of amnion

[graphic]
[graphic]

of villus

Ectoderm
of amnion

Fig. 72.-SCHEMA OF THREE STAGES IN THE FORMATION OF A CHORIONIC VIllus.

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

Afferent vessel of villus

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 cælom, 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

Plasmodial trophoblast and farther away from the embryo, the vitello-intestinal duct is elongated, and

Afferent vesse)

Cellular of villus

trophoblast it is surrounded by a hollow tube. The cavity of the tube is an elongated part of the extra-embryonic cælom, and its walls are formed by the amnion (Figs. 57,62,63).

The caudal wall of the tube necessarily consists of the elongated body-stalk

Efferent vessel (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).

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 cælom 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 fatal 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 coelom—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.

[graphic]

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 fætus 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 fætus 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

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