growth of foetal vessels into the foetal mesodermal cores. therefore, consist of a mesodermal core covered by a layer of cellular trophoblast The secondary villi, 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. Cavity of entodermal vesicle FIG. 70.-SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 77). Chorion! Plasmodial trophoblast Mesoderm lining of trophoblast, Extra-embryonic cœlom Mesoderm of amnion Ectoderm of amnion Mesoderm covering Cavity of entodermal FIG. 71.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG between the amnion and the embryonic area, is 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 ectothe cavity of the amnion; it mesodermal vesicle, enclosed 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 Cellular trophoblast Plasmodial trophoblast Plasmodial trophoblast Mesoderm Fused mesoderm Cellular trophoblast Ectoderm of amnion of chorion and amnion of amnion Fused mesoderm of amnion and chorion 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 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). 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 Afferent vessel of villus Plasmodial trophoblast Cellular trophoblast Afferent vessel of villus Mesoderm of villus Efferent vessel of villus 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 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 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 Anterior end of neural fold Plasmodial trophoblast Mesoderm lining of tropho Mesoderm of amnion - Ectoderm of amnion 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 fœtal 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. Cavity of entodermal vesicle FIG. 70.-SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 77). Chorion Plasmodial trophoblast Neural groove {Mesoderm in Cellular trophobla Amnion cavity Extra-embryonic cœlom Mesoderm of amnion 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 Plasmodial trophoblast Plasmodial trophoblast Cellular trophoblast Mesoderm Ectoderm Plasmodial trophoblast Cellular trophoblast Fused mesoderm of amnion of chorion and amnion of amnion Fused mesoderm of Famnion and chorion 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 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). The caudal wall of the tube necessarily consists of the elongated body-stalk (allantoic stalk). Plasmodial trophoblast Cellular trophoblast Afferent vessel of villus 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 afferent vessel has been cut at two points. the amniotic mesoderm fuses with the 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 colom 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 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. 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. is 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 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 |