THE MEMBKANES AND APPENDAGES. 



55 



Afferent vessel 

 of vil 



Plasmodial trophoblast 



Cellular 

 trophoblast 

 Afferent vessel 

 of villas 



Mesoderm 

 .. of villus 



Efferent 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 blood- 

 vessels 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 pro- 

 nounced, 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 neces- 

 sarily consists of the elongated body-stalk 

 (allantoic stalk). 



As the distension of the amnion still 

 continues, the walls of the tube are forced 



nrrflinQt tViP virplln inrpqfinfll rlnor anrl FlG - 73. SCHEMA OF A TRANSVERSE SECTION OF A 



SECONDARY CHORIOKIC VILLUS. A loop of the 

 the amniOtlC 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 coelom 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 extra- 

 embryonic 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 



