238 FCETAL MEMBRANES. 



appearance in the walls of the uterus 1 . The villi of the chorion 

 are covered by an epithelium derived from the subzonal mem- 

 brane, and are provided with a connective tissue core containing 

 an artery and vein and a capillary plexus connecting them. In 

 most cases they assume a more or less arborescent form, and 

 have a distribution on the surface of the chorion varying 

 characteristically in different species. The walls of the crypts 

 into which the villi are fitted also become highly vascular, 

 and a nutritive fluid passes from the maternal vessels of the 

 placenta to the fcetal vessels by a process of diffusion ; while 

 there is probably also a secretion by the epithelial lining of the 

 walls of the crypts, which becomes absorbed by the vessels of 

 the fcetal villi. The above maternal and fcetal structures con- 

 stitute together the organ known as the placenta. The mater- 

 nal portion consists essentially of the vascular crypts in the 

 uterine walls, and the fcetal portion of more or less arborescent 

 villi of the true chorion fitting into these crypts. 



While the placenta is being developed, the folding-off of the 

 embryo from the yolk-sack becomes more complete ; and the 

 yolk-sack remains connected with the ileal region of the 

 intestine by a narrow stalk, the vitelline duct (fig. 147, 4 and 5 

 and fig. 147*), consisting of the same tissues as the yolk-sack, 

 viz. hypoblast and splanchnic mesoblast. While the true 

 splanchnic stalk of the yolk-sack is becoming narrow, a somatic 

 stalk connecting the amnion with the walls of the embryo is also 

 formed, and closely envelops the stalk both of the allantois 

 and the yolk-sack. The somatic stalk together with its contents 

 is known as the umbilical cord. The mesoblast of the 

 somatopleuric layer of the cord develops into a kind of gela- 

 tinous tissue, which cements together the whole of the contents. 

 The allantoic arteries in the cord wind in a spiral manner round 

 the allantoic vein. The yolk-sack in many cases atrophies 

 completely before the close of intra-uterine life, but in other 

 cases it is only removed with the other embryonic membranes 

 at birth. The intra-embryonic portion of the allantoic stalk 

 gives rise to two structures, viz. to (i) the urinary bladder 



1 These crypts have no connection with the openings of glands in the walls of the 

 uterus. They are believed by Ercolani to be formed to a large extent by a regene- 

 ration of the lining tissue of the uterine walls. 



