MAMMALIA. 197 



to a secondary membrane round tlie ovum, known as the cliorion. 

 Since however the allantois does not always come in contact with 

 the whole inner surface of the subzonal membrane, the term chorion 

 is apt to be somewhat vague; and in the rabbit, for instance, a con- 

 siderable part of the so-called chorion is formed by a fusion of the 

 wall of the yolk-sack with the subzonal membrane (fig. 148). The 

 placental region of the chorion may in such cases be distinguished as 

 the true chorion, from the remaining part which will be called the 

 false chorion. 



The mesoblast of the allantois, especially that part of it which 

 assists in forming the chorion, becomes highly vascular ; the blood 

 being brought to it by two allantoic arteries continued from the 

 terminal bifurcation of the dorsal aorta, and returned to the body by 

 one, or rarely two, allantoic veins, which join the vitelline veins from 

 the yolk-sack. From the outer surface of the true chorion (fig. 147, 

 5, d, 148) villi grow out and fit into crypts or depressions which have 

 in the meantime made their appearance in the walls of the uterus^ 

 The villi of the chorion are covered by an epithelium derived from the 

 subzonal membrane, 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 fietal 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 foetal villi. The above maternal and fistal structures 

 constitute together the organ known as the placenta. The maternal 

 portion consists essentially of the vascular crypts in the uterine walls, 

 and the foetal 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 



^ 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. 



