594 ZOOLOGY sect. 



the amnion is developed from a series of folds of the ectoderm 

 which arise beneath the trophoblast. Tn other Mammals {B) the 

 amnion arises in the manner already described, and the portion 

 of the trophoblast immediately overlying the embryonic part of 

 the ectoderm eventually disappears. 



The allantois has, in all essential respects, the same mode 

 of development as in the Bird, arising in most cases as a hollow 

 outgrowth from the hinder part of the alimentary canal ; this, 

 growing out into the space (extra-embryonal ccelome) between 

 the chorion and the amnion, becomes in all the Eutheria applied 

 to the former, and unites with it to contribute towards the 

 formation of the j^li^centa. But in some cases the allantois does 

 not at first contain a cavity, and in some (Primates) the 



Fig. 1221.— Diagram illustrating the mode of formation of the amnion in various Mammals. 

 A, commencing formation of the amnion in Mus, Arvicola, etc. The asterisk marks what 

 corresponds to the portion of the trophoblast overlying the embryo in Fig. 1221, C ; £, mode of 

 formation of the amnion in many Mammals. The portion of the trophoblast indicated by 

 the asterisk in A disappears before the amniotic folds make their appearance. (After 

 Hnbrecht.) 



severance between the amnion and the chorion is not com- 

 pleted, and the allantois arises from the outset in continuity 

 with the latter. Sometimes, as in the Rabbit (Fig. 1219), the 

 union between the allantois (TA) and the chorion is limited 

 to a comparatively small part of the extent of the latter, but 

 in most instances the allantois spreads over the entire inner 

 surface of the chorion, and becomes united with it throughout 

 its entire extent (Fig. 1218). Villi, into which mesoderm 

 with blood-vessels penetrates, grow out from the surface of the 

 chorion and are received into depressions or crypts in the mucous 

 membrane of the uterus, which undergoes profound modification. 

 The villi become branched and enter into intimate union with 

 the uterine mucous membrane, so that a close connection is 

 established between the vascular system of the foetus and that of 

 the parent. 



The term placenta is applied to the entire structure by means 

 of which this connection is brought about ; the parts derived from 



