xm PHYLUM CHORD AT A 501 



of the blastodermic vesicle, for which it forms a complete invest- 

 ment ((7). In Mus, Arvicola, and others (Fig. 1152,^4), the amnion 

 is developed from a series of folds of the ectoderm which arise 

 beneath the trophoblast. In 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 view that the mode of development of the amnion ex- 

 emplified in the Hedgehog is the most primitive known, is 

 sustained by the consideration that since the function of the 



t/ 



amnion with its enclosed fluid is without doubt to protect 

 the delicate embryo from shocks by acting as a sort of water- 

 cushion, it would be entirely useless until the folds had all 

 coalesced, and the view that it arose phylogenetically by the 

 formation of folds, which only after a considerable time coalesced, 

 is rendered improbable. 



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



of development as in the Bird, arising as a hollow outgrowth 



from the hinder part of the alimentary canal ; this, growing out 



into the space between the serous membrane and the amnion, 



becomes in all the Eutheria applied to the former, and unites with 



it to form the chorion. Sometimes, as in the Eabbit (Fig. 1150), 



the union between the allantois (TA) and the serous membrane is 



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



but in most instances the allantois spreads over the entire inner 



isurface of the serous membrane, and becomes united with the 



latter throughout its entire extent. Yilli, 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 becomes profoundly modified. 



The villi become branched and enter into intimate union with 



the uterine mucous membrane, so that a close connection becomes 



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 



the embryo are termed the foetal placenta, those developed from the 



wail of the uterus the maternal placenta. In some Mammals the 



union between the two is not very close, so that at birth the villi of 



t/ 



the chorion are simply withdrawn from the crypts of the uterine 

 mucous membrane, no part of the latter being thrown off; such a 

 placenta is said to be non-deciduate. In other Mammals the union 

 is closer, and at birth a part of the hypertrophied mucous mem- 

 brane is thrown off in the form of a dccidua ; such a placenta is 

 termed deciduate. In the Mole and the Bandicoot not only is 

 there no decidua thrown off, but the foetal placenta with the 

 distal portion of the allantois does not pass out after the foetus, 

 yoL. ii oo 



