THE PLACENTA. 157 



grows in Placental Animals, and the longer it remains in 

 the maternal uterus, the more necessary does it become that 

 special structural arrangements should meet the increased 

 consumption of food. In this point there is a very striking 

 difference between the lower and the higher Mammals. In 

 Cloacal Animals and Pouched Animals, in which the embryo 

 remains for a comparatively brief time in the uterus, and is 

 born in a very immature condition, the circulation as it exists 

 in the yelk-sac and in the allantois suffices for nutrition, as 

 in birds and reptiles. In Placental Animals, on the contrary, 

 in which gestation is very protracted, and the embryo 

 remains much longer in the uterus, there attaining its full 

 development within its investing membranes, a new ap- 

 paratus is required to convey a direct supply of richer 

 nutritive matter; and this is admirably effected by the 

 development of the placenta. 



In order rightly to understand and appreciate the for- 

 mation of this placenta and its important modifications in 

 different Placental Animals, we must once more glance at the 

 external coverings of the mammalian egg. The outermost of 

 these was originally, and during the cleavage of the egg 

 and the first formation of the axial portion of the germ, 

 formed by the so-called zona pellucida, and by the thick 

 albuminous covering deposited externally on the latter 

 (Fig. 19, Fig. 21, z, h, vol. i. p. 178). 



We called these two outer coverings, which afterwards 

 amalgamate, the prochorion. This prochorion very soon 

 disappears (in man perhaps in the second week of develop- 

 ment), and is replaced by the permanent outer egg-mem- 

 brane, the chorion. The latter, however, is simply the 

 serous membrane, which, as we have already seen, is the 



