l6o THE EVOLUTION OF MULN. 



To tlie Iiidecidua, or lower Placental Animals, belong 

 two very comprehensive and important vertebrate groups : 

 (1) the Hoofed Animals ( Ungulata) — the Tapirs, Ilorses, 

 Swine, Ruminants, and others ; (2) the Whale-like animals 

 (Cetomorpha) — the Sea-cows, Porpoises, Dolphins, Whales, 

 and others. In all these Tndecidua the chorion tufts are 

 distributed, singly or in bunches, over the entire surface of 

 the chorion, or over the greater part of it. They are but very 

 loosely attached to the mucous membrane of the uterus, so 

 that the entire outer egg-membrane with its tufts might 

 easily and without using force be dt*awn out of the depressions 

 in the uterine mucous membrane, just as the hand is with- 

 drawn from a glove. The two " vascular cakes " do not 

 really coalesce at any point of their contact. Hence, at 

 birth the "embryonic cake" (placenta foetalis) is alone 

 removed ; the " maternal cake " (placenta uterina) is not 

 displaced. The entire mucous membrane of the gravid 

 uterus is but little altered, and, at parturition, suffers no 

 direct loss of substance. 



The structure of the placenta in the second and higher 

 division of Placental Animals, the Deciduata, is very dif 

 ferent. To this comprehensive and very highly developed 

 mammalian group belong all Beasts of Pre}^ and all Insect- 

 eaters, Gnawers (Rodentia), Elephants, Bats, Semi-apes, and, 

 lastly, Apes and Man. In all these Deciduata the whole 

 surface of the chorion is also at first thickly covered with 

 tufts. These, however, afterwards disappear from part of 

 the surface, while they develop all the more vigorously in 

 the remainder. The smooth chorion (chorion leave, Fig. 198, 

 ohl) thus becomes distinct from the tufted chorion (chorion 

 (rondosum. Fig. 198, chf). On the former there are only 



