238 EMBRYOLOGY. 



and Amphibia, either possess no accessory organs at all, or only 

 an evagination of the intestinal tube, the yolk-sac. The embryos 

 of Reptiles, Birds, and Mammals, on the contrary, are further 

 enclosed in two fugitive membranes characteristic of embryonic 

 life, the amnion and serosa. They have therefore been grouped 

 together as amniotic animals or Amniota, and the classes first 

 mentioned have been contrasted with them as non-amniotic animals 

 or A namnia. 



Among the amniotic animals a further separation into two groups 

 can be made : on the one side are the egg-laying Reptiles and Birds, 

 which HUXLEY unites into the Sauropsida ; on the other side 

 Mammals, in which (with the exception of the Monotremes) the 

 eggs develop in the uterus, and the young are further nourished 

 after birth by the secretions of milk-glands. 



In the Mammalia the foetal membranes, inasmuch as they unite 

 with the mucous membrane of the uterus to form an organ of nutrition, 

 take on a still more complicated character, and present modifications 

 which in turn can readily be utilised for systematic purposes. 



In Monotremes and Marsupials the outer embryonic membrane 

 retains an almost smooth surface, as in Reptiles and Birds ; in all 

 other Mammals there arise on the surface of the chorion villi, which 

 grow into the maternal mucous membrane. OWEN has designated 

 the one as Implacentalia, the other as Placentalia. The terms 

 Achoria and Choriata introduced for these by KOLLIKER are better. 



In the Choriata the union of the villi witn the mucous membrane 

 is either loose or firm ; corresponding to this there is either no 

 detachable layer of the mucous membrane of the uterus formed, 

 no decidua, or such a structure arises as the result of close inter- 

 growth of the placenta uterina and placenta foetalis. Thus we have 

 the Mammalia indeciduata and the Mammalia deciduata. In each 

 division there are again two sub-types in the formation of villi. In 

 the Indeciduata the villi are either evenly distributed over the 

 surface, or they are united into more or less numerous groups 

 (placentae or cotyledons), which are separated from one another by 

 smooth tracts of the chorion. In a part of the Deciduata the 

 placenta is girdle-shaped, in another part disc-shaped. 



SUMMARY. 



1. In the Mammalia there is developed, in the same way as in 

 Reptiles and Birds, a yolk-sac, an amnion, a serosa, and an allantois. 



2. Excepting in the Monotremes and Marsupials, the serosa is 

 metamorphosed into a chorion, in that it puts forth villi, and in that 



