THE FCETAL MEMBRANES OF MAMMALS 239 



the connective-tissue layer of the allantois, which is provided with 

 the umbilical blood-vessels, spreads out on its inner surface and 

 penetrates into the villi. 



3. In a part of the Mammalia certain regions of the serous 

 membrane, where the villi grow more vigorously and put forth 

 lateral branches, and sink into corresponding depressions of the 

 mucous membrane of the uterus, are converted into a placenta (when 

 many of them have arisen on one chorion they are called cotyledons).. 



4. On the placenta one distinguishes : 



(a) A placenta foetalis, i.e.. that part of the chorion which has: 



developed the tufts of villi. 



(b) A placenta uterina, i.e., that part of the mucous membrane 



of the uterus which has proliferated and is provided with, 

 depressions for the reception of the placenta foetalis. 



5. Foetal and maternal parts of the placenta can become more 

 firmly united with each other ; the result is that at birth a larger 

 or smaller tract of the mucous membrane of the uterus is also cast 

 off, and is known as the caducous membrane, or the decidua. 



6. According to the character of the embryonic membranes, the- 

 following divisions of Vertebrates may be established : 



I. Anamnia, animals without an amnion. 



(Amphioxus, Cyclostomes, Fishes, Amphibia.) 

 II. Amniota, animals with an amnion (with yolk-sac, amnion,. 

 serosa, and allantois). 



A. Sauropsida. Egg-laying, amniotic animals. 

 (Reptiles and Birds.) 



B. Mammalia. In all of them, except the Monotremes, the- 



eggs are developed in the uterus. 



(a) Achoria. The serosa develops no villi, or only a few. 



(Monotremes, Marsupials.) 



(b) Choriata. The serosa becomes the villous membrane* 



(chorion). 

 '(1) With evenly distributed villi. 



(Perissodactyla, Suidse, Hippopotamidse, Tylopoda,. 

 MammahaJ Tragu i icUe , Cetacea, etc.) 



n ~ (2) Placentalia. The serosa is at intervals metamor- 

 phosed into a placenta. 

 a. Numerous cotyledons. (Ruminantia.) 

 C ft. Placenta zonaria. (Carnivora.) 



Mammalia I Placent a discoidea. ([Man,] Apes,. Rodents, Ih- 



deciduata. I ,. -D 4. \ 



sectivores, Bats.) 



