i in: idi\i. MKMBBUntt Of MAMMALS 239 



the com:ecti\e tissue layer of the allantois, which i- |. !. i.l.-.l with 

 the umbilical blood-ve Bels, IpTtftdfl out on its inner surface and 

 j . -net r.-itt-s into tin- villi. 



3. In a part of the Mammalia certain i of I lie serous 

 membrane, where the villi ^row more ly and put forth 

 lateral branches, and sink into corresponding depre : the 

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

 many of them have arisen ononechorion they are called cot yle.i 



4. On the placenta one distinguishes: 



(a) A placenta footalis, i.e., that jurt 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 plact-nta tVtalis. 



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

 lii inly 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 embiyonic 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. ^fammal^a. 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). 

 f (l) With evenly distributed villi. 



(Perissodactyla, Suida?, Hippopotamidse, Tylopoda, 

 Mammalia *_ TJ ^ L \ 



Iragulida?, Cetacea, etc.) 



(2) Placentalia, The >--ro>a is at intervals metamor 

 deciduata. 



phosed into a placenta. 



a. Numerous cotyledons. (Ruminantia.) 



, f 8. Placenta zonaria. (Carnivora.) 

 Mammalia 

 , . , 4 y. Jrlacenta discoidea. ([Man,J Apes, Rodents, In- 



sectivores, B,r 



