44 Dynamic Tfieory. 



u sausage-shaped" bladder. In all the higher mammal embryos those 

 above the marsupials the allantois grows in length until it comes into 

 contact with the walls of the uterus of the mother, with which it forms 

 a partial union, and then it supersedes the }'elk sac and becomes the 

 vehicle of nourishment from the mother to the growing embryo. This 

 process has been described. The attachment of the allantois to the 

 uterus in man, is over a discoidal surface some C or 8 inches in diam- 

 eter, and the membranes involved in the connection, some of which are 

 furnished by the emb^o and some by the' mother, become the organ 

 called the placenta. 



While man is related to all the mammals above the marsupials, in the 

 general fact that the} r all have a placenta, he is more nearly related to 

 some than others, as shown by peculiarities regarding the placenta. 

 Thus whales, sea cows, horses, cattle, &c. , have what is called the in- 

 deciduate placenta. That is, only those membranes of the placenta 

 which were contributed by growth from the embryo, are discharged 

 from the uterus at the time of birth, while the mother retains the part 

 of the placenta contributed by her. But the placenta of man, of the 

 elephants, the beasts of prey, the rodents, the edentates, the bats and 

 apes, is a deciduate placenta, so called because the mother gives up her 

 own share in the placenta when it is delivered after the birth. 



There is still another sub-classification in the shape, of the placenta. 

 In man, apes and rodents, the placenta is circular or cake-shaped, as 

 stated heretofore, while in the elephants and in the carnivorous mam- 

 mals of land and sea, the placenta is girdle-shaped, and is, in fact, a 

 broad belt or ring of the chorion of the embryo, attached to the decidu- 

 ate membrane of the uterus of the mother. 



In the indeciduate placenta, the chorion tufts of the embryonic mem- 

 branes, enter the soft vascular mass of the maternal uterus, sheaths of 

 which form around the chorion tufts. When the birth takes place the 

 tufts of the chorion pull out of their sheaths, carrying away none of the 

 maternal uterus, as in the case of the deciduates. 



In the case of the indeciduates, the attachments to the uterus are 

 made upon all sides of the embryo, and the placenta is therefore in the 

 shape of a hollow sphere. In the indeciduate girdle placenta, the at- 

 tachment is over an equatorial belt of the spherical egg, the head and 

 tail, or polar attachments of the sphere, being abolished. 



In the discoidal form, the belt is reduced to a circular plate or disc. 

 These progressive relationships between the different kinds of placenta, 

 show the unity of the different animal races to which they belong, par- 

 ticularly, when it is considered that in the development of the chorion 

 in all cases it is first* covered with the tufts upon all sides in man and 

 oes as well as cattle and horses. But the man and the ape make no 



