The Placenta 363 



the mode of the separation or dehiscence of the fetal membranes, 

 we divide mammalia into two groups, known as the deciduata, 

 in which the maternal mucosa is torn away with the fetal mem- 

 branes, and the indeciduata, in which the separation between the 

 fetal membranes and uterus occurs between the villi of the 

 chorion and the maternal vessels. 



By some authors, the term placenta is limited to those struc- 

 tures in which the villi of the chorion are concentrated at cer- 

 tain points upon its surface. These animals are designated as 

 placental. In other animals, the viUi are scattered over the en- 

 tire surface of the chorion and these are designated as diffuse 

 placental or aplacental. 



Among our domesticated animals, in the indeciduate group, 

 we have, of the aplacental or diffuse placental type, the horse 

 (Fig. 75) swine and camel, and of the placental group we have 

 the ruminants, except the camel. 



In the deciduous group are the elephant, carnivora, rodentia 

 and monkeys. 



In man and the anthropoid apes there is a very complex de- 

 ciduous placenta, of a discoid type, with a decidua reflexa. 



Our most common and useful animals belong to the indeciduate 

 group, in which are found the ruminants and equidae, while in 

 the deciduate group we have only the dog, cat and rabbit. 



The form and extent of the placenta in the different species of 

 animals varies greatly. In equidse and swine the placentae or 

 chorionic villae are diffused over the entire surface of the chorion, 

 as small elongated tufts, which we know as diffuse placentae. 

 In ruminants the villi are restricted to comparatively small areas 

 of about 80 to 100 or more in number, which are known as 

 placentules or cotyledons. Between these cotyledons the chorion 

 is free from the uterus. In the non-gravid uterus of the 

 ruminant, there exist numerous elevations upon the surface of the 

 mucosa, which constitute the rudimentary cotyledons. When 

 impregnation takes place the cotyledons become highly vascular 

 and their mucous membrane undergoes marked development, 

 while, from the chorion, there grow out, at points corresponding 

 to these eminences, numerous branched villi which ultimately 

 sink deeply into the mucosa of the cotyledons or placentules. 

 This condition gives rise to what we know as multiple placenta. 

 (See Fig. 79-) 



