118 PRIMITIVE ANIMALS 



of carbohydrate and fatty material, which is con- 

 veyed to the foetus in the blood as occasion requires ; 

 proteid and oxygen are supplied by the maternal 

 blood, while excretory matter is also removed by 

 the maternal circulation. 



There are great variations in the form of the 

 placenta in the different groups of placental Mam- 

 mals. Various classifications have been attempted, 

 but the subject is a complicated one and by 

 no means settled. In the Indeciduate Mammals, 

 e.g. Ungulates and Lemurs, the maternal and em- 

 bryonic tissues do not come into inseparable con- 

 nection, so that at birth the foetal part of the 

 placenta formed from the trophoblast comes away 

 from the maternal portion, which is left behind and 

 subsequently absorbed in situ. In the Deciduata, 

 including the great majority of Mammals, e.g. Car- 

 nivora, Rodentia, Insectivora, Primates, the fusion of 

 maternal and embryonic tissue in the placenta is com- 

 plete, so that at birth the whole placenta is shed as 

 the " after-birth," and no separation of maternal and 

 embryonic placental tissue is possible. Besides the 

 differences of the Deciduata and Indeciduata, there 

 are also differences in the way in which the tropho- 

 blast of the embryo is attached to the uterine tissue, 

 resulting in the formation of different kinds of 

 placenta, e.g. diffuse or zonary. In the Primates 

 the mode of placentation is peculiar, because the 



