828 



PHYLUM CHORDATA : CLASS MAMMALIA 



The formation of the allantoic placenta in Perameles is in essentials 

 the same as in Eutherian Mammals, but in details there are some 

 striking differences. The most noteworthy of these is, perhaps, that the 

 cells of the uterine epithelium, instead of disappearing at an early stage, 

 as in Eutherian ]Mammals, proliferate greatly, lose their cell outlines, 

 and by the increase of the nuclei form what is known as a syncytium. 

 Later this syncytial layer becomes highly vascular, and forms the 

 maternal portion of the placenta, whereas, as already seen, in Eutheria 

 it is the uterine mucosa which forms the maternal part of the placenta. 

 Into the vascular syncytium the allantoic capillaries grow down, until 



Fig. 498. — Diagram of foetal membranes in rabbit. — In part 



after Bonnet. 



UT., Wall of uterus ; PL., embryonic part of placenta in 

 close union with the wall of the uterus ; AL., allantois, 

 line! with endoderm, growing out from the end of the 

 embryonic gut ; AM., amnion ; A.C., amniotic cavity ; 

 Y.S., yolk-sac growing out from the embryonic gut, and 

 uniting with the subzonal membrane to form a tem- 

 porary yolk-sac placenta. 



ultimately maternal and foetal vessels are separated merely by their 

 endothelial walls and a mere trace of syncytial protoplasm. The 

 connection between the yolk-sac wall and the uterus is effected in a 

 similar manner. 



General Life of Mammals 



Most Mammals live on dry land. The bats, however, 

 have the power of flight, and various forms are able to 

 take long swooping leaps from tree to tree. Thus there are 

 *' flying phalangers," such as Petaurus, among Marsupials ; 



