250 COMPARATIVE ZOOLOGY OF CHORDATES 



the outermost layer of extra-embryonic ectoderm, known as 

 the chorion in reptiles and birds, and the trophoblast in 

 mammals. 



The Allantoic Placenta. — The blood of the mother and that 

 of the embryo are never in direct communication. The 

 passage of foodstuffs, excretory and respiratory substances 

 must therefore take place by diffusion through the membranes. 

 The efficiency of the placenta is conditioned by the area of 

 mutual contact between the maternal and embryonic circula- 



+ U&&™ 



em. 



Fig. 117. — Section through a part of the allantoic placenta of Perameles : 

 embryonic tissue on the left, maternal on the right. (After Hill.) 



al, allantois ; eb, embryonic blood-vessels ; et)i, embryonic mesoderm ; 

 mb, maternal blood-vessels ; mc, maternal connective tissue ; ue, uterine 

 epithelium (which has become syncytial). 



tions, and by the thickness and number of the intervening 

 membranes. The area of contact can be increased by throwing 

 the surfaces of the maternal and embryonic tissues into folds ; 

 and the intervening membranes can be decreased by removal 

 or erosion of certain of the layers of the uterus. Four grades 

 of structure and corresponding efficiency can be seen in the 

 mammals, which will now be taken in order. 



(i) The embryonic and maternal surfaces are flat and un- 

 folded ; the area of contact is therefore small. However, the 



