THE EMBRYONIC MEMBRANES 



251 



trophoblast (embryonic outermost layer) disappears, and the 

 uterine epithelium (innermost maternal layer) becomes 

 syncytial and contains blood-vessels. Substances therefore 

 have to pass through the wall of the maternal capillaries, 

 through the uterine epithelium, across the intervening space 

 and through the wall of the embryonic capillaries. This type 

 of placenta occurs in Perameles, the only Marsupial to possess 

 an allantoic placenta at all. It was probably present in the 

 ancestors of the Marsupials, and has been lost in the other 

 living Marsupials. 



Fig. 118. — Section through a part of the allantoic placenta of the cow; 

 embryonic tissue to the left, maternal to the right. 

 The trophoblast (t) is produced into villi (v), which fit loosely into 

 crypts (c) in the uterine wall, the epithelium of which (ue) persists ; al, 

 allantois ; eb, embryonic blood-vessel ; ec, embryonic connective tissue ; 

 mb, maternal blood-vessel ; mc, maternal connective tissue ; ug, glands in 

 the wall of the uterus. 



(ii) The embryonic and maternal tissues are thrown into 

 folds ; embryonic " fingers " or villi fitting into corresponding 

 crypts in the uterine wall. In the pig, villi are distributed all 

 over the trophoblast, but in the cow they are grouped together 

 in clumps forming cotyledons. The uterine epithelium per- 

 sists. Substances therefore must diffuse through the wall of 

 the maternal capillaries, connective tissue (uterine epithelium), 

 trophoblast, and the wall of the embryonic capillaries. 



(iii) The epithelium of the uterus disappears, and the 



