THE RISE OF THE MAMMALIA 117 



conveying the nutriment from the placenta to the 

 foetus, the yolk-sac ceases to function and dwindles, 

 the foetus lying in the amniotic envelope becomes 

 completely folded off from the yolk-sac save at the 

 umbilicus, and it is attached to the uterus of the 

 mother by the allantoic stalk. 



It was for a long time a matter of dispute as to 

 whether the blood of the mother actually mingled 

 with that of the foetus in the placenta. In some 

 forms of Mammals where the fusion of embryonic 

 trophoblast and maternal tissue is very intimate 

 there can be no doubt that the maternal capillaries 

 pour their blood into spaces in the placenta which 

 actually communicate with the capillaries of the 

 foetus. In other cases where there is no absolute 

 fusion of the maternal and foetal tissues, but only 

 a close adherence or interdigitation, it seems equally 

 certain that all the interchange of fluid material is by 

 diffusion from the maternal to the foetal circulation, 

 not by commingling. This is, however, a point of 

 not very great importance. Certain it is that the 

 vascular connection between parent and foetus during 

 gestation is a very close one, and that various 

 bacterial diseases and physical states can be trans- 

 mitted from parent to offspring during this period 

 so as to simulate the appearance of the inheritance 

 of acquired characteristics. 



During gestation the placenta stores quantities 



