78 SEX AND HEREDITY 



adult. As the young animal is able to absorb food from 

 the blood of the mother it is no longer necessary to have 

 a reserve supply of food-material or yolk stored up within 

 the egg : hence we find that the egg of the mammal has 

 reverted to the condition of a very minute cell, a simple 

 sphere of protoplasm containing a nucleus, measuring 

 perhaps the yj^ of an inch (Man, see Fig. 2, B) in 

 striking contrast with the relatively huge egg of the 

 Reptile or Bird. 



Again, as the egg is to develop within the body of the 

 mother, elaborate protective coverings, such as those 

 seen in the case of the Fowl's egg, are no longer necessary 

 and have disappeared. The amnion is present as before. 

 So also is the allantois, but this has undergone a great 

 increase in complexity. It comes into close contact with 

 the lining of the enlarged oviduct or uterus. Its surface 

 is covered with a thick layer of protoplasm which fits 

 itself close to the uterine surface, insinuating itself into 

 every little crevice, and finally eating its \vay into the 

 wall of the uterus and spreading along the course of the 

 blood-vessels in this wall blood-vessels belonging to 

 the mother. In the layer of protoplasm other blood- 

 vessels develop, belonging to the embryo. 



There thus come to be associated together two sets of 

 blood-vessels through one of which courses blood be- 

 longing to the mother and through the other blood 

 belonging to the embryo. For a time these are separated 

 by a considerable thickness of the protoplasm covering 

 the allantois, but this gradually disappears and there is 

 nothing left between the two blood-streams but an 

 extremely thin, though unbroken, membrane walling in 

 the vessels of the embryo. Through this thin membrane 

 there diffuse into the blood of the embryo from that of 



