I08 HOW ANIMALS DEVELOP 



and more folded so as to increase the area over which 

 the absorption of food can take place. In man the 

 placental wall forms thin branching fingers, con- 

 taining blood-vessels, which dip down into pools of 

 maternal blood left by the breakdown of the uterine 

 blood-vessels. 



Most of the embryo's food passes into its blood 

 stream through the placenta, but the process is not 

 really a simple filtration. The wall of the placenta, 

 or trophoblast, breaks down the proteins offered to 

 it by the maternal blood and passes them on to the 

 embryo in a digested form ready to be built up 

 again into the particular substances which the 

 embryo requires for its growing organs. At the same 

 time the placenta passes back to the mother the 

 waste products of which the embryo has to rid itself; 

 in the more elaborate types of placenta this back- 

 ward passage is so efficient that the embryo never 

 develops a functional embryonic kidney, which other 

 embryos have evolved to deal with their excretion 

 before the final adult kidney is ready. 



Not quite all the embryo's food comes from the 

 mother's blood. Some is provided by the so-called 

 uterine milk, which is secreted by glands in the 

 uterus, chiefly in animals with simple placentas. 

 In the more complicated types, the embryo derives 

 a certain amount of nourishment from the maternal 

 tissues — the wall of the uterus, the connective 

 tissue and the lining of the blood-vessels — ^which it 

 digests and absorbs. This process goes on particularly 

 rapidly in early stages of development. In man, 



