156 THE EVOLUTION OF MAN. 



tive urine must be chiefly the product of the primitive 

 kidneys. The intestinal fibrous layer of the allantois is 

 traversed by large blood-vessels which accomplish the nutri- 

 ment and, especially, the respiration of the embryo ; these 

 are the navel-vessels, or umbilical vessels (vol. i. p. 400). In 

 all ReptUes and Birds the allantois becomes an immense 

 sac, which encloses the embryo with the amnion, and which 

 does not coalesce with the outer covering of the egg 

 (chorion). InCloacal Animals {Monotremata) and Pouched 

 Animals {Marsupialia) the allantois is also of this nature. 

 It is only in Placental Animals that the allantois develops 

 into that very peculiar and remarkable formation, called 

 the placenta, or "vascular cake." The nature of the placenta 

 is this : the branches of the blood-vessels which traverse the 

 wall of the allantois, penetrate into the hollow tufts of the 

 chorion, which are inserted into corresponding depressions 

 in the mucous membrane of the maternal uterus. As this 

 mucous membrane is also abundantly supplied with blood- 

 vessels, which conduct the mother's blood into the uterus, 

 and as the partition between these maternal blood-vessels 

 and the embryonic vessels in the chorion-tufts soon becomes 

 extremely thin, a direct exchange of substance is soon de- 

 veloped between the two sets of blood-vessels, which is of 

 the utmost importance for the nutrition of the younc 

 Mammal. The maternal blood-vessels do not, however, 

 pass directly (anastomosis) into the blood-vessels of the 

 embryonic chorion-tufts, so that the two kinds of blood 

 simply mix, but the partition between the two sets of 

 vessels becomes so thin, that it permits the passage of the 

 most important food-materials, freed from unnecessary 

 matter (transudation, or diosmosis). The larger the embryo 



