378 THE PHYSIOLOGY OF REPRODUCTION 



absorbs nutritive material from the mucosa and from the 

 maternal blood. The nutriment serves in part for the nutrition 

 of the trophoblast itself, and in part for the growth and de- 

 velopment of the embryo. In the earhest stages there are as 

 yet no embryonic vessels, and the nutriment is transmitted 

 from cell to cell. But as the embryo increases in size and its 

 requirements grow in proportion, such a path becomes in- 

 adequate, and a vascular channel is developed in connection 

 with the two foetal membranes — the yolk-sac or umbilical vesicle, 

 and the allantois. 



The mammalian yolk-sac has only a secondary importance 

 for the nutrition of the embryo. The blastodermic vesicle 

 at an early stage of development is divided into an embryonic 

 and a non-embryonic area. The latter is the yolk-sac which 

 gradually becomes folded ofi from the embryo. Its relations 

 are the same as those of the yolk-sac in Sauropsida, but the 

 contents are an albuminous fluid instead of yolk. It is com- 

 monly believed that the placental Mammals are descended from 

 ancestors in which the ovum had a large supply of yolk, but 

 that, when the fertihsed ovum found a new supply of food in 

 the uterus, the yolk was reduced and ultimately disappeared. 

 At the same time the envelopes, which were developed under 

 the influence of the vitelline contents, have been preserved and 

 modified in different ways to aid uterine nutrition.-^ 



In the early stages the development proceeds, as in birds 

 and reptiles, with the gradual extension of the hypoblast round 

 the wall of the blastocyst, which thus becomes didermic. The 

 mesoblast grows out between the epiblast and hypoblast, 

 starting at the embryonic area and gradually extending for a 

 variable distance round the wall of the blastocyst. Near the 

 embryo appears the area vasculosa, in which blood-vessels and 

 blood are developed from the ceUs of the mesoblast, while at 

 the same time the embryo begins to be folded off from the 

 yolk-sac by anterior and posterior folds. The area gradually 

 extends further and further round. Its outer boundary is 

 marked by the sinus terminalis which communicates with the 

 vitelline veins. The blood is brought from the dorsal aortse by a 



' According to Hubreoht's views, the mammalian ovum is not descended 

 from the ovum of Sauropsida. 



