THE RISE OF THE MAMMALIA 115 



forms a rather large yolk-sac, which extends outside 

 the region from which the foetus is actually formed ; 

 so that as in a Reptile the embryo floats, so to speak, 

 on its yolk-sac. But this yolk-sac is much smaller 

 in the Mammal than in the Reptile or Bird, and 

 instead of containing yolk, it contains a small 

 quantity of albuminous material. The yolk-sac, 

 however, plays some part in the nutrition of the 

 embryo after the latter with its investing trophoblast 

 has been embedded in the uterine wall. The tropho- 

 blast becomes thickened at this stage ; at the same 

 time the uterine tissues of the mother become 

 highly charged with blood, in consequence of which 

 they swell and the activity of the uterine glands is 

 greatly increased. The tissue of the embryonic 

 trophoblast becomes closely adherent to or may 

 even completely fuse with the uterine tissues to 

 form a common mass in which the origin of cells 

 from the parent or embryo can no longer be deter- 

 mined. While this condition of things is being 

 established, the yolk-sac, which is applied to the 

 trophoblast over a large part of its area, absorbs 

 nourishment from the trophoblast and conveys it to 

 the growing foetus lying on its surface. As soon, 

 however, as the fusion of trophoblast with the 

 maternal uterus is completed, which happens with 

 considerable speed, the yolk-sac ceases to be an im- 

 portant organ of nutrition, its place being taken by 



82 



