MAMMALIA. 481 



exceedingly characteristic stage in most mammals and it is 

 also present in the chick; but whilst in the latter the 

 mesoblast extends to the lower pole and then splits all 

 round to form a completed serosa and yolk-sac, each with 

 its mesoblast wall, in the mammal the mesoblast remains at 

 this stage throughout foetal life. 



The lower disc (3) forms the prokalymnia or absorptive 

 disc for albuminous nutrition^ the zone (2) forms later the 

 yolk-sac placenta for hcemal nutrition and the upper half (i) 

 will undergo further changes. Whilst this development has 

 been going on within the blastocyst, the serosa has been 

 pushing out processes which come in contact with the wall 

 of the uterus and moor the blastocyst to the uterus. They 

 may in some mammals extend all over the surface and 

 seem in some cases to assist in absorption of nutritive fluid ; 

 hence this serosa, without its mesoblastic sheath, has been 

 termed the " trophoblast!' In others they form a girdle, or 

 they may be concentrated at one part. 



In the region of the prokalymma both epiblast and 

 hypoblast become modified into thickened active layers, 

 probably to subserve albuminal nutrition. Meanwhile from 

 the hind-gut of the embryo there arises in the mid-ventral 

 line a small outgrowth, which grows rapidly and pushes out 

 into the space between serosa, amnion and yolk-sac. As it 

 is a production of the gut-wall^ it has from its first origin an 

 inner wall of hypoblast and an outer wall of mesoblast. It 

 is known as the allantois and soon spreads over the dorso- 

 posterior part of the embryo, coming to lie in close con- 

 tact wdth the serosa in this region. In the chick it grows 

 till it covers practically the upper half of the blastocyst-wall 

 or serosa, and in Prototheria it occupies the whole right 

 half of the cavity. (See below.) The mesoblast of the 

 allantois and that of the yolk-sac now develop complete 

 systems of arteries and veins, the former being the allantoic 

 arteries and veins and the latter the vitelline. 



The vitelline blood-vessels ramify all over the placental 

 zone, and vascular villi or processes are thrust out into the 

 serous villi, coming into intimate contact with the uterine 

 blood- system. Thus is formed the true yolk-sac placenta 

 and a haemal nutrition, which rapidly replaces in function 

 the prokalymma and its albuminal nutrition. 



M. 32 



