MAMAMALTA. 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 feetal life. 
The lower disc (3) forms the prokalymma or absorptive 
disc for albuminous nutrition, the zone (2) forms later the 
yolk-sac placenta for hemal nutrition and the upper half (1) 
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 “ ¢vophoblast.” 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 az 
inner wall of hypoblast and an outer wall of mesoblast. It 
is known as the a//antois and soon spreads over the dorso- 
posterior part of the embryo, coming to lie in close con- 
tact'with 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 a//antoic 
arteries and veins and the latter the wze//ine. 
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 hemal nutrition, which rapidly replaces in function 
the prokalymma and its albuminal nutrition. 
M. 32 
