484 CHORDATA. 
It must be remembered that these types of placenta, 
depending for distinction upon the degree of quantitative 
or qualitative production of the villi, are gradational. 
The amnion remains as a thin membrane enveloping the 
embryo and containing the Zguor amanii, a colourless indif- 
ferent fluid. Its walls are said to contract rhythmically and 
rock the embryo. At birth the amnion is ruptured and its 
remains are thrown off with the placenta. 
The yolk-sac, as before stated, never contains yolk and, 
after the prokalymma has ceased to be functional, it either 
shrivels and folds up at the lower pole or its outer wall, the 
prokalymma, is shed, and the inner wall remains as a 
vascular membrane. 
In this general account there are a series of striking 
differences from the lower types (the Sauropsida), and yet at 
the same time. there is a great degree of similitude. Here 
are the same four foetal organs, the serosa, the amnion, the 
allantois‘and the yolk-sac (or umbilical vesicle), but their 
origin and function are different. The actual development 
of the embryo and its organs is very similar, although it 
commences much later. 
The main differences are as follows :— 
MAMMAL, 
1. The egg is minute (g4,> in. 
diam. in rabbit) with little or no yolk 
and segmentation is total and equal. 
2, The development of the em- 
bryois very slow, but that of the 
membranes is rapid, hence the ser- 
osa, amnion and yolk-sac are formed 
at first without their mesoblastic 
sheaths. 
3. The serosa early becomes an 
attaching organ, possibly also nutri- 
tive, and the yolk-sac never contains 
yolk, but it becomes an organ for 
interchange of blood, hence a nutri- 
tive, excretory and respiratory organ, 
to be replaced in the Eutheria by 
the allantois similarly modified. 
SAUROPSIDA. 
1. The egg is large (about 1 inch 
diam. in chick), has a mass of yolk 
and segmentation is partial. 
2. The development of embryo 
commences first and that of mem- 
branes is slower, hence the mem- 
branes have their mesoblastic 
sheaths from their inception. 
3. The serosa remains simple but 
is covered by a porous shell, the 
yolk-sac contains plentiful yolk, and 
the allantois is a respiratory organ, 
its cavity forming an excretory re- 
servoir. 
How are we to explain these important differences ? 
The lowest mammals or A/onotremata have large eggs, with 
