This can only be due to its absorbing the white of the
egg, which indeed is diminishing rapidly.
During the eighth, ninth, and tenth days the
amnion does not undergo any very important changes.
Its cavity is still filled with fluid, and on the eighth
day its pulsations are at their height, henceforward
diminishing in intensity.
The splitting of the mesoblast has now extended to
the outer limit of the vascular area, viz. over about
three quarters of the yolk-sac. The somatopleure at
this point is continuous (as can be easily seen by
reference to Fig. 9) with the original outer fold of
the amnion.
It thus comes about that the further splitting of the
mesoblast merely enlarges the cavity im which the
allantois lies. The growth of this organ keeps pace
with that of the cavity in which it is placed. Spread
out over the greater part of the yolk-sac as a flattened
bag filled with fluid, it now serves as the chief organ of
respiration.
Hence it is very vascular, the vessels on that side of
the bag which is turned to the serous membrane and
shell being especially large and numerous.
The yolk now begins to diminish rapidly in bulk.
The yolk-sac becomes flaccid, and on the eleventh day
is thrown into a series of internal folds, abundantly
supplied with blood-vessels. By this means the surface
of absorption is largely increased, and the yolk is more
and more rapidly taken up by the blood-vessels, and in
a partially assimilated condition transferred to the body
of the embryo.
