EMBRYO AND EMBRYONIC MEMBRANES 231 
umbilical field, through which dried remnants of the inner wall 
of the allantois, which is likewise attached to the distal pole of 
the yolk-sac, protrude for a short time. On the inner side the 
yolk-sac is attached to the umbilicus by its distal pole, and by 
its stalk to the intestine. The absorption of the yolk-sac then 
goes on with great rapidity, being reduced from a weight of 
5.34 gr. twelve hours after hatching to 0.05 gr. on the sixth day 
after hatching, according to a series of observations of Virchow. 
The Amnion. The amnion invests the embryo closely at the 
time of its formation, but soon after, fluid begins to accumulate 
within the amniotic cavity, which gradually enlarges so that the 
embryo les within a considerable fluid-filled space, which in- 
creases gradually up to the latter part of the incubation, and 
then diminishes again, so that the embryo finally occupies most 
of the cavity. The connections of the amnion with the chorion, 
and later with the allantois, albumen-sac, and yolk-sac, have 
been already described. 
Muscle fibers appear in the walls of the amnion on the fifth 
or sixth day and gradually increase in number; though they 
subsequently degenerate over the area of fusion with the allan- 
tois. They persist elsewhere, however, and are active in the 
inclusion of the yolk-sac in the manner already described. Shortly 
after the appearance of the muscle fibers slow vermicular or 
peristaltic contractions of the amnion begin, and the embryo is 
rocked within the amniotic cavity. Apparently, adhesions are 
thus prevented, but they are sometimes formed and lead to various 
malformations of the embryo. In some cases the amnion fails 
to develop; in such cases, the embryo usually dies at a relatively 
early stage, though Dareste records an anamniotic embryo of 
thirteen days, apparently full of life and vigor. 
The amnion apparently acts first as a protection against all 
mechanical shocks and jars which are taken up by the fluid; 
second, by protecting the embryo against the danger of desicca- 
tion; third, by protecting it against adhesions with the shell- 
membrane and embryonic membranes, and lastly by providing 
space for the expansion of the allantois and consequent increase 
of the respiratory surface. It also has secondary functions in 
the chick in connection with the absorption of the albumen and 
the inclusion of the yolk-sac. It will be readily understood, 
then, why anamniotic embryos usually do not develop far. 
