EMBRYO AND EMBRYONIC MEMBRANES 231 



umbilical field, through which dried remnants of the inner wall 

 of the allantois, w^hich 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 

 w^ithin the amniotic cavity, which gradually enlarges so that the 

 embryo lies 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 cavitv. 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 w^alls 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, how^ever, 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. 



