II.] THE AMNION. 43 



body of the chick is continuous with the narrow space 

 between the two investments of the yolk-sac. 



At a comparatively early period the canal of the 

 splanchnic stalk becomes obliterated, so that the 

 material of the yolk can no longer pass directly into 

 the alimentary cavity, but has to find its way into 

 the body of the chick by absorption through the blood- 

 vessels. The somatic stalk, on the other hand, remains 

 widely open for a much longer time ; but the somatic 

 shell of the yolk-sac never undergoes that thickening 

 which takes place in the somatic walls of the embryo 

 itself; on the contrary, it remains thin and insignificant. 

 When, accordingly, in the last days of incubation the 

 greatly diminished yolk-sac with its splanchnic invest- 

 ment is withdrawn into the rapidly enlarging abdominal 

 cavity of the embryo, the walls of the abdomen close 

 in and unite, without any regard to the shrivelled, 

 emptied somatopleuric investment of the yolk-sac, 

 which is cast off as no longer of any use. (Fig. 9. Com- 

 pare the series.) 



The Amnion. Very closely connected with the 

 cleavage of the mesoblast and the division into soma- 

 topleure and splanchnopleure, is the formation of the 

 amnion, all mention of which was, for the sake of 

 simplicity, purposely omitted in the description just 

 given. 



The amnion is a peculiar membrane enveloping the 

 embryo, which takes its origin from certain folds of 

 the somatopleure, and of the somatopleure only, in the 

 following way. 



At a time when the cleavage of the mesoblast has 

 somewhat advanced, there appears, a little way in front 



