II.] THE AMNION. 39 



stalk ; but this stalk must, like the embryonic sac itself, be 

 a double stalk, and consist of a smaller inner stalk within a 

 larger outer one, Fig. 8, E,H. The folds of the splanchnopleure, 

 as they tend to meet and unite in the middle line below, 

 give rise to a continually narrowing hollow stalk of their 

 own, a splanchnic stalk, by means of which the walls of the 

 alimentary canal are continuous with the splanchnopleuric 

 investment of the yolk-sac, and the interior of that canal is 

 continuous with the yolk inside the yolk-sac. In the same 

 way the folds of the somatopleure form a similar stalk of 

 their own, a somatic stalk, by means of which the body- 

 walls of the chick are continuous (for some time; the con- 

 tinuity, as we shall see, being eventually broken by the 

 development of the amnion) with the somatopleuric invest- 

 ment of the yolk-sac; and the pleuroperitoneal cavity of the 

 body of the chick is continuous with the narrow space be- 

 tween the two investments of the yolk-sac. 



At a comparatively early period the canal of the splanch- 

 nic 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 absorp- 

 tion through the blood-vessels. The somatic stalk, on the 

 other band, remains widely open for a much longer time ; but 

 the somatic shell of the yolk-sac never undei'goes that thick- 

 ening which takes place in the somatic walls of the embiyo 

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

 When accordingly in tlie last days of incubation the greatly 

 diminished yolk-sac with its splanchnic investment is with- 

 drawn 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 invest- 

 ment of the yolk-sac, which is cast off as no longer of any 

 use. (Fig. 8. Compare the series.) 



8. Very closely connected with the cleavage of the meso- 

 blast and the division into somatopleure 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 takes its origin from certain folds of the 

 somatopleure, and of the somatopleure only, in the following 

 way. 



