AVES. 197 



amnion, that is, between the amnion proper and the false amnion 

 or serous envelope. It is filled with fluid, so that in spite of its 

 flattened form its opposite walls are distinctly separated from 

 each other. 



The vascular area has become still further extended than on 

 the fifth day, but with a corresponding loss in the definite cha- 

 racter of its blood-vessels. The sinus terminalis has indeed by 

 the end of the seventh day lost all its previous distinctness ; and 

 the vessels which brought back the blood from it to the heart are 

 no longer to be seen. 



Both the vitelline arteries and veins now pass to and from 

 the body of the chick as single trunks, assuming more and more 

 the appearance of being merely branches of the mesenteric 

 vessels. 



The yolk is still more fluid than on the previous day, and its 

 bulk has (according to von Baer) increased. This can only be 

 due to its absorbing the white of the egg, which indeed is dimin- 

 ishing 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, i.e. over about three-quarters of the 

 yolk-sack. The somatopleure at this point is continuous (as can 

 be easily seen by reference to fig. 121) with the original outer 

 fold of the amnion. It thus comes about that the further splitting 

 of the mesoblast merely enlarges the cavity in 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-sack as a flattened bag filled with fluid, it now serves as the 

 chief organ of respiration. It is indeed very vascular and a 

 marked difference may be observed between the colour of the 

 blood in the outgoing and the returning vessels. 



The yolk now begins to diminish rapidly in bulk. The yolk- 

 sack becomes flaccid, and on the eleventh day is thrown into a 

 series of internal folds, abundantly supplied by large venous 

 trunks. By this means the surface of absorption is largely in- 

 creased, and the yolk is more and more rapidly taken up by the 



