278 THE SIXTH DAY. [CHAP. 



This can only be due to its absorbing the white of the 

 egg, which indeed is diminishing rapidly. 



During the eighth, ninth, and tenth days the 

 aninion 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, viz. over about 

 three quarters of the yolk-sac. The somatopleure at 

 this point is continuous (as can be easily seen by 

 reference to Fig. 9) 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-sac as a flattened 

 bag filled with fluid, it now serves as the chief organ of 

 respiration. 



Hence it is very vascular, the vessels on that side of 

 the bag which is turned to the serous membrane and 

 shell being especially large and numerous. 



The yolk now begins to diminish rapidly in bulk. 

 The yolk-sac becomes flaccid, and on the eleventh day 

 is thrown into a series of internal folds, abundantly 

 supplied with blood-vessels. By this means the surface 

 of absorption is largely increased, and the yolk is more 

 and more rapidly taken up by the blood-vessels, and in 

 a partially assimilated condition transferred to the body 

 of the embryo. 



By the eleventh day the abdominal parietes though 



