202 THE SIXTH DAY. [CHAP. 



Botli the omphalo-mesaraic 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 

 tlie 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 diminishing rapidly. 



3. 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, 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. 8) 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 chorion 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. 



4. By the eleventh day the abdominal parietes though 

 still much looser and Jess firm than the Avails of the chest 

 may be said to be definitely established, and the loops of 

 intestine, which have hitherto been hanging down into the 

 somatic stalk, are henceforward confined within the cavity 

 of the abdomen. The body of the embryo is therefore 

 completed ; but it still remains connected with its various 



