164 F(ETAL MEMBRANES. 



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 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 still much looser 

 and less firm than the walls 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 

 appendages by a narrow somatic umbilicus, in which run the stalk of 

 the allantois and the solid cord suspending the yolk sack. 



The cleavage of the mesoblast is still progressing, and the yolk is 

 completely invested by a splanchnopleural sack. 



The allantois meanwhile spreads out rapidly, and lies over the 

 embryo close under the shell, being separated from the shell mem- 

 brane by nothing more than the attenuated serous envelope, formed 

 out of the outer primitive fold of the amnion and the remains of the 

 vitelline membrane. With this membrane the allantois partially 

 coalesces, and in opening an egg at the later stages of incubation, 

 unless care be taken, the allantois is in danger of being torn in the 

 removal of the shell-membrane. As the allantois increases in size 

 and importance, the allantoic vessels are correspondingly developed. 



On about the sixteenth day, the white having entirely disappeared, 

 the cleavage of the mesoblast is carried right over the pole of the 

 yolk opposite the embryo, and is thus completed (fig. 121). The yolk- 

 sack now, like the allantois which closely wraps it all round, lies loose 

 in a space bounded outside the body by the serous membrane, and 

 continuous with the pleuroperitoneal cavity of the body of the embryo. 

 Deposits of urates now become abundant in the allantoic fluid. 



The loose and flaccid walls of the abdomen enclose a space which 

 the empty intestines are far from filling, and on the nineteenth day 

 the yolk-sack, diminished greatly in bulk but still of some considerable 

 size, is withdrawn through the somatic stalk into the abdominal 

 cavity, which it largely distends. Outside the embryo there now 

 remains nothing but the highly vascular allantois and the bloodless 

 serous membrane and amnion. The amnion, whose fluid during the 

 later days of incubation rapidly diminishes, is continuous at the um- 

 bilicus with the body-walls of the embryo. The serous membrane 

 (or outer primitive amniotic fold) is, by the completion of the cleavage 



1 For details on this subject vide A. Courty, " Structure des Appendices Vitellins 

 chez le Poulet." An. Sci. Nat. Ser. III. Vol. ix. 1848. 



