198 FCETAL MEMBRANES. 



blood-vessels, and in a partially assimilated condition transferred 

 to the body of the embryo 1 . 



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 

 membrane 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 dis- 

 appeared, 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 nine- 

 teenth 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 some considerable size, is withdrawn through the somatic stalk 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 



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

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



