220 



THE DEVELOPMENT OF THE CHICK 



states that, after this connection is estabhshed, the amniotic 

 fluid coagulates in alcohol, "just like the fluid in the albumen- 

 sac; owing, presumably, to the presence of albumen which has 

 found its way through the perforations into the amniotic fluid." 

 This observation is confirmed by Fiilleborn. 



The AUantois. The part of the w^all of the allantois that 

 fuses with the chorion may be called the outer wall; the remainder 

 of the sac of the allantois constitutes the inner w^all. The distal 

 intermediate part of the allantois is specialized as the wall of the 

 albumen-sac. 



AlhAm. 



AlIC. 



jSliS. 



Fig. 129. — Twelfth day of incubation. The conditions 

 represented in Fig. 128 are more advanced. The albu- 

 men-sac is closing; its connection with the cavity of 

 the amnion by way of the sero-amniotic connection 

 will be obvious. The inner wall of the allantois has 

 fused extensively with the anmion. The umbilicus of 

 the yolk-sac is much reduced, and some yolk protrudes 

 into the albumen (sac of the yolk-sac umbilicus). 



In the outer wall there are three layers, viz., an internal epi- 

 thelial laver, formed bv the entoderm of the allantois; a thick 

 very vascular middle or mesodermal layer, formed by fusion of 

 the mesoblast of allantois and chorion; and a thin, outer, ecto- 

 dermal laver derived from the chorion. 



