220 THE DEVELOPMENT OF THE CHICK 



states that, after this connection is established, 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 Fulleborn. 



The Allantois. The part of the wall 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 wall. The distal 

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

 albumen-sac. 



,All.+Aff>. 



AI/.C. 



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 amnion. 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 layer, formed by 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 layer derived from the chorion. 



