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 sae of the allantois constitutes the inner wall. The distal 
intermediate part of the allantois is specialized as the wall of the 
albumen-sac. 
All+Am, 
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-sae is much reduced, and some yolk protrudes 
into the albumen (sae of the yolk-sae 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. 
