EMBRYO AND EMBRYONIC MEMBRANES 223 
Structure of the Allantois. (1) Inner wall. The inner wall 
of the allantois consists primarily of two layers, an inner ento- 
dermal and outer mesodermal layer. The latter soon becomes 
differentiated into two layers, an external, delicate, limiting layer 
of flat polygonal cells, with interlocking margins, and an inter- 
mediate layer of star-shaped cells embedded in a homogenous 
mucous ground substance. Parts of the inner wall become 
extremely thin, and in these regions the intermediate layer may 
become entirely absent. Elsewhere, particularly around the 
larger arteries and veins, the intermediate layer may attain 
considerable thickness. The entoderm becomes reduced to a 
layer of flat, interlocking cells. On the eighth day, spindle- 
shaped muscle cells begin to appear in the mesoderm of the 
inner wall, and undergo rapid increase in numbers. Their dis- 
tribution is somewhat irregular; in certain places they may even 
form several layers, and in others are practically wanting. 
On the seventh day the inner wall of the allantois begins 
to fuse with the amnion in the neighborhood of the sero-amniotic 
connection, and this fusion rapidly extends over the area of 
contact between the two membranes. Within the area of fusion 
the muscle layers of the allantois and amnion mutually reinforce 
each other, and in places no boundary can be found between 
them (Filleborn). But during the latter half of incubation the 
musculature of the fused area of allantois and amnion degener- 
ates almost completely. 
Towards the end of incubation, part of the inner wall of the 
allantois fuses also with the yolk-sac, and is therefore carried 
with the latter into the body-cavity of the chick. 
(2) The Outer Wall of the Allantois. As already noted, the 
outer wall of the allantois fuses with the chorion. The compound 
membrane, which is respiratory in function, must be considered, 
therefore, as one. Over the entire respiratory area the ectoderm, 
belonging primarily to the chorion, which is elsewhere two layers 
of cells in thickness, becomes reduced to an exceedingly thin 
layer in direct contact with the walls of the capillaries internally 
and the shell membrane externally. According to Fulleborn, 
the ectoderm cannot be distinguished as a separate layer in the 
latter half of incubation, and the capillaries appear to be in 
immediate contact with the shell-membrane. No muscular 
tissue appears to develop in the outer wall of the allantois. 
