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 (Fi.illeborn). 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 Fiilleborn, 

 the ectoderm cannot be distinguished as a separate layer in the 

 latter half of incubation, and the capillaries appear to be in 

 immediate contact Avith the shell-membrane. No muscular 

 tissue appears to develop in the outer wall of the allantois. 



