144 THE DEVELOPMENT OF THE CHICK 
arises from the neck of the allantois both behind and also at the 
sides. (cf. Fig. 82.) 
During the fourth day the distal portion of the allantois 
pushes out into the portion of the extra-embryonic body-cavity 
beneath the hind end of the embryo and rapidly expands to form 
a relatively large sac. But the neck of the allantois remains 
embedded in the ventral mesentery and does not expand; the 
terminal portion of the intestine has in the meantime formed 
Am. Amesy. Ect. 
ie 2 
eel } 
av 
£etam. 
Mesam. 
Fic. 80. — Sagittal section through the tail of an embryo of about 35 s. 
All., Allantois. An. pl., Anal plate. ¢. C., Central canal of the neural 
tube. Cl, Cloaca. Ectam., Eetoderm of the amnion. Mesam., Mesoderm 
of the amnion. _p’a. G., Post-anal gut. p. i. p., Posterior intestinal portal. 
s. A., Segmental arteries. Other abbreviations as before. 
the primordium of the cloaca, from which, therefore, the neck of 
the allantois appears to arise (Fig. 183); at all stages of incuba- 
tion the neck of the allantois forms an open connection between 
the cloaca and the allantoic sac. 
The Umbilicus. The closure of the body-wall progressively 
reduces the communication between the embryonic and extra- 
embryonic body-cavity to a narrow chink between the yolk-stalk 
