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 



Atn.cav. Ect. 



T.B. 



BJ.v. 



Mesam. 



FIG. 80. Sagittal section through the tail of an embryo of about 35 s. 



All., Allantois. An. pi., Anal plate, c. C., Central canal of the neural 

 tube. Cl., Cloaca. Ectam., Ectoderm 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 



