318 



THE DEVELOPMENT OF THE CHICK 



shadow its final rupture. Tlie lower end of the large intestine 

 is perfectly solid, and higher up, somewhat vacuolated. (The 

 solid stage begins on the seventh day.) The urinogenital ducts 

 open into the urodseum above the solid end of the large intestine. 

 It will be seen, therefore, that the urodaeum is transformed into 

 a passageway between the urinogenital ducts and the allantois, 

 being closed anteriorly by the solid large intestine and posteriorly 

 by the urodaeal (cloacal) membrane. 



Rect, ,^ 



Afc/?. 



':dac/.A. 



Ufd. An 



Fig. 185. — Photograph of the region of the cloaca in a median sagittal 



section of an 8-day chick. 



All., Allantois. An., Anus. B. F., Bursa Fabricii. caud. A., Caudal 

 artery. Int., Intestine. N'ch., Notochord. p. P., Perianal papilla. Rect., 

 Rectum. Ur'd., Urodaeum. 



During the twelfth and thirteenth days, the vacuoles in the 

 upper part of the large intestine flow together and re-establish 

 the cavity, but the lower end still remains closed by a solid plug 

 of cells; immediately anterior to the latter the large intestine is 

 dilated, and this apparently corresponds to the coprodaium of 



