ALIMENTARY TRACT AND ITS APPENDAGES 



315 



from the intermediate section of the cloaca or urodseum; this is 

 a relatively short division of the cloaca which receives the renal 

 and reproductive ducts in its dorsal wall by two pairs of openings; 

 it is bounded from the larger anterior division, coprodseum, by 

 a rather low circular fold; the coprodseum passes gradually, with- 

 out a sharp line of division, into the rectum. 



The early embryological history of these parts has been con- 

 sidered in the preceding chapters. The condition on the fourth 

 day is shown in the accompanying figure (Fig. 183) representing a 



Fig. 183. — Median sagittal section of the hind end of a chick embryo 



on the fourth day of incubation. (After Gasser from Maurer.) 



All., Allantois. Am., Tail fold of amnion, cl. M., Cloacal mem- 

 brane. CI., Cloaca. N'eh., Notochord. n.T., Neural tube. R., Rec- 

 tum. Y. S ., Wall of yolk-sac. 



sagittal section of the hind end of the embryo. The cloaca is the 

 large terminal cavity of the intestine, closed from the exterior 

 by the cloacal membrane, in which the entoderm of the floor of 

 the cloaca is fused to the superficial ectoderm at the base of the 

 tail. The line of fusion is a long, narrow median strip, extending 

 from just below the neck of the allantois to the hinder end of the 

 cloaca. Leading out from the cloaca ventrally, in front of the 



