ALIMENTARY TRACT AND ITS APPENDAGES 



319 



the adult cloaca. Even on the seventeenth day the large intes- 

 tine appears to be still closed at its lower end, and the urodaeal 

 membrane still persists as a plug of vacuolated cells. (Gasser.) 

 Both plugs must, however, disappear soon after. 



It would thus appear that the urodseum only of the adult 

 cloaca corresponds to the embryonic cloaca; the proctod2eum is 

 certainly derived from an ectodermal pit, and it is probable that 



Fig. 186. —Chick embryo of 11 days, sagittal section 

 through the region of the cloaca. Reconstructed from 

 several sections. (After Minot.) 

 Air., Ascending limb of the allantois. AH"., Descend- 

 ing limb of the allantois. An., Anal invagination. An.pl., 

 Urodeal membrane. Art., Umbilical artery. B. F., 

 Bursa Fabricii. b. f.. Duct of the bursa. Clo., Cloaca. 

 Ec, Ectoderm. Ent., Entoderm of the rectum. Ly., 

 Nodules of crowded cells, probably primordia of lym- 

 phoid structures in the wall of the large intestine. W. D., 

 Wolffian duct. 



the coprodaeum represents the enlarged lower extremity of the 

 embryonic large intestine. The bursa Fabricii is an entodermal 

 structure derived from the posterior portion of the embryonic 

 cloaca. 



IV. The Development of the Liver and Pancreas 

 The Liver. The anterior and posterior liver diverticula, de^ 

 scribed in Chapter VI, constitute the rudiments from, which tne 



