314 THE DEVELOPMENT OF THE CHICK 



are formed as folds and culs-de-sac excavated in the thickness of 

 the original epithelial wall, by elevations of the subjacent con- 

 nective tissue. It should be noted finally, that from the eighth 

 day on, the surface of the mucosa, both in the proventriculus and 

 in the gizzard, is covered with a thick layer of secretion; subse- 

 quently replaced in the gizzard by the corneous lining. 



A.o.m 



FIG. 182. Photograph of a transverse section of an 8-day chick through 

 the region of the proventriculus and tip of the heart. 

 A. coel., Coeliac artery. A. o. m., Omphalomesenteric artery. Cav. om., 

 Cavum omenti. Cav. pc., Pericardial cavity. Coel., Coelome. Gon., Gonad. 

 Lig. g-h., Gastro-hepatic ligament. M. D., Miillerian duct. Mtn., Metaneph- 

 ros. p'c., Membranous pericardium. Pr'v., Proventriculus. S'r., Supra- 

 renal. V. c. i., Vena cava inferior. Ven., Ventricle of heart. V. h. 1., Left 

 hepatic vein. V. s'c., Subcardinal vein. V. umb., Umbilical vein. 



Large Intestine, Cloaca, and Anus. The cloaca of the adult 

 is a large chamber opening to the exterior by the anus; it consists 

 of three divisions: the proctodseum or terminal chamber is capable 

 of being closed by the sphincter muscle, the bursa Fabricii opens 

 into its dorsal wall, and it is separated by a strong circular fold 



