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. 
Fic. 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. pe., Pericardial cavity. Coel., Coelome. Gon., Gonad. 
Lig. g-h., Gastro-hepatie ligament. M.D., Miillerian duct. Mtn., Metaneph- 
ros. p’e., 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’e., Subeardinal 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 
