THE CLOACA IN BIRDS val 
of discontinuous holes where complete resorption of the epithelium 
has taken place (fig. 13, from a wax model of the same embryo). 
The perforated walls of the cloaca at this period thus simulate 
in appearance a fenestrated membrane. Almost immediately, 
however, the holes run together, forming a continuous rift along 
the cloaca and caudal intestine. In this manner the dorsal wall 
of the cloaca becomes detached from the sides and thus isolated 
as a trough-shaped structure, is slowly resorbed. Its histologi- 
cal appearance will be described later in the paper. 
An invasion of the caudal intestine also occurs from another 
region in chick embryos and, to a lesser extent, in ducks. This 
is an extension of the degeneration process going on in the primi- 
tive-streak mass (fig. 5, ps. v.) into the ventral wall of the caudal 
intestine, and involves only that part of the intestine which is 
adjacent to the primitive streak. Thus, in the undifferentiated 
epithelium of the inner curvature of the caudal intestine are 
found phagocytes (again represented by periods, fig. 5) which 
are coextensive and continuous with the primitive-streak mass, 
which is itself undergoing rapid phagocytosis. The occurrence 
of these has nothing to do with the imvasion of the caudal 
intestine from the cloacal end, except that the two processes 
cooperate in destroying that end of the gut. 
The resorption of the caudai intestine in birds can now be sum- 
marized as follows. In chick embryos the flanks of the caudal 
intestine are invaded by a degenerative process originating in the 
cloaca, which removes the epithelium before the cavity of the 
anal gut can be occluded. In duck embryos the two processes 
take place nearly simultaneously, the cloacal invasion slightly 
preceding the occlusion of the caudal intestine. Finally, in 
terns, the cloacal fenestra is not present at all, and the caudal 
intestine undergoes reduction by the method already described 
as common to most amniotes. 
formed. ‘The side walls are the first to thicken. As development proceeds, the 
latter are brought closely together, buckling the flat, thin-walled area into a steep- 
pitched roof. But for some time there is an abrupt transition between thick- 
and thin-walled portions, and it is along this thin area, and its continuation into 
the cloaca, that resorption of epithelium first appears. 
