THE CLOACA IN BIRDS ie 
The degenerating epithelial cells bordering the gap may then be 
studied in less crowded condition. Such a picture is presented 
in figure 19—an obliquely frontal section passing through the 
fenestrated area at right angles to the back lines of the cloaca; 
that is, in a plane cutting the allantoic duct lengthwise. In 
this figure the following features should be noted: the isolated 
roof of the cloaca, rows of necrotic epithelial cells on either flank, 
the concentration of mesenchyma about the gap on either side, 
and the rounded margins of the epithelium conspicuous by their 
failure to regenerate. In the epithelium bordering the gap are 
occasional pycnotic nuclei, and here and there a phagocyte, 
indicating a slow resorption in contrast to the sudden removal 
characteristic of initial stages. When the degenerative process 
slows down and finally comes to an end, a single large foramen 
is left in the dorsal wall of the cloaca extending from behind the 
level of the wolffian duct to the site of the caudal intestine, hav- 
ing a lenticular shape when viewed from below (fig. 15). As 
seen in microscopic section (fig. 20) the epithelium of the roof 
of the cloaca has been entirely removed, leaving in its place a line 
of mesenchymal cells which have flattened out into a surface 
layer as if under compression by the fluid in the cavity, in a 
manner recalling the formation of the false epithelium which 
lines the joint cavities. 
Even before degeneration stops, however, the process of closure 
setsin. This consists of a fusion of the epithelial margins of the 
gap beginning at the caudal angle of the aperture, so that in 
the space of another twelve hours, only a slender cleft remains at 
the anterior end of what was once a big fenestra (fig. 23, fen.). 
This process of closure seems to be aided if not caused by 
a progressive approximation of the sides of the cloaca, beginning 
at the anal plate, which results in the fusion of opposite walls and 
the formation of a urodaeal membrane. Figure 21, of a cross- 
section of the fenestra in the last stage of closure, shows that even 
to the end of closure no regeneration of the cloacal lips has taken 
place, but that rather the free margins of the walls have been 
pushed down into the mesenchymal cavity, as if by lateral com- 
pression exerted upon the side of the cloaca. By the middle of 
