THE CLOACA IN BIRDS 169 
tion becomes more slender the lumen becomes occluded and the 
solid strand thus formed soon after ruptures (fig. 3, 2). At 
least some of the cells disintegrate and are removed by phagocytes, 
but pyenotic nuclei are inconspicuous here as compared with the 
abundance of necrotic cells to be found in the degenerating cau- 
dal intestine of the chick. This process, which begins at the 
cloacal end of the gut, progresses slowly in a craniocaudal direc- 
tion until the entire caudal intestine disappears. In duck, 
pheasant, and chick embryos, however, the reduction of the 
caudal intestine is greatly complicated by the disintegrating 
process going on in the primitive-streak mass, asreferred to above, 
and by the disintegration of the adjacent cloacal wall, the latter 
process resulting in the formation of the cloacal fenestra. 
The developmental history of this foramen, which is thus 
intimately associated with the removal of the caudal intestine, 
is divided into two phases, a period of active disintegration, 
beginning at about the 41-somite stage (chick embryo, 2 days, 
18 hours), and lasting approximately twelve hours, and a period 
of closure, beginning somehere near the 50-somite stage (7-mm. 
embryos, of approximately 3 1/3 days), and ending in embryos 
of about 9 mm., incubated 3 days and 18 hours. Expressed in 
terms of embryonic growth, the first trace of the process appears 
just before the wolffian ducts fuse to the cloaca. The final 
stage in closure occurs about the time the ultimate somite is 
formed (I have found as many as fifty-three) ; that is, before the 
resorption of caudal somites begins. 
The initial phase, as illustrated by the first text plate (figs. 
1 to 5), is based upon two embryos. In consequence of the 
great rapidity with which the degenerative process is initiated, 
a far greater number of specimens of the same age than were 
available would have had to have been sectioned in order to 
have provided more than the two stages referred to. For there 
is not the slightest indication of the process in an embryo only 
one somite younger than the one shown in figure 5, where the 
entire area of the cloacal wall which is to be denuded has already 
begun to degenerate. 
