THE CLOACA IN BIRDS 175 
able that the development of the corresponding region in the 
adjacent cloacal wall has likewise been interfered with, and that 
when reduction of the caudal intestine does occur, both of these 
areas are subjected to a retrograde process more rapid and exten- 
sive than obtains in other vertebrates. 
DEVELOPMENT OF THE UROGENITAL APPARATUS 
Anomalies arising in connection with the wolffian ducts 
About the time that the primary excretory ducts reach the 
level of the cloaca in their downgrowth from the pronephros, 
an eruption of diverticula appears on each flank of the cloaca 
opposite the distal portion of the ducts. Since these outpocket- 
ings of the cloaca seem to develop in response to the presence of 
the wolffian ducts, and later fuse with them, I have named them 
complemental diverticula. A surface view of this stage, such as 
is shown in figure 13 of a 41-somite chick embryo (62 hours), 
reveals the presence of two groups of diverticula—a circlet of 
five or six small ones opposite the terminal portion of the duct, 
and a single larger one farther up on the shaft, as broad as the 
whole field of smaller ones. In this embryo the duct of the left 
side has not fused with the cloaca, although fusion on the right 
side has taken place. In a 40-somite embryo neither duct.has 
fused. My observations would therefore differ somewhat in de- 
tail from the statement of Lillie that the wolffian duct ‘‘reaches 
the cloaca (with which it unites) about the 3l-som. stage’’ and 
that ‘‘at about the sixtieth hour the ends of the ducts (described 
in the preceding sentence as solid) fuse with broad lateral diver- 
ticula of the cloaca, and the lumen extends backwards until the 
duct becomes viable (?) all the way into the cloaca (at about 72 
hours, 35 somite stage).’”’ For a frontal section (fig. 6) of the 
cloaca shown in figure 13, at the place where the left wolffian 
duct makes the nearest approach, shows that the duct has not 
yet fused with the cloaca, that its terminal portion is patent, 
and that the mesial wall of the duct is thinning out in anticipa- 
tion of fusion. The section through the left side happens to 
