180 EDWARD A. BOYDEN 
by the confluence of three originally separate elements. The 
first of these to appear is the median diverticulum designated 
as diverticulum c in the reconstructions shown in plate 3. It 
arises as early as the beginning of the fourth day and main- 
tains its identity as a distinct and conspicuous feature of the 
cloaca as late as the seventh day, at which time it is incorporated 
in the urodaeal sinus. This structure has been figured in descrip- 
tions of the avian cloaca as far back, at least, as the work of 
Bornhaupt (’67). But I question whether its existence as a sepa- 
rate rounded diverticulum has ever been appreciated. Pomayer, 
in the Fleischmann series, labeled it “‘Urogenitaltasche” in a 
sagittal section of a duck, giving it the same designation as the 
paired urogenital pockets of the snake, Tropidonotus, which are 
dilated outpocketings on the dorsal wall of the cloaca into which 
the wolffian ducts empty. A median diverticulum occurs in the 
same place (as diverticulum c) in the turtle embryos modeled 
by R. F. Shaner (fig. 3, an. s.), and has been interpreted by 
that author as the primordium from which the respiratory sacs 
(bursae anales) of turtles develop. In view of its position between 
the two wolffian ducts in both chicks and turtles, it seems 
not improbable that diverticulum c represents the dorsal out- 
pocketing of the cloaca of reptiles from which the wolffian 
ducts have shifted in course of their migration to the allantois. 
The second and third components of the urodaeal sinus arise 
more or less together. As seen in figures 14 and 6, the wolffian 
ducts, when they first reach the level of the cloaca, fuse to the 
cloaca along a broad area extending from the caudal margin to 
near the allantois (a to c). The fusion at ¢ approximates the 
primary position of the excretory ducts in lower vertebrates. 
In consequence, however, of the fusion of the two side walls of 
the cloaca, beginning with the anal plate, to form the urodaeal 
membrane, the outlet of the wolffian ducts at c and 6 in figure 
6 is suppressed. The broad complemental diverticulum (fig. 
6, a) thus becomes the main channel, and in course of develop- 
ment is enlarged into a wing-like expansion of the cloaca connect- 
ing the wolffian duct with the neck of the allantois (fig. 16). 
Meanwhile the segment of the wolffian duct between the orifice 
