THE CLOACA IN BIRDS 179 
offered by G. H. Parker that “the physiological purpose of this 
arrangement is to secure the transmission of the excretion from 
the embryonic kidney to the allantois, and to prevent the escape 
of the excretion, either into the intestine or into the amniotic 
cavity, where it might prove injurious to the embryo.” ‘That 
the urodaeal sinus is a mechanism inherited directly from rep- 
tiles was revealed two years later by the comparative studies of 
Fleischmann and his students on the cloaca and phallus of liz- 
ards, snakes, turtles, birds, and mammals. He notes that in 
the Sauropsida the urodaeum is divided into two poriions, a 
distended oral portion always in relation to the wolffian ducts, 
and an elongated caudal portion which forms an open passage- 
way (even in young embryos) to the anus. The shutting off of 
the urodaeal sinus from below in birds is due to the fact that the 
second half of the urodaeum never elongates, but remains short 
and impervious through the formation of a urodaeal membrane. 
While the posterior portion of the urodaeum becomes elongated 
and subject to great modification in various reptiles, the anterior 
chamber (urodaeal Kammer of Unterhéssel) is always associated 
with bladder formation. It becomes chiefly dilated in a dorso- 
lateral direction, so that the entire cavity and associated meso- 
dermal ducts assume the appeance of a dorsal bladder (cf. Fleisch- 
mann, Taf. VIII, figs. 1, 2 and 4). This striking feature appears 
temporarily in bird embryos as the urodaeal sinus, and is as con- 
vincing a repetition of reptilian ancestry as the allantoic bladder 
previously referred to in figure 39. But since it was studied 
chiefly in older embryos, and then largely by means of sagittal 
sections, its extent and composition was not fully appreciated 
even by Fleischmann. 
As seen in figures 40 and 41, the urodaeal sinus (wrod.) is a greatly 
inflated segment of the cloaca, placed athwart the main axis of 
the hind-gut, between the occluded rectum and the urodaeal 
membrane. Its lumen from front to back is reduced to the size 
of a fissure, but is greatly expanded laterally and dorsoventrally, 
extending from the wolffian duct of one side to that of the other 
and from the dorsal side of the cloaca to the allantois. Although 
existing as a single structure at this stage, it has been formed 
