178 EDWARD A. BOYDEN 
Formation of the wrodaeal sinus 
In discussing the origin of urinary bladders Felix defines four 
main types: 1) mesodermal bladders, arising from the fusion or 
dilation of the caudal ends of the wolffian duct; 2 and 3) dorsal 
and ventral cloacogenic bladders, outgrowths or dilations of the 
dorsal and ventral walls of the cloaca, respectively, and, 4) 
allantoidogenic bladders formed by the retention of the proxi- 
mal end of the allantois. The first type in its pure form is real- 
ized only in selachians, the second type only in amphibians, 
both groups being devoid of an allantois. The bladders of all 
other vertebrates, according to Felix, are of mixed origin. When 
we examine birds, it appears that they are the only class among 
amniotes without one or more bladders, yet curiously enough, 
reptiles, from which birds have descended, constitute the class 
with the greatest number and diversity of bladders. Thus, 
according to Felix, lizards derive their bladders from three sources, 
dorsocloacogenic, allantoidogenic and mesodermal; and in 
turtles the bladder is formed from dorsocloacogenic, ventro- 
cloacogenic, allantoidogenic, and mesodermal origins (Keibel 
and Mall, II, p. 869). It would be strange, then, if the bird did 
not exhibit some traces of bladder formation in its ontogeny, 
and such, in fact, may be found. The most conspicuous of 
these is the intra-embryonic expansion of the allantois shown in 
figure 39. It is almost identical at this stage with the primor- 
dium which develops into the ventral bladder in most reptiles. 
But it is completely resorbed in adult birds. 
The other structure in bird embryos which recalls the reptil- 
ian bladders (this time those of dorsocloacogenic and mesoder- 
mal origin) is the wrodaeal sinus, a name which I have applied to 
the cavity of the urodaeum at its maximum extent (figs. 40 and 
41 urod.). Minot in 1900 called attention to the peculiar 
relations of this cavity as follows: ‘‘From the closure of the 
intestinal opening by the entoderm (occluded rectum), and of 
the anal opening by the anal plate (meaning urodaeal membrane), 
there is left a clear passage from the wolffian duct across (to) 
the opening of the allantois.’”’ And he quotes the suggestion 
ox 
