THE CLOACA IN BIRDS 191 
but occurs in most other reptiles. Another objection to this 
homology is based upon the fact that the anal sacs arise on the 
cephalic rather than on the anal side of the caudal intestine. 
They are thus more nearly comparable to diverticulum c, which 
unquestionably represents the urodaeal Kammer or dorsal 
bladder of the saurian cloaca, than to the bursa of Fabricius. 
In Unterhéssel’s account of the saurian cloaca another divertic- 
ulum is represented which, as a possible homologue of the bursa, 
seems much more promising. This is an invagination of the 
dorsal wall of the cloaca, defined by Unterhdssel as lying at the 
junction of the urodaeum and the proctodaeum. It is figured 
in models of late embryonic stages of three different species, 
and would seem to be a modification of the same structures. 
The first is a vaulted portion of the roof of the urodaeum of the 
lizard Platydactylus guttatus (Taf. VIII, fig. 1, st). The second 
is a comb-shaped diverticulum occupying the. same position in 
the cloaca of the snake Anguis fragilis (Taf. VIII, fig. 2, not 
labeled). The third consists of a pair of dorsal diverticula lying 
behind the urodaeal chamber and described as outpocketings 
of the proctodaeum in the snake Tropidonotus natrix (Taf. VIII, 
fig. 4, s). But it will be remembered that the bursa for a long 
time was described as an outgrowth of the proctodaeum, and 
the author in this case admits the lack of younger stages. From 
an examination of the account of the saurian cloaca I am con- 
vinced that the key to the homology of the bursa of Fabricius 
lies in the study of the reptilian cloaca, and am optimistic enough 
to believe that such a careful study of the younger stages of the 
reptile cloaca as Fleischmann and his students have made of 
older stages will bring the desired results. The comparison 
which Schumaker has lately made with the tonsiloid tissue dis- 
covered by Keibel in the cloaca of the mammal Echidna does not 
seem to meet the problem. At best it can only be considered a 
vestige of a reptilian prototype, and to reptiles we must again 
direct our attention for interpretation of the bursa of Fabricius. 
