16 



ON THE BURSA FABRICII IN BIRDS. 

 Fig. 7. 



A. 



Cloaca of Casuarius picticottis, adult male ; viewed from before. 



A. Circular folds of mucous membrane, being the last remains of the Bursa. B. 

 Rectum, b. Recto-cloacal valve. 0. Cut surface of external sphincter muscle. 

 c. Vesical sphincter. D. Urino-genital papillge. E. Glandular pore. P. Penis. 



Although at first sight the relation of these parts in Rliea, Struihio, 

 &c. seems so different from that which obtains in other birds, yet a 

 little reflection will, I think, convince one that it is not so in reality. I 

 have represented diagram matically (figs. 8 and 9) what I conceive to 

 be the true relations of the parts in question, denoting the homologous 

 regions in the two forms (Serpentarius as illustrating the normal type, 

 Casuarius the abnormal one) by similar letters. If we imagine B in 

 fig. 8 to lose the constriction at its aperture into D, and D to become 

 P. Z. S. 1877 proportionately deeper, we should have a form corresponding to fig. 9, 

 p. 317. in which B passes uninterruptedly and without constriction into D, the 

 cavity B D thus formed receiving the aperture of C. In such a form 

 as Plotus, where the opening is but very slightly constricted, we have a 

 type connecting the two extremes ; and I have seen RJiea in a stage 

 very similar to that mentioned above in Cygnus olor. In confirmation of 

 this view as to the true relation of these parts, in the Ostrich &c. the 

 lower part of the bursa, in the region corresponding to D in figs. 8 

 and 9, is not glandular (vide fig. 6, p. 15, where the non-glandular part 

 of the bursa is seen beneath the pores H H'). 



"With regard to the function and homologies of the bursa Fabricii, 

 great differences have prevailed amongst authors. Thus Milne-Edwards 

 says*, " Fabrice d'Acquapendente, a qui Ton doit la decouverte de cette 



* Phys. et Anat. Comp. vol. viii. p. 514. 



