18 ON THE BURSA FABRICII IN BIRDS. 



patches of lymphatic follicles (which do not, however, in this instance 

 project outside the mucous membrane of the intestine) in the appendix 

 to the caecum of the rabbit (described by Frey, ' Untersuchungen iiber 

 die Lymphgefasse des Darmkanales,' Leipzig, 1863). An organ still 

 more closely corresponding in its general shape and position with the 

 bursa Fabricii is the sac-like pouch which opens into the dorsal wall of the 

 cloaca in many Elasmobranchs *. The glands of this, however, differ in 

 structure from those of the bursa Fabricii ; so that at present it seems to 

 me that we can assign no very definite analogue or homologue for the 

 latter, but that it is a glandular outgrowth of the cloaca peculiar to 

 birds. 



In conclusion, I may briefly recapitulate the chief conclusions arrived 

 at in this paper : 



(1) That the bursa Fabricii exists in both sexes, and probably in all 

 species, of birds. 



(2) That it is most developed in young birds, but becomes atrophied 

 and more or less obliterated in adults, the period, however, of the com- 

 mencement and conclusion of this process differing greatly in various 

 birds. In some it probably persists, though in a state of functional 

 inactivity, throughout life. 



(3) That in the majority of birds the bursa is a moderate-sized or 

 small sac, that opens by a narrow aperture on the dorsal wall of the 

 cloaca into the lowest " chamber " of that organ. 



(4) That in the Struthious birds, on the contrary (the state of Apteryx 

 as regards these points being doubtful), the cloaca opens into the bursa 

 by a posterior aperture, owing to the fact that the bursa is not constricted 

 off at the neck, but is commensurate in extent with the third or outer 

 chamber of the cloaca, the two being united into one. This condition, 

 however, is only to be found in young birds. 



(5) That the bursa is a glandular organ, of which lymphatic follicles 

 are the essential constituents, but has no exact homologue in other 

 classes of Yertebrata. 



* Signer Alesi, in his paper, s. <?., alludes to this pouch as being ventral in position, 

 which it certainly is not. It is figured in Squafina vulgaris by Gegenbaur (Vergl. 

 Anatomic, fig. 267 c, & p. 798). It is absent in Chimcera. Leydig has described its 

 structure (' Beitrage z. microscop. Anat. u. Entwickel. der Rochen u. Haie,' Leipzig, 

 1852), and found that it consisted of collections of glands similar to the glands of 

 Brunner. 



