CHAPTER XIV 



THE FUNCTION OF THE THYMUS 



A. Comparative Anatomy and Development of the 



Thymus 



1. NOTHING is certainly known of the thymus in the Cyclo- 

 stomata. 



In Elasmobranchs the thymus arises on each side as epi- 

 thelial outgrowths of the dorsal gill-pockets. The number of 

 clefts which give rise to thymus elements varies in different 

 species, but it is probable that thymus buds originally arose 

 from all the clefts. 



A similar origin may be assigned to the thymus in Dipnoi, 

 Ganoids, and Teleosts : but in these groups modifications 

 occur in the directions of resorption and fusion of originally 

 separate portions. In Teleosts the separate rudiments unite 

 into a single mass which, in contrast with the course of events 

 in Elasmobranchs, remain in connection with the gill epithe- 

 lium. Growth is generally in a backward direction dorsal to 

 the branchial arches, but the position varies in different species. 



2. In Urodela the thymus arises in the form of compact 

 outgrowths from the epithelium of the dorsal gill-pockets 

 from 1 to 5. 



In the Anura the organ arises exclusively from the second 

 cleft. 



In adult Amphibians the thymus lies behind and above 

 the mandibular articulation. In the frog the gland is found 

 behind the annulus tympanicus, covered by the depressor 

 mandibube muscle. It is a small, longish, oval body, which 

 may be 2 to 3 millimetres in length. 



3. The thymus of reptiles has been specially investigated 

 by de Meuron, Van Bemmelen, and Maurer. The organ 



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