IN TERNAL ANA TOM Y. 



2 9 



Evolution of tJic Tliymns Gland. 



Tongue-bars, like those occurring in the gill-slits of 

 Amphioxus, are only known otherwise to occur in the 

 remarkable worm-like creature, Balanoglossus. In the 

 higher Vertebrates they appear to be entirely absent, but 

 in the course of the development of the higher forms 

 there is a structure which arises from the dorsal wall 

 of the gill-slits which may very well be the homologue 

 of the tongue-bars of Amphioxus. This structure is the 



olf vel ph.b nph 



nt 



ch 



c 



eiid 



Fig. it. Anterior region of young Amphioxus from left side. (After WlLLEY ; 

 the renal tubules inserted after BOVERI.) 



at. Atrium, ci. Buccal cirri, ch. Notochord. d.f. Dorsal fin-chambers, e. Eye- 

 spot. end. Endostyle. hep. Outgrowing coecum ; the index line passes through 

 one of J. Miiller's renal papillae. met. Metapleural fold. nph. Nephridia or renal 

 tubules, nt. Spinal cord. olf. Olfactory pit. ph.b. Peripharyngeal ciliated band. 

 tb. Tongue-bars, vel. Velum. 



thymns gland. The thymus is one of those enigmatical 

 ductless glands which are so eminently characteristic of 

 the Vertebrate organisation, and are of the utmost phys- 

 iological and pathological importance to the individual. 

 In their structure and development they give clear indi- 

 cations of having undergone an extensive change of 

 function in the course of their evolution. 



The thymus, therefore, is presumably the derivative of 

 an ancestral organ, which formerly possessed an active 

 function as opposed to the apparently passive function 

 which this gland, and others like it, exercise in the exist- 



