INTER.VAL ANATOMY. 



29 



Evolution of the Thyuins Gland. 



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

 Amphioxus, are only known otherwise to occur in the 

 remarkable worm-like creature, Balanoglossiis. 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 



ci end tb met al he;p 



Fig. II. — Anterior region of voiing AinphioxLis from left side. (Attcr W'lIXEY ; 

 tlie renal tuljules inserted after Bo\'1-:k[.) 



at, Atritim, ci. Buccal cirri, ch. Notochord. d.f. Dorsal fin-eliambers. f. Eye- 

 spot, end. Endostyle. hep. Outgrowing eia^cum ; the index line passes through 

 one of J. Mtillcr's renal papilln^. inct. Metapleural fold. iiph. Ne[3hridia or renal 

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

 tb. 'I'ongue-bars. vel. Velutn. 



thymus 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- 



