30 



ANATOMY OF AMPHIOXUS. 



ing Craniota. Amphioxus has hitherto been regarded 

 as forming a marked exception among the Vertebrates 

 in having no thymus, whereas one might reasonably have 

 expected to find here the representative of the thymus 

 in full activity. Although contrary to the prevailing 

 impression, I would suggest that the thymus is repre- 

 sented in Amphioxus by the very actively functional 

 tongue-bars. 



DoHRN has shown that in the Selachian (shark) 

 embryo the thymus arises by a series of distinct cell- 

 proliferations from the epithelium of the dorsal wall of 



the successive gill-slits witli 

 the exception of the first, 

 which is the spiracle (Fig. 

 12). Sometimes these pro- 

 liferations cause a small pro- 

 jection downwards into the 

 gill-slit, comparable to an 

 incipient tongue-bar. Event- 

 ually these separate thymus 

 rudiments pass inwards and 

 come together so as to form 

 the definite thymus gland. 

 J"f^ " Dohrn concluded from its 



Fig. 12. — Horizontal section through mode of Origin that the 

 the branchial region of an embrvo of , 



&v///«m fa;»«/a to show the rudiments thymus resulted from the 

 of'the thymus. (After DoHRN.) inetamorphosis and intro- 



sp. Spiracle, cav. CavUy (ccelom) of _ m^L^j 



branchial bar. I, II, III. First, second, Version of gill-filaments ; and 



and third gill-pouches, jug.v. Jugular . ■ ,, r r , . 



vein. %. Thymus rudiments. m point ot tact, this View of 



its morphological nature is 

 probably correct. But the tongue-bars of Amphioxus 

 which correspond closely in position to the thymus rudi- 

 ments in the Selachian embryo, and are, like the latter 



%; 



