478 



does not appear to atrophy, for I find it still present and possessing 

 the characters of normal thymus in embryos of large size. 



The views I have been led to adopt as to the nature of the 

 thymus and its close connection with the organs of respiration in fishes 

 naturally caused a search, already made by others, for thymus elements 

 in connection with the spiracle and mouth. In the spiracle of Raja, 

 although the epithelium of the dorsal wall of the cleft becomes thickened, 

 there is never any proliferation of true thymus elements. Nor are any 

 such formed in connection with the mouth. 



This is quite in parallel with the behaviour of the thymus in the 

 gill-region itself. The reduction of the number of gills, even in the 

 Elasmobranchii in which the last cleft only possesses one half-gill on 

 its anterior surface, leads to a corresponding lessening in the number 

 of thymus elements. Thus in Teleosteans there are only four such 

 elements on each side (Maueer), and from the researches of von 

 Koelliker, de Meuron and others it is known that this number under- 

 goes still further decrease in higher forms. 



If thymus elements ever existed in connection with the once gill- 

 bearing spiracle, it appeared just possible that traces of such in some 

 altered form might be met with. And this was perhaps true of the 

 mouth-cleft. 



Recalling the interesting researches of Van Bemmelen ] ), it seemed 

 not unlikely that some of the structures he described as "vermutliche 

 rudimentäre Kiemen" might bear an interpretation such as that just 

 indicated. 



Reference to his paper showed that the author himself had al- 

 ready discussed such a possibility — albeit only to reject it. 



Since 1884, when Van Bemmelen's work was written, the quest 

 after rudimentary gill-clefts has somewhat abated, and morphologists 

 are less inclined to set down any and every problematical organ in 

 the Vertebrate head to modified remains of such structures. I incline 

 certainly to the view which would consider both the vesicular follicle 

 of the spiracle and that of the angle of the mouth as possibly the 

 rudimentary aequivalents of thymus elements of these parts. 



At any rate the whole history of these structures harmonises with 

 this supposition 2 ). 



1) J. F. Van Bemmelen, Ueber vermutliche rudimentäre Kiemen- 

 spalten bei Elasmobranchiern. Mitteil. a. d. Zool. Station zu Neapel, 

 Bd. 6, p. 165—184. 



2) Van Bemmelen, op. cit. p. 173 — 179. 



