363 



error, but, noue the less, Dohrn's observation is incorrect, if it be 

 meant to apply to the developmental origin of the thymus in any of 

 the Scylliidae. When this first origin of the thymus in these fishes 

 is traced, the thymus-elements are seen to be derivatives of the super- 

 ficial epithelium, exactly as in Raja. 



Summing up my observations on the structure, identified as the 

 thymus of the spiracle in Raja, it agrees with an ordinary thymus- 

 element in the following points: 



1) It arises as a placode of the gill-pouch, and with the rupture 

 of this latter it comes to be "epiblastic" in position. 



2) In late phases it acquires a covering of ordinary epiblast. 



3) Connective-tissue septa grow into it. 



4) Blood-capillaries penetrate it. 



5) Its epithelium gives origin to leucocytes. 



6) At a later period it becomes more or less constricted off from 

 the branchial epithelium, but, apparently, unlike a thymus-element, 

 not completely. 



That is to say, it passes through all the evolutions of a true 

 thymus-element. 



As already stated, this rudimentary thymus-element of the spiracle 

 is undoubtedly identical with the structure, described by Van Bem- 

 MELEN under the name of the ventral vesicular follicle of the spiracle ^). 

 From his account it seems pretty clear, that Van Bemmelen's re- 

 searches on it begin at about the period where mine cease. It is 

 strange, though perhaps not inexplicable, that he found no leucocytes 

 within it. He does, indeed, take into consideration its possible thymus- 

 character, albeit only to reject it in favour of the supposition, that 

 it may represent the rudiment of a gill-cleft. 



My own recognition of its thymus-nature dates back several years, 

 and the supposition of that time is now backed up by overwhelming 

 proofs of its correctness. 



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

 spalten bei Elasmobranchiern. Mitteil. a. d. Zool. Stat. zu Neapel, Bd. 6, 

 1885, p. 173. 



