477 



of embryos of Raja batis, I feel that at any rate something can be 

 added to our knowledge of the organ. 



A detailed description of the development of the organ will not 

 be given. Such an account would recapitulate many of the results 

 of Dohen, de Meueon and Maueee. 



Early Development of the Thymus. 



As Dohen 1 ) and Maueee 2 ) have shown the thymus first appears, 

 at no very early stage, as a slight thickening of the dorsal aspect of 

 each of the true branchial clefts. This thickening of the epithelium 

 is only very slightly within the cleft itself, and thus it is somewhat 

 difficult to say if it be of epiblastic or hypoblastic origin. Since, how- 

 ewer, the gill-pouches grow towards and break through the epiblast 

 without the cooperation of any appreciable depression of the latter, 

 there can, I think, be little doubt that the organ is, in the main, hypo- 

 blastic in nature. 



In the part of "Studie no IV" treating of this subject, Dohen 1 ) 

 states "An diesen knospenförmigen Wucherungen nimmt nur die innere 

 Schicht des Epithels Anteil, die äußere zieht in dünner Lage darüber 

 weg". 



There seems to me to be an error in this. If it were correct, the 

 thymus would be a derivative of a deep layer of cells lining the cleft. 

 In none of my sections does an outer layer of epithelium pass over 

 the thickening which represents the commencing thymus. More pre- 

 cisely, the whole thickness of the epithelium in the circumscribed area 

 in which the thymus of each gill cleft develops is concerned. 



The point must be emphasised, because it proves that the thymus 

 is actually a portion of the lining of the cleft. The thymus 

 in Raja arises, as Dohen states, from all five functional gill-clefts. 

 In Mustelus, Scyllium and Pristiurus (Dohen p. 43) the thy- 

 mus of the first three visceral cleft grows to a large size, while that 

 of the fourth remains small, and that of the fifth degenerates shortly 

 after its appearance. 



In Raja the thymus of the first four branchial clefts attains 

 large dimensions, while that of the fifth remains small 3 ). The latter 



1) Mitteil. a. d. Zool. Station zu Neapel, 1884, p. 42. 



2) F. Maueee, Schilddrüse und Thymus der Teleostier. Morphol. 

 Jahrb., Bd. 11, 1885, p. 150. 



3) Compare Dohen, Studie No. IY p. 43 : "Bei den genannten Rochen 

 (Torpedo, Raja) wird dagegen auch die Wucherung der vierten Spalte 

 bedeutend, und die fünfte bleibt klein." 



