484 



Gustav Killian 1 ). This memoir closes with a long and interesting 

 discussion which cannot be summarised without depriving it of much 

 of its cogency. 



Killian's observations and reasoning — and the latter seems to 

 be convincing — show that in the "Gaumentonsillen" and "Rachen- 

 tonsillen" we deal with structures concerned in the "Abwehr des Orga- 

 nismus gegen eindringende Schädlichkeiten, namentlich bakterieller 

 Natur". 



If the higher Vertebrates be provided with such organs in connec- 

 tion with their respiratory passages, would it appear strange if corre- 

 sponding arrangements were met with in association with the gills of 

 fishes ? 



These are certainly very delicate organs, and by reason of their 

 build structures liable to injury and desease of various sorts. No 

 organs of fishes are more frequently the seat of parasites. 



In default of a better explanation the thymus of fishes may there- 

 fore be regarded as a lymphoid organ specially differentiated from 

 the gills themselves or from the gut-outgrowths which preceded the 

 latter, for the protection of the gills in the ways above suggested. 



Hassall's Corpuscles of the Thymus. 



The consensus of opinion regarding these is that they are epi- 

 thelial and epiblastic in nature. This may be at once conceded — 

 a superficial examination of sections of epitheliomata shows the close 

 similarity between the two. Dohrn and Maurer both look upon these 

 bodies as, in the words of Maurer, "von Epithelzellen gebildet, welche 

 die Reste der epithelialen Anlage des Organes zum Teil darstellen" 

 (p. 170). Dohrn (p. 51) goes much further than Maurer in this 

 matter. As I unterstand him, these curious structures would represent 

 the elements, or their descendents, which had been proliferated from 

 the cleft. Maurer speaks more cautiously, and seems to incline to 

 the opinion that they correspond to some only of such elements. 



As yet I have met with no true and undoubted corpuscles in the 

 thymus of the skate, but in sections from advanced embryos one sees 

 here and there in the organ a cell (or a few) not of lymphoid nature 

 but bearing a close resemblance to an ordinary epiblastic cell of the 

 same animal. 



1) Gustav Killian, Ueber die Bursa und Tonsilla pharyngea. Morph. 

 Jahrb., 1888, Bd. 14. — A separate copy is quoted from. 



