485 



When it is borne in mind that the thymus arises just where 

 epiblast and hypoblast unite together, it is, I think, conceivable that 

 some few true epiblastic cells might get drawn into the proliferation 

 of hypoblast. 



It is of such cells thus accidentally drawn in that I believe 

 Hassall's corpuscles to be derivatives. 



They are structures which in any case appear comparatively late, 

 and this circumstance, as well as their obvious epiblastic nature, lends 

 support to the view of such a mode of origin. Morphologically they 

 cannot possess any significance. 



The supposed Thymus of Petromyzon. 



In his paper Dohrn referred to a thymus in this form once only, 

 and promises further information about it at a subsequent stage. 

 Apparently this promise was forgotten or overlooked. The only posi- 

 vite statement we possess emanates from Stannius who describes it 

 as a paired organ behind the gills in the region of the heart. There 

 can be no doubt to my mind that the structure here referred to is 

 the degenerate pronephros, for it is exactly in this position that one 

 finds this body in Myxine. Of course the supposition must be tested 

 by observation. 



Above the gills of the adult lamprey, of the Ammocoete, and of 

 very young larval forms no thymus is to be found. 



The search was a very disappointing one, for it was to be hoped 

 that the condition of the organ in such lowly forms might throw more 

 light upon its nature. At present I incline to the belief that the 

 Marsipobranchii possess no well-defined thymus. Perhaps this stands 

 in relation to the very different structure of their gills, which, moreover, 

 here wide blood sinuses along their outer margins. 



The thymus of fishes is an organ derived from the hypblast of 

 the gill-clefts. Its original elements become converted into leucocytes, 

 which probably serve for the protection of the important organs of 

 piscine life, the gills. 



With the disappearance of the gills above fishes and Perenni- 

 branchiate Amphibians it undergoes a restriction in the area of its 

 formation and a transfer of its functions unto the service of other 

 organs than those for whose behoof it was originally formed. Probably 

 these functions remain the same as before, but they also experience 



