571 



with the necessary quotations from their works and in their own words, 

 in a more extended memoir. 



The history of the histogenesis of the thymus — and a sad one 

 it is — begins with Koelliker's researches, published in 1879. In 

 these KoELLiKER stated, that the thymus of the rabbit arose from (the 

 wall of) a gill-cleft, and that its original epithelial cells became con- 

 verted into leucocytes. 



This latter discovery is among the most brilliant ever made by 

 the honoured Nestor, who has occupied a pre-eminent position in 

 the embryological and histological research of the past sixty years. 

 But its fate has been the very reverse of that, to which its importance 

 in embryology, histology, physiology, pathology, and practical medicine 

 fully entitled it. 



Against his results were placed the conjectures and assertions of 

 Stieda and His. And, although these never obtained a satisfactory 

 basis of support in the researches of the two anatomists just named 

 or of others, although they were never really based in actual finds at 

 all, they sufficed to dominate our ideas as to the nature of the thymus 

 for nearly twenty years. According to them leucocytes wandered into 

 the thymus from the mesoblast, and the original epithelial cells of the 

 thymus gave rise to Hassall's corpuscles. 



Now, in the skate, as we have seen, Hassall's corpuscles have 

 not yet been found, and the original epithelial cells give rise to leuco- 

 cytes, as is now abundantly proved for this and several other verte- 

 brates, fishes and mammals, by the researches of Koelliker, Maurer ^) 

 (in part), Prenant, Oscar Schultze and myself. 



Moreover, how can leucocytes wander into the thymus from the 

 mesoblast, or from any other part of the embryonic body, at a period, 

 when there are none in existence elsewhere ? How can this happen, when 

 the very first source of leucocytes in the body is the epithelium of 

 the thymus itself? 



The formation of leucocytes is, therefore, as Koelliker first 

 showed, a function of the thymus. But, as my researches prove, one 

 can now go further, and state with the utmost confidence, that it is 

 the function of the thymus, not merely to form leucocytes, but to be 

 the parent-source of all the leucocytes of the body. 



In 1891 Gulland stated that the first leucocytes of the body 



1) Since the above was written, in December 1898, Maurer in his 

 latest memoir (Morph. Jahrb , Bd. 27, 1899, p. 156) has announced his 

 complete adhesion to KoELLrKER's original view. 



