406 JOHN BEARD, 



GuLLAND, and, until quite recently, of Maurer, as well as by the writers 

 of almost every text-book of Embryology and Comparative Anatomy, 

 published since 1879. 



On the other hand, Kölliker has stoutly maintained his original 

 position, and in recent years his conclusions have been emphatically 

 confirmed by every new investigator of the development of the thymus ; 

 thus, by Prenant, Oscar Schultze, the writer, Maurer (finally), and 

 Josef Nusbaum and T. Prymak. 



According to the views of Stieda and His, the function of the 

 thymus is still absolutely unknown, and Hassall's concentric corpuscles 

 are supposed to arise from the original epithelial cells. No serious 

 attempt has ever yet been made to convert this latter supposition 

 into fact by systematic investigation. It assumes these degenerative 

 structures to be essential and integral parts of the thymus, and this 

 is not the case. It ignores the fact, that even in mammals, 

 such as the common rabbit, the epithelial cells of the 

 thymus become converted into leucocytes, thus trans- 

 forming the original epithelial organ into a lymphoid 

 structure, long before Hassall's corpuscles appear upon 

 the scene! 



Under this view by hypothesis the original epithelial cells, or their 

 remains, become transformed into concentric corpuscles, by hypothesis 

 again leucocytes wander in from the surrounding mesoblast, where in 

 some not yet understood way they have arisen from mesoblastic cells. 

 Therefore, of the leucocytes themselves of the thymus nothing of the 

 history is known. 



From first to last, from beginning to end, in this 

 series of supposed events nothing is certain, all rests 

 upon — hypothesis! 



On this view nothing of the least practical use and value is at 

 present known regarding the nature of the thymus. According to 

 Kölliker, the original epithelial cells give rise to lymph-cells or 

 leucocytes. And, although he made no attempt to explain Hassall's 

 corpuscles, it must be evident, that, if his conclusion be correct, 

 something of positive value is thereby established. As already stated, 

 the most recent students of the thymus have with one accord accepted 

 Kölliker's result. 



None the less, in the latest edition of Wiedersheim's "Ver- 

 gleichende Anatomie" (1898) the author felt obliged to write: "über 



