551 



the thymus -placode of the spiracle. The original preparations have 

 been re-worked some half dozen times, in 1899 a complete new set 

 of sections of Raja up to embryos of 42 mm was made, 70 figures 

 have been carefully drawn under a 2 mm apochromatic of Zeiss, and 

 many pages of notes have been written. 



The result of the repeated study of the older and of the newer 

 sets of preparations has invariably been the same, and it does not 

 permit of the alteration of anything of the least importance in the 

 manuscript of 1898. 



There is hardly an organ in the vertebrate body about whose 

 functions in the embryo and at later periods so little is really estab- 

 lished as the thymus. Since Koelliker discovered its mode of origin 

 in Mammals from the epithelium of a gill-pouch, and since he stated 

 that the original epithelial cells gave rise to lymph-cells or leucocytes, 

 two views have been held concerning it. On the one hand, Stieda 

 and His have asserted, that the leucocytes, which are undoubtedly 

 present in the thymus of any late embryo or foetus, as well as in 

 that of older animals, have migrated thither from the exterior, pos- 

 sibly from the mesoblast. In this conclusion they have been supported 

 by the researches of Dohrn, Gulland and Maurer, and by almost 

 every text-book of Embryology and Comparative Anatomy published 

 since 1879. On the other hand, Koelliker has stoutly maintained his 

 original position, and in recent years the results of his investigations 

 have been emphatically confirmed by Prenant, Oscar Schultze, and 

 myself, and, finally, by Maurer. 



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 yet been made to convert this latter supposition into fact 

 by systematic investigation. 



On this view, then, nothing of the least practical use and value 

 is at present known regarding the nature of the thymus. 



According to Koelliker the original epithelial cells of the thymus 

 give rise to lymph-cells or leucocytes. And although he made no at- 

 tempt 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 three most recent students of the thymus have 

 entirely accepted Koelliker's result. 



