573 



from the thymus or as the direct descendants of such, have therefore 

 no basis of fact in their favour. 



3) The thymus alone is sufficient to account for all the leuco- 

 cytes of the body, and since Schaffer demonstrated its existence in 

 Marsipobranch fishes, a discovery which I can fully confirm, we now 

 know it to be an organ characteristic of all true vertebrate animals. 

 Any other source of leucocytes is superfluous. 



4) Except in the cases of organs, which are paired or form parts 

 of a metameric segmentation, we do not seek for two organs with 

 the same function in any vertebrate animal. The thymus is a paired 

 metamerically repeated organ, if the gill-region be metameric; but its 

 development is confined to that region in all known cases. In the 

 trunk-region of no vertebrate is the homologue of any organ in its 

 branchial region encountered. 



It must, therefore, be heid — and in my humble opinion the 

 contrary is impossible of proof — that the thymus is the 

 parent-source of all the lymphoid structures of the 

 body^). This conclusion throws light upon one of the teachings of 

 embryologists, histologists, and pathologists, that the thymus is an 

 example of organ, which, after assuming function in early life, atrophies 

 at a later period. 



This is only certainly known to happen in mammals, and from it 

 the inference is drawn, that in later life the organ ceases to exist. 

 It no more ceases to exist than would the Anglo-Saxon race disappear, 

 were the British Isles to sink beneath the waves. 



The simile is a real one, for just as the Anglo-Saxon stock has 

 made its way from its original home into all parts of the world, and 

 has there set up colonies for itself and for its increase, so the original 

 leucocytes, starting from their birth-place and home in the thymus, 

 have penetrated into almost every part of the body, and have there 

 created new centres for growth, for increase, and for useful work for 

 themselves and for the body. 



1) Compare Flemming's prophetic words, written in 1895, i. e. : "es 

 wird . . . kaum zu bezweifeln sein, daß es (i. e. die Thymus) während 

 der Periode seiner vollen Ausbildung der Neulieferung von Lymph- 

 ziellen ebenso dient, wie später die wahren Lymphdrüsen und lymphoiden 

 Organe". Here, however, it is not stated, that the thymus must be 

 regarded as the source of all the leucocytes of the body. 



