THE THYMUS 331 



but throughout life, the thymus has the characters of an 

 epithelial organ. But there are undoubtedly leucocytes 

 present, and Hammar believes, as did the older observers, 

 that these are of mesodermal origin, invading the gland 

 secondarily, and there undergoing further proliferation. 



Stohr believes in the epithelial origin and nature of the 

 small thymic cells. These arise in situ from the repeated 

 multiplication of the reticular epithelium, and though morpho- 

 logically they come to assume a structure indistinguishable 

 from that of the true lymphocytes, they remain throughout 

 true epithelial elements. 



We have, then, four prevailing theories as to the nature of 

 the thymus element : (l)That the original epithelial elements 

 are entirely replaced by a leucocytal invasion from the meso- 

 derm, and that the thymus in its fully developed condition 

 is a lymphoid organ ; (2) that the original epithelial elements 

 give rise to lymphoid cells in situ, and that the thymus becomes 

 a lymphoid organ ; (3) that the thymus remains an epithelial 

 organ, but that there are lymphoid elements which have 

 invaded it ; (4) that there are no true lymphoid elements, 

 but that the small thymic cells, which appear to be of a 

 lymphoid nature, are, in reality, epithelial. This last is the 

 view of Pappenheimer. 



If this last view be correct, we should expect to find some 

 differences between these cells and lymphoid elements. The 

 present writer has given some little attention to the structure 

 of the thymus (apart from embryological considerations), 

 and it would seem that many of the thymus cells are indis- 

 tinguishable from the lymphoid cells. But the matter is 

 still in a very doubtful state, and needs further investigation. 

 The question is not only of morphological, but also of con- 

 siderable physiological, importance. If the thymus cells are 

 in truth epithelial throughout the life of the organ, we may 

 with reason look for an internal secretion in the sense in 

 which we defined it in an earlier chapter. On the other 

 hand, if the cells be finally included among lymphoid ele- 

 ments, then we shall expect to find that the functions of the 

 thymus are allied to, if not identical with, those of lymphatic 

 glands. 



Hammar's most recent view is that the thymus is an epithe- 

 lial organ with a leucocytal infiltration. Hassall's corpuscles 



