322 ALWIN M. PAPPENHEIMER 



CONCLUSIONS 



Minute granulae of a type not hitherto described, were demon- 

 strated in the frog's thymus, by the use of Benda's mitochondrial 

 method. Larger gentianophile granules and droplets were found 

 in some of the cells of this type. Whether these were secretory 

 or degenerative in nature was not determined. 



Granulae, possibly of the same nature as those demonstrable 

 by the mitochondrial methods, were shown to be present in 

 the living cells by the use of vital stains. 



The small thymic cells also contain granulae, and in this 

 respect, the small thymus cells are identical with the lymphocytes 

 of the blood. This observation is in direct opposition to that of 

 Scliridde (29), that the small thymus cells, which he believes 

 with Stohr (20) to be of epithelial origin, may be differentiated 

 from true lymphocytes by the absence of granulae. 



Tn clotted plasma cultures, there is a radical difference in the 

 behavior of the small and large thymus cells. The former 

 show practically no capacity for further proliferation, but after 

 a period of active motility, undergo degeneration; the latter 

 exhibit active growth, often in the form of syncytial cell masses. 

 They are actively phagocytic towards the degenerating small 

 thymus cells. 



This characteristic difference in the behavior of the two types 

 of cells is opposed to Stohr's view that the small cells are modi- 

 fied epithelial reticular cells and that transitions between the 

 two normally occur. 



The small cells of the rat thymus show absolutely no rnorpho- 

 logical differences from the lymphocytes of the lymph nodes; 

 they exhibit the same active motility and the same proneness 

 to undergo degeneration when kept in vitro. 



The growth of rat thymus differs from that of lymph nodes 

 (1) in the early rarefaction of the implanted fragment, with the 

 appearance of numerous large phagocytic cells; (2) in the for- 

 mation of tissue-like planes composed of epithelial reticular cells 

 differing in their appearance from the fusiform or stellate cells 

 which grow from the connective tissue capsule or reticulum of 



