318 ALWIX M. PAPI'ENHEIMER 



cells of this tj'pe, nor IVoiii pieces of spleen, heart muscle or 

 intestinal wall used as controls. 



That these cells were derived from capillar}^ endothelial cells 

 seemed improbable. The manner of their j];rowth and their fre- 

 quent origin from small masses of cells in which the absence of 

 capillaries could be determined with certainty, seems to exclude 

 this possibility. More positive evidence in favor of their origin 

 from the reticular cells, is that they may, in thin portions of 

 the culture, resemble closely the normal protoplasmic cellular 

 framework of the gland. Although the protoplasm shows a 

 fibrillated structure, evident especially in fixed material, there is 

 no formation of definite fibrils upon the surface of the cells, nor 

 do the plasmatic processes of the cells, no matter how long- 

 drawn-out, resemble the more rigid and refractile processes of 

 the growing connective tissue cells. In their power of phago- 

 cyting the small cells, they function as do the normal reticular 

 cells of the thymus. But since other cells in vitro may assume 

 phagocytic powers, too much emphasis should not be laid upon 

 this point. 



If the assumption be correct that the growing cells of these 

 cultures are derived almost wholly from the reticular cells, then 

 we must, if we accept the prevailing view as to the histogenesis 

 of these elements, hold them to be epithelial in nature. The 

 epithelial origin of the thymic reticulum is believed in by all 

 the recent workers on the structure of the thymus, including 

 Hammar (27), Stohr (28), Schridde (29), Maximow (30), and 

 C'remieu (31). Dustin (32) and Pigache and Worms (33), 

 amongst recent writers, still hold to the old view that the thy- 

 mus, like the lymph-glands, has a fibrous reticulum.'' Salkind 

 (34), in a recent paper, takes an intermediate position, claiming 

 to have demonstrated by special methods, a fibrous reticulum 

 analogous to that of lymph-glands, coexisting with the epithelial 

 reticulum and giving origin to the lymphoid cells. If his views, 



' For a complete discussion of the histogenetic origin of the thymus elements, 

 the reader is referred to the exhaustive reviews of Hammar (27) (Ergcbnisse d. 

 Anat. u. Entwick., 1910, Bil. 19, p. 1) and of Wicscl (35) (Lubarsch and Ostertag's 

 Ergcbnisse d. Allg. Path. u. Path. Anat., 1911, Bd. 15, 2, p. 416). 



