HISTOLOGY OF THE THYMUS 319 



which are counter to the prevalent conceptions of the thymic 

 reticuhim, prove correct, the interpretation of these growing 

 elements as epithelial in nature, would be rendered less probable. 

 The cells in question might then be derived either frona the 

 fibrous or the epithelial reticulum. Nevertheless, it is certain 

 that in the mammahan thymus at least, it is impossible to put 

 in evidence such a fibrous reticulum, either with Mallory's ani- 

 line blue method or with the silver impregnation method of 

 Bielschowsky, both of which bring out with great clearness the 

 reticular fibers of the lymph nodes. Until Salkind's work re- 

 ceives further confirmation, it seems unwise to revise again the 

 current and well founded conception of the exclusively epithelial 

 origin of the thymus reticulum. 



Much emphasis has been laid by Lambert and Hanes (36) 

 upon the differences in the character of in vitro growth of epi- 

 thelium and connective tissue. The former tend to form solid 

 coherent growths, while in the latter, the cells, though often 

 connected by plasmatic processes, tend to remain isolated from 

 one another. As a general distinction, this has undoubtedly 

 proven to be true. The tissue which we have been studying 

 forms at times an apparent exception since, though epithelial, 

 it may grow either in coherent sheets of closely apposed cells, 

 or as a loose anastomosing network, or indeed, these cells may 

 lose their plasmatic connections with adjacent cells and wander 

 isolated far out over the surface of the plasma. This variability 

 in the manner of growth is not surprising if we recall the normal 

 development of the parent tissue, and the changes which it 

 undergoes. Beginning as a solid outgrowth of cells, the thymic 

 epithelium early becomes rarefied into a protoplasmic syncytium 

 in the meshes of which lie the small cells. It is probable also 

 that under certain abnormal conditions, the reticular cells of the 

 mammalian thymus assume a rounded form and become rela- 

 tively independent of their cellular connections, often, too, exer- 

 cising a phagocytic function upon the small thymic cells (Rud- 

 berg (37), Pappenheimer (7), Cremieu (31) and others). That 

 the thymic epithelium should differ widely in the manner of its 

 growth from the epithelium of glandular organs or of solid epi- 



