HISTOLOGY OF THE THYMUS 321 



appear among the growing cells at the margin. Where these 

 lie against the cover-glass, they may retain their irregular shape 

 and plasmatic processes, but the majority are globular (fig. 10). 

 Transitions between these phagocytic cells and the growing 

 healthy cells are evident, and their origin from the reticular cells 

 could be definitely proven by studying serial sections of the 

 growing tissue. 



The lymph-nodes also show a growth of the fixed elements 

 begining usually on the second or third day, and almost invari- 

 ably as radially directed sprouts. As the growth proceeds, the 

 fragment becomes surrounded by a halo of spindle or stellate 

 cells, which resemble in all respects, growing connective tissue 

 cells from other organs (fig. 9). As compared with the growing 

 reticular cells of the thymus, the cell processes are often more 

 numerous and rather of the dendritic type with secondary branch- 

 ing. There is often a fibrillar differentiation on the surface of 

 the protoplasm. The nuclei are smaller than those of the thy- 

 mic cells and stain more intensely. Rarefaction of the central 

 fragment does not occur, or at least not until a much later stage 

 of growth. It is never so pronounced as in the thymic cultures. 

 While it is occasionally possible to find a few lymphoid cells 

 enclosed within a larger connective tissue cell, phagocytosis is 

 far less conspicuous than in the thymus, and large globular cells 

 stuffed with ingested small cells are never seen. Either the 

 reticular cells of the thymus possess phagocytic properties which 

 the lymph gland reticular cells do not have to the same degree; 

 or the small thymic cells are more susceptible to phagocytosis 

 than the lymphocytes of the lymph gland. The former expla- 

 nation is the more plausible. The behavior of the small cells 

 in thymus and lymphoid tissue, has been found to be the same 

 in all other respects; whereas the difference in structure and 

 origin of the reticular cells in the two tissues makes a difference 

 in function the more probable. 



