314 GEO. A. THIEL AND HAL DOWNEY 



small or medium-sized Ijaiiphocytes or as histogenous wandering 

 cells which become transformed into small and large lympho- 

 cytes. Some may also form granulocytes. Sabin also believes 

 in the possibility of the derivation of small lymphocytes di- 

 rectly from the mesenchyme in developing lymph nodes, although 

 she admits that the evidence for this is not very convincing. 



The writers find, howe\'er, that in the embryonic spleen of the 

 pig e\'idence for the derivation of free lymphoid cells from the 

 fixed cells is by no means lacking. Figures 6 and 7 show reticular 

 cells at widely separated stages of embryonic development, in 

 both of which numerous small lymphoid cells are being cut off 

 from the local reticulum, many of them being transformed into 

 tj'pical small lymphocj'tes without the intervention of the large 

 lymphocyte stage. Not man}^ of these smaller lymphocytes can 

 be proliferative products of large hemocytoblasts, for the latter 

 are rarely observed in the early follicular rudiments. 



According to Weidenreich and Downey (pp. 367-369), lym- 

 phocytes of various types, including the typical small lympho- 

 C3^te, and large mononuclears or macrophages can be cut off 

 directly from the reticulum of the Ij-mph nodes of adult animals. 

 Their figures 2, 3, and 4 give the details of the process as seen in 

 the interfollicular tissue of guinea-pig lymph node and in a germ 

 center of a follicle of cat node. From this it is evident that the 

 process of isolation of free lymphoid cells from the fixed tissue 

 which was seen in the early spleen persists in the hematopoietic 

 organs of adult mammals. In the adult animal all types of lym- 

 phoid cells may be derived from the fixed tissue of the same region 

 of the organ, while in the early spleen large lymphoid cells are 

 derived from the mesenchyme of the pulp and small lympho- 

 cytes from that of the future lymphoid sheaths of the arteries, 

 while the large mononuclears do not appear until later. 



The fact that the earliest small lymphocytes exhibit marked 

 ameboid properties has led different workers to believe that they 

 reached the primary folhcular rudiments by migration from the 

 pulp portion of the organ. With this interpretation in mind, a 

 careful study was made of various spleens in which the first small 

 lymphocytes were evident. In all cases the ameboid activity is 



