322 GEO. A. THIEL AND HAL DOWNEY 



for the condensation of mesenchyme about the arteries, but also 

 for the products derived from the mesenchyme of the two regions. 



Cells of the 'myeloid' series are usually not found in the follicles 

 or lymphoid sheaths, but we are not justified in stating that they 

 never occur in these regions. In a 17-cm. pig embryo the writers 

 noted a few myelocytes in the peripheral portion of an arterial 

 sheath. The surrounding portion of the pulp contained none of 

 these granulocytes. At this stage, and up to 30 cm., the lafter 

 are usually differentiated from hemocytoblasts immediately after 

 they are cut off from the condensed mesenchyme in the extreme 

 marginal portions of the organ, far removed from either arteries 

 or venous sinuses. The occasional occurrence of myelocytes 

 within the arterial sheaths, therefore, seems to indicate that they 

 are differentiated from lymphocytes within the sheath, i.e., within 

 the lymphoid part of the spleen. 



Attention may be called to the fact that Hertz, working with 

 experimental myeloid metaplasia, saw accumulations of large 

 lymphocytes around arteries of the pulj). These were inter- 

 preted as the beginnings of new follicles, and since he saw pro- 

 myelocytes and myelocytes in these collections he believes that 

 myelocytes can be found in the follicles. Werzberg, on the other 

 hand, working with similar material, never found myelocytes in 

 the follicles. Dominici states that the myeloid metaplasia of the 

 spleen caused by repeated hemorrhages in rabbit may also involve 

 the periphery of the follicles. In his experiments small lympho- 

 cytes seemed to be the mother cells of the other forms of lympho- 

 cytes. Weidenreich ('11) also states that during myeloid meta- 

 plasia the peripheral portion of the follicles may be involved, 

 and that to the extent that the large lymphocytes come under 

 the special conditions of the pulp they may differentiate into 

 myelocytes in place of forming small lymphocytes. 



AH of the facts enumerated above seem to indicate clearly 

 that the lymphoid portion of the spleen is not composed of tissue 

 which is fundamentally different from that of the pulp, but that 

 the special conditions or 'environment' of the pulp cause or permit 

 differentiation along myeloid lines of cells which are identical 

 with and derived from the same source as the cells of the follicular 



