EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 293 



appearance seems to be purposeful, I do not feel right to take 

 them for such. It seems more correct to attribute their appear- 

 ance to a reaction similar to that which characterizes the develop- 

 ment of macrophages — reaction to particulate matter, Evans (15). 

 The considerable hypertrophy of their cytoplasm is brought by 

 digestion of phagocytozed material. Smaller necrotic centers can 

 undergo a complete resorption by giant cells. The formation of 

 giant cells around necrotic centers can serve as an example of a 

 new differentiation potency assigned to mesenchymal embryonic 

 cells. 



Besides deficiencies in the development of veinous vasculariza- 

 tion, irregularities in the development of arterious vasculariza- 

 tion may also be observed. A partial lack of development of 

 arterious ramifications in districts of the spleen is a frequent 

 occurrence under the conditions of the experiments. A partial 

 deficiency in the veinous vascularization usually coincides and 

 this leads to a nearly complete lack of vascularization in parts 

 of the spleen. In such regions mesenchymal cells are still capable 

 of proliferation, but they soon develop very differently than they 

 do normally, namely^ they become fibroblasts. Large regions of 

 the spleen can be transformed into typical connective tissue (fig. 

 23). The elongated cell bodies may appear pressed closely 

 together, in other instances they are separated by accumulations 

 of collagenous fibers. A few mitoses are usually observed in 

 these cells. Sometimes the presence of a few lymphoid hemocyto- 

 blasts or granular leucocytes may indicate a more intense 

 hematopoietic differentiation which previously has taken place 

 in such regions. 



The development of fibrous tissue in the embryonic spleen 

 completes a .whole range of transformations to which the young 

 mesenchymal cells are capable. The leucocyte, the erythrocyte 

 and perhaps the fibroblast and the macrophage are under normal 

 conditions final non interchangeable diffei-entiation stages; the 

 endothelial, the giant and reticulum cells are different morpho- 

 logical structures, which a mesenchymal cell may assume. All 

 these cells derive from one stemcell, and its differentiation depends 

 upon the various conditions to which the polyvalent cell is 

 submitted. 



