300 VERA DANCHAKOFF 



small lymphocytes are differentiated. According to new envi- 

 ronmental conditions the mesenchymal cells may develop into 

 typical connective tissue. Again giant cells appear as a reaction 

 of the mesenchymal cells around the necrotic centers. Thus the 

 study of the spleen development in normal conditions and after 

 stimulation adds strong evidence for the conception that at least 

 one of the factors for the differentiation of the polyvalent stem 

 cells consists in the physicochemical conditions to which the 

 cell is submitted. 



If the environmental conditions which determine the vari- 

 ous differentiation of the stem cells are easily traced, it is much 

 more difficult to understand how differentiation of the stem cells 

 takes place simultaneously with their uninterrupted multipli- 

 cation. The process of differentiation affects both the cyto- 

 plasm and the nucleus. Specific substances are developed in 

 the cytoplasm. The nucleus during differentiation process 

 loses gradually its typical, nucleolus and accumulates chromatin, 

 which permanently remains in the form of intensely basophylic 

 particles. Maximow's recourse to the ''Gleichgewichtsstorung" 

 during mitosis cannot explain the persistence of the stock of the 

 young stem cells. May the persistence of the young stem cells 

 be explained perhaps by a higher rate of cell multiplication in 

 comparison with their differentiation? I do not think so. If 

 the differentiation is a result of the influence of certain condi- 

 tions upon the cell, whatever rate of cell proliferation may be 

 admitted, certain kinds of cells under definite conditions will 

 all differentiate simultaneously. The simultaneous and per- 

 manent differentiation and multiplication of the lymphoid 

 hemocytoblasts cannot be explained by a high rate of cell pro- 

 liferation . 



A group of hemocytoblasts (let us say a group of similar A 

 cells) develops in the loose connective tissue. Some of these cells 

 differentiate into B cells, or granulocytoblasts; others continue 

 to multiply as such. If the environmental conditions, as they 

 seem to appear, are similar for all these cells, how is the differ- 

 ence in their behavio.ur to be explained? Is a difference in the 

 constitution of different cells involved and does the group of 



