No. 607] DIFFERENTIATION IN THE ORGANISM 



427 



groups of cells appeared in the spaces between the mesen- 

 chymal cells, which already had be^n their erythro- 

 blastic differentiation, while within the vessels. These 

 cells, now outside the vessels, proliferate and continue 

 their differentiation into erythroblasts, and their cyto- 

 plasm is gradually transformed into or substituted by 

 homogeneous hemoglobinic substance. 



The changes undergone by a polj-v^alent hemoblast 

 within the vessels are thus no longer reversible outside 

 of them. The differentiation determined by environ- 

 mental conditions has been rendered specific, i. e., uni- 

 valent and irreversible. The specificity of tissue and 

 cells can not therefore be the result alone of segregation 

 of different cytoplasmic materials during cleavage. The 

 process of segregation, of course, transfers different 

 materials to different cell groups, the presence of which 

 impedes their permutability, but these cell groups are still 

 polyvalent and may, under various conditions, undergo 

 various development. 



The relations between these cell groups, the structures, 

 effected by them, the different products of their metabo- 

 lism, form the external factors of the environment which 

 gradually render the cells of a jwlyvalent group specific, 

 univalent and in-eversible in their potencies. This speci- 

 ficity is tnnisiniitcd ])y inollicr cells to their daughter 

 cells ii'i'('s|n'cti\-('ly of tlio ciivi I'oiiinotital conditions, to 



A few woids coiiccniiim' the structural changes of the 

 cells during their definitive si>ecialization. Differentia- 

 tion during elt'avau'e is et't'eeted by transmission of differ- 

 ent cytoplaMiiic niat.'i-ial^ to different cell group<^. What 

 kind of .'lintmt^ in tin- <vll Mru.-tiire are induced by the 

 (A-tcnml lactnr-- <.r liie environment.' ( \un])ai-e tlu-l nic- 

 tinv ot' t'.ie niatnic univalent bloo<l cdU N\ith that of their 

 motli.-r ceiU in the Ma-e of a heinohlaM ('Ntopia^n. 

 structni-c of the M-Min- nnclcn^. <-l,roninM,nic conn -Icxei 

 dnrinu mito-i^. a^ demonstrated hv oni mici o^cpical 



