DIFFERENTIAL GROWTH I43 



tunity to produce their specialties, either by suppressing further pro- 

 liferation in vitro or by transplanting them back into an organism. 

 Under such conditions they were essentially true to the kind from which 

 they stemmed: those derived from pigment epithelium would resume 

 the production of pigment granules (8) ; those from thyroid, of colloid 

 (13) ; those from gut, of digestive enzymes (14). Cultured cells re- 

 turned to an organism, but into atypical locations (42), moreover, 

 showed no tendency to differentiate in conformity with their new en- 

 vironment, proving that the culture period had not restored to them the 

 ability of younger cells to switch into a new local course of differentia- 

 tion. 



All this adds up to the realization that the measure of true differ- 

 entiation is the absence of true dedifferentiation, if dedifferentiation 

 connotes not the mere assumption of a semblance of primitivity but the 

 actual return to a state of wider potency, i.e. the recuperation of some- 

 thing that had ostensibly been lost. By the testimony of the reported and 

 similar experiments, vertebrate cells are incapable of such true dedif- 

 ferentiation ; hence, for them, true differentiation is the rule. 



Tissue Differentiation 



All our statements thus far refer to cytodifferentiation. Histodif- 

 ferentiation and organ differentiation require some additional comment. 

 Tissues and organs consist of cells and matrix in definite mutual rela- 

 tions and groupings. What happens during the differentiation of a 

 tissue is in large measure a direct reflection of the differentiation of its 

 constituent cells. Their order of proliferation, their movements, trans- 

 formations, affinities, secretions, etc., and the spatial arrangements and 

 restraining conditions resulting from group occupancy of a common 

 space determine, for the main part, the character and organization of the 

 tissue or organ. By their collective activity the cells set up conditions for 

 one another that would not develop if they remained single and inde- 

 pendent. For this reason, what is commonly called "histodifferentia- 

 tion" implies an added element of order and novelty over the mere 

 "cytodifferentiation" of the unit cells. This is particularly striking in the 

 formation of composite organs where the combination of cells from 

 diverse sources leads to novel results which neither of the contributing 

 components could have produced by itself. In addition to material contri- 

 butions, there are dynamic influences of mechanical, chemical, and 

 electrical nature acting from one tissue or organ upon another. 



