GRADIENTS, FIELDS, AND DETERMINATION 303 



dedifferentiation occurs very often and have given many examples. 

 Schaxel (191 5, 192 1) and Hammerling are certain that it never occurs. 

 Weiss (1939) makes a distinction between dedifferentiation, which he be- 

 Ueves does not take place, and "modulation," which is believed to occur. 

 However, it is not evident from his discussion that modulation is any- 

 thing but a lesser degree of dedifferentiation. According to Schotte, de- 

 differentiation occurs in many cases. '^ 



Doubtless; the differences of opinion will continue until we know more 

 about what constitutes differentiation in any particular case. At present, 

 however, there is a large body of evidence indicating that more or less 

 dedifferentiation does occur in many organisms and many tissues, even 

 to some extent among the higher vertebrates. Probably cells previously 

 subjected to a developmental environment producing less stable differen- 

 tiation are more capable of reaction to altered environment than others. 

 Dedifferentiation does not necessarily involve complete loss of determi- 

 nation of cells but is alteration of structure and behavior in the direction 

 toward the more general characteristics of cells of earher stages of devel- 

 opment. Such changes certainly do occur in many cells. The assumption 

 that certain cells in adult organisms are undifferentiated, irrespective of 

 their structure and function, because they are able to become, or give 

 rise to, cells with other structure and function requires not merely asser- 

 tion but rigid proof. 



'8 O. Schotte, 1939, "The origin and morphogenetic potencies of regenerates. First sym- 

 posium on development and growth," Growth, Suppl. 1939. 



