THE LIFE CYCLE 57 



accumulated as a structural component of the cell is now broken 

 down, oxidized, and eliminated, may bring about dedifferentiation, 

 but it is not necessarily a reversal of reaction in the chemical sense, 

 for the breakdown and ehmination of the substance may be a 

 different process dependent upon different factors from its syn- 

 thesis out of nutritive substances. 



In order then to avoid the possibihty of confusion, it is prefer- 

 able to regard development, not as reversible, but as regressible. 

 Differentiation is a progression from one condition to another, 

 dedifferentiation a regression, but perhaps through stages very 

 different from the stages of progression. 



Apparently not all differentiated cells are capable of dediffer- 

 entiation to the embryonic condition; at least dedifferentiation 

 fails to occur in many cases under any conditions with which we 

 are familiar. In general, less highly differentiated cells undergo 

 dedifferentiation more readily and more completely than more 

 highly differentiated; consequently dedifferentiation is much 

 more conspicuous in the lower than in the higher forms, although 

 even in man some cells are capable of more or less dedifferentiation. 

 This limitation of dedifferentiation, as well as the advance of differ- 

 entiation, in the course of individual development and evolution, 

 suggests again an increase in the physiological stabiHty of the 

 cellular substratum. 



Dedifferentiation may be brought about in cells capable of it 

 either by forcing the cell to use up its own substance as a source 

 of energy and so undergo reduction, as in starvation, or by isolating 

 the cell from the action of the correlative factors which have 

 brought about differentiation, and in some cases, and to a certain 

 degree, simply by increasing the rate of metabohsm of the cell by 

 stimulation or otherwise. Reduction, except perhaps in embryonic 

 cells, is probably impossible without some degree of dediiTerentia- 

 tion, but dedifferentiation may occur without reduction. Since the 

 differentiated cell has in general a low rate of metabohsm as com- 

 pared with the embryonic cell, and since the decrease in rate is 

 associated with differentiation, we should expect that an increase 

 in rate would occur during dedifferentiation, and this, as will appear, 

 is apparently the case. 



