S8 SENESCENCE AND REJUVENESCENCE 



If the suggestions of the preceding section concerning the nature 

 of differentiation are correct, we should expect the most recently 

 developed morphological features of the cell to disappear first in 

 dedifferentiation, since these are, under the conditions existing in 

 the cell, the least stable of the substratal constituents. As these 

 are removed the rate of metabolism rises and other parts of the 

 substratum become relatively unstable and disappear, and so on, 

 until the cell once more approaches the embryonic condition. So 

 far as the course of morphological dedifferentiation has been fol- 

 lowed, it seems in general to proceed in this way and so to reverse the 

 course of differentiation. But this does not necessarily involve a 

 reversal of reaction any more than the removal of a previously 

 deposited sand-bar, by acceleration or change of course of the cur- 

 rent of a river, involves a reversal of its flow. 



The dedifferentiating cell is apparently capable at any stage 

 of resuming the process of differentiation, and if dedifferentiation 

 proceeds far enough it may, under altered correlative conditions, 

 begin a new course of differentiation and become a different kind 

 of a cell from that which it was originally. As the sand-bar formed 

 in the stream tuider certain conditions may under others be re- 

 moved and its place taken by a deep channel, and again the chaimel 

 may give place to a mud flat or a beach, so the original morpho- 

 logical differentiation of the cell may disappear and give place to 

 other kinds of differentiation as the physiological conditions change. 



THE BASIS OF SENESCENCE AND REJUVENESCENCE 



The association of a colloid substratum with a chemical reaction- 

 system and the occurrence of growth and reduction and of differ- 

 entiation and dedifferentiation lead us to a conception of senescence 

 and rejuvenescence which, as will appear in following chapters, 

 seems to be the only one which is in full agreement with the facts 

 of experiment and observation. According to this view, senescence 

 is primarily a decrease in rate of dynamic processes conditioned by 

 the accumulation, differentiation, and other associated changes of 

 the material of the colloid substratum. Rejuvenescence is an 

 increase in rate of dynamic processes conditioned by the changes 

 in the colloid substratum in reduction and dedifferentiation. 



