DIFFERENTIATION AND REJUVENATION 167 



To be exact, it must be added that the relative in- 

 crease of the nuclei may be prolonged beyond the 

 period of segmentation as commonly defined. Then 

 begins the other change ; the protoplasm slowly pro- 

 ceeds to grow, and as it grows, differentiation follows, 

 and so the cycle is completed. Whether other na- 

 turalists will be inclined to accept this conception that 

 the process of the segmentation of the ovum is that 

 which we must call rejuvenation or not, I cannot say, 

 for the matter has as yet been very little discussed, 

 but you must admit that the conception hangs as a 

 theory well together with the main facts of senescence 

 as now known to us. We have first an explanation 

 of the process of the production of the young material, 

 and next out of that young material the fashioning of 

 the embryo. The cycle of life has two phases, an 

 early brief one, during which the young material is 

 produced, then the later and prolonged one, in which 

 the process of differentiation goes on, and that which 

 was young, through a prolonged senescence, becomes 

 old. I believe these are the alternating phases of 

 life, and that as we define senescence as an increase 

 and differentiation of the protoplasm, so we must 

 define rejuvenation as an increase of the nuclear ma- 

 terial. The alternation of phases is due to the alter- 

 nation in the proportions of nucleus and protoplasm. 

 In the next lecture I shall be able to convince you, 

 I hope, that this conception of the relation of the 

 power of growth to the proportion of nucleus and 

 protoplasm enables us to understand various problems 



