THE EVOLUTION OF DEATH 73 



differentiation possible the protoplasm must grow in order 

 to furnish the basis for the differentiation. Hence I should 

 /like to give the above conclusion the following form: 

 Senescence is caused by the increase and differentiation of pro- 

 toplasm. 



The correctness of this conclusion is strengthened by the 

 fact that we find the opposite relations in young cells which 

 have characteristically a nucleus with little undifferentiated 

 protoplasm. During the development of the ovum there 

 arise at first relatively large cells which develop further, and 

 through numerous generations became steadily smaller. 

 Since the ovum usually contains a nutritive yolk, the cells grow 

 by assimilating the yolk. The brilliant investigations of 

 Conklin 32 have shown that during the segmentation of the 

 ovum not only is the total amount of nuclear substance in- 

 creased, but also the total amount of protoplasm in the strict 

 sense. It comes about, however, that the increase of the 

 nucleus is relatively greater than the increase of protoplasm. 

 Conklin determined in Crepidula that in the two-celled stage 

 the nuclei form only 0.0117 of the total volume of the ovum, 

 but in the twenty-four-celled stage they form 0.0255 of the 

 volume. Soon there follows a stage with really young cells, as 

 I have above defined them . We distinguish two chief periods 

 of development. The first is much the shorter and is char- 

 acterized by the preponderating increase of the nuclei. The 

 second is much longer and is marked by the growth and dif- 

 ferentiation of the protoplasm. The first is the period of 

 rejuvenation, the second the period of senescence or growing 

 old. 



A remark must be here intercalated. The rate of growth 

 and of the division of the cells does not depend solely upon the 

 organization of the cells itself for the time being. The degree 



