2 3 o AGE, GROWTH, AND DEATH 



comparable to the death of one cell in our own bodies. 

 It is not properly comparable to the death of the 

 whole body, to the ending-up of the cell cycle. August 

 Weissmann was led to a series of erroneous notions 

 concerning death by his failure to distinguish between 

 the death of a cell and the death of a cycle of cells. 

 Let him serve as a warning to us. Is there anything 

 like a cell cycle among the lower organisms ? among 

 the protozoa, as the lowest animals are called? It 

 has been maintained by a French investigator, by the 

 name of Maupas, that such a cycle does exist, that 

 even in these low organisms there is a cell which 

 begins the development, and that gradually the loss 

 in the power of cell multiplication goes on until the 

 cycle gives out and has to be renewed by a rejuvenes- 

 cent process, and this rejuvenating process he thinks 

 he has found in the so-called conjugating act of these 

 animals, in which there occurs a curious migration of 

 the nucleus of one individual into the cell body of 

 another. Whether he is right or not remains still to 

 be determined. It means much that Professor G. N. 

 Calkins, one of the world authorities on protozoa and 

 easily the foremost American master of this branch 

 of zoology, thinks that cyclical development rules the 

 protozoa, each cycle ending with natural death. You 

 will recognise, I hope, from what I have said, that we 

 have now some kind of measure of what constitutes 

 old and young. We can observe the difference in the 

 proportion of protoplasm and nucleus, the increase or 

 diminution, as the case may be, of one or the other. 



