124 THE STORY OF THE LIVING MACHINE. 



child inherits equally from father and mother, and 

 hence we must find the hereditary traits handed 

 down in some element which the offspring obtains 

 equally from father and mother. As we have seen 

 (Figs. 34-44), the only element which answers this 

 demand is the nucleus, and more particularly the 

 chromosomes of the nucleus. Clearly enough, 

 then, we must look upon the nucleus as the special 

 agent in reproduction of cells. 



Again, we have apparently conclusive evidence 

 that the nucleus controls that part of the assimila- 

 tive process which we have spoken of as the con- 

 structive processes. The metabolic processes of 

 life are both constructive and destructive. By the 

 former, the material taken into the cell in the form 

 of food is built up into cell tissue, such as linin, 

 microsomes, etc., and, by the latter, these products 

 are to a greater or less extent broken to pieces 

 again to liberate their energy, and thus give rise 

 to the activities of the cell. If the destructive 

 processes were to go on alone the organism might 

 continue to manifest its life activities for a time 

 until it had exhausted the products stored up in 

 its body for such purposes, but it would die from 

 the lack of more material for destruction. Life 

 is not complete without both processes. Now, in 

 the life of the cell we may apparently attribute 

 the destructive processes to the cell substance and 

 the constructive processes to the nucleus. In a 

 cell which has been cut into fragments those pieces 

 without a nucleus continue to show the ordinary 

 activities of life for a time, but they do not live 

 very long (Fig. 25). The fragment is unable to 

 assimilate its food sufficiently to build up more 

 material. So long as it still retains within itself a 

 sufficiency of already formed tissue for its destruc- 



