144 THE STORY OF LIFE'S MECHANISM. 



is that the nucleus is connected most intimately 

 with the process of reproduction and especially 

 with heredity. This has long been believed, but 

 has now been clearly demonstrated by the ex- 

 periments of cutting into fragments the cell 

 bodies of unicellular animals. As already noticed, 

 those pieces which possess a nucleus are able to 

 continue their life and reproduce themselves, 

 while those without a nucleus are incapable of 

 reproduction. With greater force still is the fact 

 shown by the process of fertilization of the egg. 

 The egg is very large and the male reproductive 

 cell is very small, and the amount of material which 

 the offspring derives from its mother is very great 

 compared with that which it derives from its father. 

 But the child inherits equally from father and 

 mother, and hence we must find the hereditary 

 traits handed down in some element which the off- 

 spring 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 nu- 

 cleus 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 limn, 

 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 



