552 



CONTINUITY OF LIFE 



■'^s^^"ppr^<^^^?T^^ 



' -j>m^- i.s, :' i!~m!ths.!v;i:%-^--^VjFfMjL'M« f: 





i 



Fig. 22-1. Mitosis in the whitefish early embryo. 



somes, which under the most powerful light 

 microscopes appear to be string-like masses 

 of dark-staining bodies (Fig. 24-4). The 

 most careful studies with both light and 

 electron microscopes lead us to believe that 

 the genes are arranged in a linear fashion 

 within the chromosomes which are enclosed 

 in a thin sheath. 



When reproduction occurs in a chromo- 

 some, a new string of genes forms along- 

 side the original, and it is an absolute dupli- 

 cation of the first in every detail. The new 

 chromosome apparently forms by accumu- 

 lating the proper constituents from the sur- 

 rounding protoplasm under the influence 

 of the original chromosome. It is as if the 

 chromosome were a template from which 

 another, exactly like itself, can be pro- 

 duced; the newly formed one then becomes 



a template itself from which another can 

 be formed, and so on. Just how this is done 

 is still unknown. 



DUPLICATION OF CELLS- 

 MITOSIS 



The duplication of genes and chromo- 

 somes is the first step in the ultimate dupli- 

 cation of cells. This is immediately followed 

 by a division of the cytoplasm, resulting in 

 two cells equal in respect to the genes 

 which each bears. This process is called mi- 

 tosis. The most significant aspect of cell 

 division is the remarkably equal distribu- 

 tion of the chromosomes to the daughter 

 cells. This is essential because the genes 

 control the future metabolism and ultimate 

 success or failure of the cell. This is 



