676 General and Applied Biology 



equal, or nearly so, in both sexes of most organisms ensures an equal 

 opportunity of inheritance from each parent. It is worth noting that 

 the cytoplasm of most sex cells is much greater in volume in the female 

 sex cell. If heredity were fundamentally dependent upon the cytoplasm, 

 there would not be the necessary equal opportunity suggested above. 

 The role played by the chromosomes during the process of reduction- 

 division, when sex cells are formed, also suggests their value in heredity. 

 (5) The entire field of Mendelian heredity and its modern interpreta- 

 tions and modifications all tend to prove the chromosome theory of 

 inheritance. 



Fig. 334. — Giant chromosomes as photographed from the salivary gland of the 

 fruit fly (Drosophila melanogaster) . (Copyright by General Biological Supply 

 House, Inc., Chicago.) 



Chromosome Polyploidy (Increase in the Number of Chromosomes): 



Under ordinary conditions, chromosomes do not change their normal 

 number. However, in some instances, changes take place naturally or 

 can be induced by artificial methods. One of the earliest examples of 

 natural change in chromosomes was giv^n by the Dutch botanist, Hugo 

 de Vries (1895), in which he discovered the origin of a new species of 



