THE CELL AND CELL DIVISION 



69 



are found chiefly in those forms where sex differences are found ; 

 in such cases one or more chromosomes are unpaired, or the 

 members of the pair may be dissimilar in size. And further 



D 



in the germ cells with chromosomes, none is paired all are 



single, each somatic pair is represented, and the groups in eggs 

 and sperm are alike. 



FIG. 34. Changes in the volume of chromosomes in the egg of the Elasmo- 

 branch, Pristiurus. All drawn to same scale. From Wilson, "Cell," after 

 Riickert. A. In egg of 3 mm. diameter. Chromosomes at maximal size and 

 minimal staining capacity. B. In egg of 13 mm. diameter. C. In fully grown 

 ovarian egg. Minimal size and maximal staining capacity. 



Such facts as those given above taken in connection with the 

 precision with which each chromosome is halved in mitosis, lead 

 almost irresistibly to the supposition that the chromosomes 

 must be qualitatively unlike. Such qualitative differences 

 cannot be observed directly, and can only be inferred, but as 

 we shall see in connection with the relation of the chromosomes 

 to heredity, this inference seems to be justified from the results 

 of the experimental or accidental modification of the chromo- 

 somal content of the nuclei and the character of the resulting 

 cells or cell groups (Chapter VII). 



