MATERIALS OF HEREDITY 1 5 



save that one of those present in the females is lacking in 

 the cells of the males. 



This has remarkable results in the production of the 

 germ cells from which new individuals are to develop. In 

 producing these germ cells, the original cells from which 

 they come divide each into two germ cells (figure 4), half 

 the chromosomes going into one of the new cells, half into 

 the other. In the female (B), since the number of chromo- 

 somes is even, all of the cells get the same number of 

 chromosomes. If the number in the mother is fourteen, 

 the number in each germ cell from the mother is seven. 



But in the male (A), since the number of chromosomes 

 is odd, the results are different. At the division in which 

 the chromosomes are distributed to the germ cells, half 

 of the germ cells receive one more chromosome than the 

 other half. In half of the sperms, one chromosome is 

 omitted, while it is present in the rest of the sperms. 



This presents an opportunity to determine what difference 

 it makes if one chromosome is omitted. The egg cells (ova) 

 are all alike in the number of their chromosomes. Half of 

 them unite with sperms that have the full set of chromo- 

 somes, half unite with the sperms that have one chromo- 

 some omitted (figure 5). That is, there is one set of ferti- 

 lized eggs with a full set of chromosomes, another set with 

 one chromosome lacking. Both kinds develop into indi- 

 viduals. What difference does the omission of one chromo- 

 some make? 



Differences between the Sexes. — It makes a tremendous 

 difference. It makes a difference to practically every charac- 

 teristic of the individual. It brings about all the differences, 

 physical, physiological, temperamental, mental, that dis- 

 tinguish the male and the female. It brings about many of 

 these differences rather directly, many others indirectly, 

 through a long chain of processes, but in the end all the 



