17 Life and Death, Heredity and Evolution 



When mating is to occur we know that these paired 

 packages of each nucleus separate into two groups, each 

 group containing one member of each pair (Figure 45, B, 

 C, D). These two groups are then separated by cell divi- 

 sion into different cells. Each of these cells therefore con- 

 tains a group with half the number of packages that were 

 present in the parent nucleus. It is such cells with half 

 the original number of packages or chromosomes in their 

 nuclei that form the germ cells, the two cells that are to 



* 



Figure 45. The separation of the two groups of paired chromosomes 

 into different germ cells, in the insect Nezara hilaris, after Wilson, 

 1911. A, the 14 chromosomes in a single cell, before the germ cells 

 are formed. B, the 14 gathered into 7 pairs. C, the members of the 

 seven pairs separating as the division to form the germ cells occurs. 

 D, the two groups of 7 chromosomes each, in different germ cells, 

 formed by the separation of the 14 shown in A and B. 



unite in mating (Figure 45, D). After the two half nuclei 

 have united, of course the original number of chromosomes 

 is restored. 



As before remarked, we know that the different chromo- 

 somal packages present in a nucleus are diverse. The 

 evidence for this is complete, but cannot be given here. 

 Now, when these diverse packages separate into two half 

 groups, different half groups are formed in different cases, 

 depending on which member of any given pair goes into a 

 given group. In this way a great number of diverse com- 



