186 Life and Death, Heredity and Evolution 



characters in mating. Thus, in Figure 41, we see that after 

 the separation of the original nucleus into two, each of these 

 two divides twice, giving off two very small nuclei, which 

 are absorbed and disappear. 



It is worth while to notice just how new combinations are 

 formed in these cases in which the two nuclei that mate 

 originally came from the same single nucleus. Let us sup- 

 pose that the original single nucleus had four pairs of the 

 chromosomal packets ; these we may designate as follows : 



A B C D 

 abed 



Now when this nucleus divides into two, the division is of 

 the usual sort, in which each single packet divides into two 

 like itself, so that each of the two nuclei produced has the 

 same set of chromosomal packets that its parent had. 



But now each of these two goes through the "reducing 

 division," in which the set of eight divides into two sets of 

 four each, one member of each pair going to each resulting 

 set of four. Then evidently many different combinations 

 may be formed, depending on how the members are dis- 

 tributed. In one nucleus the group of four will be A B C D, 

 in another A B C d, in another A b c D, in another a b C d, 

 and so on (16 different combinations are possible). Now 

 two of these combinations of four unite. It is practically 

 certain that they will have different combinations ; let us 

 suppose that one contained the combination A b c D, the 

 other the combination a b C d; then the nucleus formed by 

 their union will show the combination 



A b c D 



a b C d 



That is, from a nucleus showing the combination 



A B C D 

 abed 



