388 Evolution by Jumps 



An explanation for the occasional irregularities in the 

 behavior of chromosomes is found in a study of their divi- 

 sion during cell formation, and especially during the forma- 

 tion of germ cells. When the two halves of each chromo- 

 some begin to separate from each other, before the germ 

 cell is formed, it may happen that the halves of one of the 



Fig. 98. Non-disjunction 



In the formation of new cells the chromosomes (1—6 in Ai) normally split 

 lengthwise (Ao), move toward opposite poles and become the chromosomes of the 

 daughter-cells (A3). Various irregularities have been observed to occur, however. 

 Sometimes a cell that appears to be perfectly normal (Bj) as to its chromosomes 

 proceeds to divide in the usual way, but the two halves of one of the chromosomes 

 (4 in B2) instead of passing to the two daughter-cells, remain together, with the 

 result that the two daughter-cells (B3, which may be two germ cells), are unlike, 

 one of them having a deficiency of chromosome numbers, the other having an excess 

 of chromosome substance. 



chromosomes fail to come apart. Such non-disjunction 

 would result in producing one sperm, let us say, with a 

 shortage of chromosomes, and one with an excess. In the 

 OEnothera, for example, instead of each germ cell having 

 seven chromosomes, the normal number, there would be one 

 with six and one with eight. The latter, on combining with 



