138 CLELAND 



vice versa. The behavior of the chromosomes at the time of reproductive 

 cell formation also fitted exactly that of the genes, so that the chromosomes 

 were distributed to the germ cells according to the same rules which gov- 

 erned the distribution of the genes (fig. 2). Furthermore, whenever a case 

 was found where the chromosomes behaved in an unorthodox manner in this 

 regard, it was found that the genes behaved in corresponding fashion, and 

 whenever genes were found to have changed places, via inversion or transfer 



(a) 



(b] 



(c) 



<. > 



(d) 



ie) 



(f) 



Fig. 2. Diagrams to show the peculiar type of mitosis which occurs at the forma- 

 tion of reproductive cells. — a. A nucleus with four chromosomes. — b. Chromosomes 

 descended from different parents and carrying corresponding genes form pairs with 

 each other. Each chromosome also becomes split. — c. The spindle is formed and 

 paired chromosomes become aligned across its center. Note that the same chromo- 

 somes are present as in fig. Ic, but their arrangement is different. — d. Correspond- 

 ing chromosomes are separated. In this case, the separation is not between half 

 chromosomes but between whole chromosomes. — e. A second division separates the 

 half chromosomes from each other. — /. As a result, from each cell which has under- 

 gone this process of "reduction," four cells result, each with only a single set of 

 chromosomes, instead of the two complete sets found in the parent cell. These cells 

 become or give rise to sperm or egg cells. When sperm and egg unite, each carrying 

 a single set of chromosomes, a fertilized egg with two sets will be formed. From 

 this, by ordinary mitosis, multitudes of cells will be produced, each with two sets, 

 and the body will thus consist of cells with two full sets. Note: The diagrams in 

 fig. 1 and 2 are oversimplified with respect to certain details, but the essential fea- 

 tures and the end result are correctly pictured. 



