Cell Division 



39 



in the salivary glands from the fruit fly larva, which have enormous 

 chromosomes with a banded structure [Plate 4] can be identified 

 with a particular place in the chromosome. 



The process just described is that of simple cell division. It is a 

 common method by which unicellular organisms multiply and it 

 occurs in the growth of larger organisms. Sexual reproduction is 



Two chromosomes, one 

 coming from male and 



one from female parent. 



The chromosomes line 

 up near each other 



/■. 



♦ * 



^ 



/ 



... 



>> ' 



and exchange parts 



so that each thread 



contains contributions 



from both parents. 



FIG. 5. The reduction process by which the germ cells are formed 



One of the"mixed"chromo- 

 somes is rejected and the 



germ cell then is left with 

 a single chromosome. 



more complicated. The organism sets aside germ cells, viz- the 

 sperm in the male and the egg in the female, which unite in fertiUza- 

 tion. In the ordinary body cells of the organism there are two of 

 each kind of chromosome, one coming from the male parent and one 

 from the female. When these germ cells are formed, this number is 

 reduced to one by a process of reduction (see Fig. 5). This is itself a 

 very wonderful provision which ensures that the chromosomes in the 

 germ cells contain parts derived from both parents. The two similar 



