23O FRANK A. HARTMAN. 



There is no question then that unequal division does sometimes 

 occur in chromosomes. 



IV. DISCUSSION. 

 i. Chromosome Variations and Their Causes. 



The difference in size of chromosomes of nymphs as compared 

 to adults depends upon the age of the nymphs. It also bears a 

 direct relation to the size of the cells, the smaller cells of nymphs 

 containing the smaller chromosomes. 



Cells of young nymphs are dividing more rapidly than are 

 those of adults, for that reason they probably have not the chance 

 to grow so large. In addition to this the body cells in young 

 animals undoubtedly develop at the expense of the germ cells 

 because it is more important that the body organs become 

 established before the germ cells are brought into activity. Then 

 when the body has almost reached its growth the germ cells 

 get their share of nutrition and become larger as do the chromo- 

 somes. 



A study of the growth stages of a corresponding chromosome, 

 in the same individual shown in Figs. 68 to 82, has demonstrated 

 that the number and size of the granules differ. Either is 

 sufficient cause for a variation in size of corresponding chromo- 

 somes. 



Unequal division likewise produces unequal chromosomes. 



These two facts lead us to expect unequal corresponding 

 chromosomes in the same animal. A careful study proves that 

 this is of common occurrence, regardless of the species concerned. 



2. The Role of Chromosome Variations in Heredity. 

 Theoretical. It has been believed for a long time that the 

 chromosomes of the germ cells play the most important part in 

 heredity. Their individuality seems to be established as the 

 result of the work of many investigators. The picture which we 

 now have of each individual germ cell chromosome is a body 

 which is able to transmit a certain group of characters in the 

 formation of a new organism. Each corresponding chromosome 

 in different cells would represent the same group of characters 

 while the different chromosomes of each cell would represent 

 different groups of characters. 



