CHAPTER III 



THE MECHANISM OF HEREDITY 



THE statement of the theory of the gene at the end 

 of the first chapter is derived from purely numeri- 

 cal data without respect to any known or assumed 

 changes in the animal or plant that bring about, in the 

 way postulated, the distribution of the genes. However 

 satisfactory the theory may be in this respect, biologists 

 will seek to discover in the organism how the orderly 

 redistribution of the genes takes place. 



During the last quarter of the last century, and con- 

 tinuously through the first quarter of the present century, 

 the study of the changes that take place during the final 

 stages in the maturation of the egg and sperm-cell have 

 revealed a remarkable series of events that go far toward 

 furnishing a mechanism of heredity. 



It was discovered that there is a double set of chromo- 

 somes in each cell of the body and in the early stages of 

 the germ-cells. The evidence of this duality came from 

 observations on differences in the sizes of the chromo- 

 somes. Whenever recognizable differences exist there are 

 two chromosomes of each kind in the somatic cells and 

 one of each in the germ cells after maturation. One mem- 

 ber of each kind has been shown to come from the father 

 and the other from the mother. At the present time the 

 duality of the chromosome complex is one of the best 

 established facts of cytology. The only striking exception 

 to the rule is sometimes found in the sex-chromosomes, 

 but even here the duality holds for one sex, and often for 

 both. 



