HEREDITY AND EVOLUTION 179 



of heredity. This chromatin does not contain black hair or 

 anything resembling the other characters which it passes 

 on from parent to child. It is simply material which causes 

 development to go in a certain manner. The chromatin is 

 said to be a determiner of characters though in itself it 

 does not possess the characters. 



It will be recalled that when cells divide by mitosis, the 

 chromatin becomes arranged in minute bodies called chro- 

 mosomes (Figs. 71, 72). The ability to determine a great 

 number of hereditary characters must be present in each 

 chromosome. For instance, in the fruit fly (Drosoph'ila) more 

 than four hundred different hereditary characters have been 

 studied. Yet this fly has but four chromosomes in each germ 

 cell. This gives some idea of how very numerous and how 

 extremely small each mass of chromatin for determining a 

 single character must be. The name gene is applied to that 

 particle of chromatin which determines a single hereditary 

 characteristic. No one has ever seen a gene. From the way 

 chromosomes behave in heredity, it is necessary to assume 

 that these minute units exist in the chromosomes, though 

 they are probably so small that even the best microscopes 

 could not reveal them in the chromosomes. 



In the ordinary cells of the body (except the germ cells) 

 the chromosomes are recognizable as two sets. This may be 

 understood if we remember that each cell of the body is 

 formed by mitosis of the fertilized egg. Therefore each cell 

 has the same number of chromosomes as were present in the 

 fertilized egg. Two sets of chromosomes were united to form 

 the fertilized egg. One of these was present in the egg before 

 fertilization and the other was contributed by the sperm cell 

 when it joined with the egg at fertilization. If both parents 

 are alike with regard to a given character, both germ cells 

 carry the same kind of a gene and the offspring will be homo- 

 zygous. In the case of hair color mentioned above, if one 



