HEREDITY 309 



in each variety of organism. This number is called the somatic or 

 DIPLOID number, or merely designated as S. From this point on, it 

 will be convenient to distinguish between the reproductive or germ 

 cells and those of all other tissues of the body, designating the 

 gametes as germ plasm and all other tissues as soma. For example, 

 the somatic chromosome number is S and that of the germ plasm 



IS S , The terms diploid and haploid are also used to refer to S 



and -. 



2. 



Chromosomes are in pairs. That is to say, for example, the forty- 

 eight chromosomes in the soma of the human female consist of 

 twenty-four pairs, each member of a pair having been derived from 

 a separate parent. Members of a pair are spoken of as homologous 

 chromosomes, or synaptic mates. One pair in the human female is 

 composed of two chromosomes that are concerned with the devel- 

 opment of the female sex characters. These are designated as the X 

 chromosomes; the other twenty-three pairs are referred to as auto- 

 somes. In the human male there are twenty-three pairs of auto- 

 somes, and one X or sex chromosome, whose synaptic mate is dis- 

 tinguishably different from an X chromosome and is designated 

 as the y chromosome. In the males of many of the lower forms the 

 y chromosome is completely lacking, so that in these organisms 

 the chromosome number of the male is always one less than that 

 of the female. The number of chromosomes is a species character. 

 Thus in Man the number is forty-eight; in certain grasshoppers the 

 females have twenty-four and the males twenty-three chromosomes; 

 in some of the Nemathelminthes there are but four chromosomes. 

 In moths and butterflies, in birds, and in some varieties of fish the 

 male has two chromosomes which correspond to the X chromo- 

 somes, while the female has but one X and a synaptic mate which 

 corresponds to the y chromosome. 



