THE PHYSICAL BASIS OF MENDELISM 403 



germ cells of Drosophila contain four chromosomes as the result of a 

 reduction which takes place in such a manner that each germ cell con- 

 tains one member of each pair of chromosomes. As a consequence, 

 the germ cell of Drosophila contains two large curved autosomes, 

 representing the two pairs of these chromosomes, one small autosome, 

 and one X- or one Y-chromosome. The same thing is true for other 

 species of plants and animals — in the reduction divisions the 

 chromosomes are distributed in such a manner that each germ cell 

 receives one member of each pair of chromosomes. It follows from 

 this that in general a definite number of pairs of chromosomes is 

 characteristic of the body cells of individuals of a given species, and, 

 taking the chromosomes by pairs, one member of each pair is derived 

 from one parent and the other from the other parent. 



From the standpoint of interpretation the chromosomes are aggre- 

 gates of chromatin material which in itself is definitely and highly 

 organized. Our conceptions of this feature of cell organization are 

 based on appearances of the cytological preparations from certain of 

 the more favorable plants and animals and further interpreted by 

 investigations on heredity. Accordingly the entire chromatin con- 

 tent of the nucleus is regarded as made up of a definite number of indi- 

 vidual chromatin elements called chromomeres. The number of 

 chromomeres in a cell of any species must run into the thousands. A 

 certain definite group of these elements make up each chromosome, 

 and at every cell division this chromosome is reformed from the same 

 group of chromomeres, but the chromosome is definitely organized 

 with respect to the position or locus occupied by each chromomere. 

 At certain stages in the history of chromosomes, they are simply lines 

 of chromomeres, very much like single strings of beads with each bead 

 corresponding to a chromomere. Now it appears probable that all 

 the chromomeres in a chromosome are different, as though our string 

 of beads had no duplicates throughout its length. Moreover, each 

 chromomere has a definite place or locus in the particular chromosome 

 in which it belongs and it is always found at that particular locus. 

 The chromomeres of this discussion are identified with the factors of 

 Mendelian heredity, and how closely this conception of the nature of 

 chromatin and its complex organization corresponds to the modern 

 view of Mendelian phenomena will be pointed out as each new phase 

 of Mendelism is taken up. 



Somatic cell division. — The phenomena of cell division (called 

 mitosis) are represented in outline in Fig. 71, for a species having four 



