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 fhis 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 
