THE PHYSICAL BASIS OF MENDELISM 59 
and, taking the chromosomes by pairs, one member of each pair is de- 
rived from one parent and the other from the other parent. 
From the standpoint of interpretation the chromosomes are aggre- 
gates of chromatic material which in itself is definitely and highly or- 
ganized. 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 content of the nucleus 
is regarded as made up of a definite number of individual chromatin 
elements called chromomeres. The number of chromomeres in acell 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 chro- 
mosome is reformed from the same group of chromomeres. The individu- 
ality of the chromosome, therefore, depends on the individuality of 
the chromatin elements of which it is made up. Not only is each chro- 
mosome made up of a definite 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 par- 
ticular 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 mi- 
tosis) are represented in outline in Fig. 28, for a species having four 
chromosomes in its body cell. Bearing in mind the description which 
has just been given of the organization of the chromatin material we may 
follow the steps involved in mitosis as they are outlined in this figure. 
In the “‘resting” cell at A the chromatin is scattered throughout the nu- 
cleus in clumps or knots loosely strung together to form an irregular 
network. As the cell prepares for division the chromatin elements 
appear in more definite form until at B the chromomeres have arranged 
themselves in a single row in a long, continuous spireme-thread. This 
spireme-thread may be considered to be made up of the four chromosomes 
united end to end with the chromomeres arranged in a linear series. As 
mitosis progresses to the next stage represented at C, each chromomere 
of the spireme-thread divides into two so that a double spireme-thread 
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