THE PHYSICAL BASIS OF MEN DELI SM 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 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 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 



