THE PH\'SIC.\L BASIS OF MENDELISM 



363 



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 through- 

 out the nucleus 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 



Fig. 61. — Diagram of mitosis in a species having four chromosomes in its 

 cells, yl, the "resting" cell; 5, formation of the spireme thread; C, longitudinal 

 division of the spireme thread and transverse segmentation into four chromosomes; 

 D, separation of the daughter chromosomes formed by longitudinal spHtting 

 of spireme thread; E, beginnings of nuclear reconstruction and di\dsion of the cell 

 body; F, cell division complete and daughter nuclei in the "resting" stage. 

 (From Babcock and Clausen.) 



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 results from the longitudinal splitting of the 

 original thread. Both parts of the thread are quantitatively and quali- 

 tatively equal, for, by the spHtting of all the chromomeres both of the 



