THE CELL THEORY OF ORGANIZATION 



165 



division the chromatin becomes organized into a number of 

 bodies called chromosomes, each of which splits, and the halves 

 are distributed in two sets to the daughter nuclei. The distri- 

 bution of the chromosomes is effected by an interesting appa- 

 ratus called a spindle (Fig. 170, B), which consists of delicate 

 fibers (spindle fibers) formed in the early stages of nuclear 

 division. The two sets of daughter chromosomes (Fig. 170, C) 

 collect at the poles of the spindle to organize the daughter 

 nuclei, which then pass into the resting condition (Fig. 170, _D), 

 and a cell wall is formed between, that divides the original 



. ,---->) 



FIG. 170. Stages in nuclear and cell division from the root tip of an onion 



A, resting nucleus with the chromatin in the form of a network and two nucleoles ; 

 B, a spindle with the divided chromosomes gathered in the middle region and 

 ahout to separate into two groups of daughter chromosomes ; C, the two sets of 

 daughter chromosomes at the poles of the spindle ; D, formation of the new wall 

 between the daughter nuclei 



cell into two daughter cells. It is a remarkable fact that the 

 number of chromosomes in the nucleus is fixed for different 

 plants, a point which we shall have occasion to consider in 

 other connections. 



Chromatophores and chloroplasts reproduce themselves by 

 fission, or splitting, plainly illustrated in the cells of the moss 

 leaf (Fig. 169, C), and are thus passed on with each cell division. 



200. The cell theory of organization. The process of growth 

 and development of a many-celled organism is through con- 

 tinuous cell multiplication. Development generally begins with 

 a cell, which both in animals and in plants is called the egg. 



