74 GENERAL BOTANY 



The goal of cell division, therefore, is probably not simply a 

 division of a mother cell into two essentially equal daughter 

 cells but also the distribution of exactly equivalent masses of 

 chromatin, called chromosomes, to each daughter nucleus of the 

 newly formed cells. We shall learn that this equal division and 

 distribution of the chromatin substance to all the cells of a com- 

 plex organism is the basis for the modern theories concerning 

 reproduction and inheritance. We may therefore anticipate with 

 interest the discussion of these important and complicated proc- 

 esses, concerned with the division of the cell and the nucleus, 

 which are comprehended under the general term mitosis. 



Process. It is customary, for the sake of clearness, to describe 

 the various processes of mitosis under certain phases or stages 

 (Fig. 42). These phases, taken in the order of their occurrence, 

 are the prophase, metaphase, anaphase, and telophase. The stu- 

 dent should bear in mind, however, that the processes included 

 under the above phases are continuous processes, and that one 

 phase graduates insensibly into the next phase, which follows it 

 in orderly sequence. Some of these phases, such as the meta- 

 phase, are undoubtedly longer than others, which are passed 

 through more quickly. This is indicated by the fact that in 

 prepared slides certain phases are much more prominent than 

 others, for the probable reason that their longer duration makes 

 it easier to fix a larger number of nuclei in these stages. All 

 the phases of mitosis are passed through rapidly, however, and 

 the entire process of cell and nuclear division never occupies 

 more than a few hours. 



The prophase is the preparatory phase of nuclear division, 

 during which changes take place in the nucleus and the cell 

 preparatory to the equal division of the chromatin substance, 

 which is the principal goal of nuclear division. These prepara- 

 tory changes involve two structures which play an important 

 part in the ultimate division of the nucleus. These structures 

 are the chromatin, which we have learned is a permanent portion 

 of the nucleus, and the spindle, which is a temporary structure 

 apparently designed as a framework on which the chromatin 

 gathers and finally becomes distributed in equal amounts to the 



