CELL DIVISION 139 



mosomes take up a position in the spindle, halfway between the 

 centrioles, to form the so-called equatorial plate. 



During the metaphase, the longitudinal division of each chromo- 

 some into halves becomes conspicuous. This splitting of the 

 chromosomes occurs at different periods in the prophase, but is 

 more easily observed during the metaphase as the half-chromo- 

 somes he side by side on the equatorial plate of the division spindle. 

 The fact that the chromosomes divide in this manner is of theoret- 

 ical significance, because regions of chromosomes are believed to 

 differ from one another along the longitudinal axis. The genes, or 

 determiners of hereditary characteristics, located in the chrome- 

 somes are believed to be arranged like beads on a string. The 

 longitudinal division of chromosomes is, therefore, thought to be 

 equal with respect to quality as well as quantity of chromatin. 

 The importance of the chromosomes and their behavior in correla- 

 tion with the mechanism of heredity will be more fully discussed 

 in the chapter on Genetics. 



Following the period of inactivity of the chromosomes as they 

 lie on the equatorial plate, the halves of each chromosome begin 

 to separate and move toward opposite centrioles. This migration 

 constitutes the anaphase. Xo satisfactory explanation of the 

 physical principle underlying the movement of the half-chromo- 

 somes toward the ends of the spindle has been proposed. That 

 certain of the spindle fibers are attached at specific points to the 

 chromosomes is an established fact. 



The telophase, or reconstruction of nuclei, begins when the 

 half-chromosomes approach the centrioles. The chromosomes 

 more or less reverse the process of their formation during the pro- 

 phase. They become irregular in outline, and are redistributed as 

 chromatin granules on the linin network of the new nucleus, which 

 is set off by the appearance of a nuclear membrane. In this way, 

 daughter nuclei, containing chromatin equal in amount and kind, 

 are established at each end of the mitotic spindle. As nuclear 

 reconstruction goes on, the astral rays and spindle fibers disappear, 

 and in some types of cells each centriole divides. At the same time 

 the cytosome becomes constricted so as to form approximately 

 equal halves, each of the daughter cells containing one of the 

 daughter nuclei and a centrosphere with one or two centrioles. 



This process of indirect cell division is regarded as essentially a 

 mechanism fcr insuring equal qualitative and quantitative dis- 



