THE DIVISION OF THE PROTOPLAST 



59 



(Fig. 39). This indicates that the plane of cell division is determined by 

 factors acting at an early stage throughout the cell and not by the nucleus 

 alone. 



Mitosis. — Mitosis is a process in which each of the chromosomes, the 

 principal constituents of the nucleus, undergoes a longitudinal doubling, 

 the halves of all the chromosomes then separating into two similar groups 

 which reconstitute two new nuclei (daughter nuclei). Only rarely or 

 under very exceptional circumstances does a nucleus, without respect to 

 the chromosomes as individuals, divide b}^ simple constriction {amitosis); 

 mitosis is the almost universal method of nuclear division. 



In the root tip of a plant with large chromosomes the course of mitosis 

 is essentially as follows (Fig. 40). The nucleus in the metabolic stage 



^ 



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. . •; ' . i .4jj j gjfe» ' • ■-i .a a 



E F G H 



Fig. 39. — Division of vacuolate pith cell, showing the cytoplasmic diaphragm (phrag- 

 mosome) present before mitosis. Semidiagrammatic. {After E. W. Sinnott and R. 

 Block.) 



preceding a division contains numerous chromonemata which, because of 

 their number, length, and coiled or contorted condition, can seldom be 

 traced far as individuals. The prophase comprises all the changes that 

 transform the chromosomes from this metabolic condition, in which their 

 chromonemata have little or no matrix about them and are all uniformly 

 dispersed in the nucleus, into the compact separate individuals seen at the 

 mid-point of mitosis. In the early part of the prophase the mass of 

 chromonemata becomes less uniform, so that the threads belonging to 

 different chromosomes stand apart more clearly as indi^•iduals, though 

 their length at this stage usually precludes following them from end to 

 end. They are more or less spirally coiled, and close examination shows 

 them to be longitudinally double ; hence at this stage each chromosome is 

 actually represented by two chromonemata running closely parallel. 

 The two longitudinal halves of a chromosomt^ at this stage or at any other 

 are known as chromatids. As the prophase ad^'ances to its middle stage, 

 the chromonemata tend to relax their coils and thicken somewhat, so that 

 the doubleness appears more plainly. In the later prophase the second 

 chromosomal constituent, the matrix, accumulates about each chromo- 



