How Cells Divide — Indirect Cell Division 67 



nucleus is to divide, the chromatic strands thicken and shorten, finally 

 forming a definite number of strands, known as chromosomes. Each 

 chromosome contains two parallel chromatids which are more or less in 

 contact with each other. At times the chromatids are surrounded by 

 a lighter matrix; at other times the matrix is not visible. 



The first visible signs of mitosis in a plant cell is the shortening and 

 thickening of the granular chromatic strands to form a specific number 

 of chromosomes (Fig. 22, 5, 6). The number of chromosomes in the 

 normal cells of any one species of plant is the same. In the onion, for 

 instance, there are sixteen chromosomes in each cell. In other plants 

 the number may be different but again specific for that particular plant. 

 The chromosomes gradually become thicker and shorter (Fig. 22, 6, 7) . 

 During this stage the nuclear membrane completely disappears, thus 

 permitting the chromosomes to move somewhat freely within the cell. 

 In the nuclear region appears a spindle-shaped group of fibers, the 

 spindle, extending from one end of the cell (cell pole) to the opposite 

 pole. No asters and no centrosomes are present in higher plants as they 

 are in animals. 



B. Metaphase (Fig. 22, 6) 



In this stage the chromosomes localize themselves on the middle or 

 equatorial plane of the spindle. The halves of each chromosome, formed 

 by longitudinal dimsion, now move along the spindle toward opposite 

 poles of the cell. One-half of each original chromosome with its con- 

 tents goes to each pole. 



C. Anaphase (Fig. 22, 7, 8) 



In this phase the newly formed chromosomes continue their migration 

 toward the poles where they will eventually be localized. 



D. Telophase (Fig. 22, 9) 



In this final stage the chromosomes are crowded together at their 

 respective poles. Each chromosome changes into a fine network of 

 chromatin material which somewhat resembles that of the original parent 

 cell. A new nuclear membrane is formed in each new cell. The remain- 

 ing nuclear contents are also formed. A partition called the cell plate 

 forms across the middle of the original cell. The cell plate splits into 

 two parallel plates between which is formed the new cell wall. The lat- 

 ter divides the original cell into two daughter cells. There is no indenta- 



