How Cells Divide — Indirect Cell Division 63 



B 



Fig. 20. — Animal cell mitosis in which the chromosome number is assumed to 

 be 8. A, Prophase (beginning), with chromatin granules in netlike arrangement; 

 centrosome dividing and surrounded by the astral rays (asters) ; B, prophase 

 (early), with the chromatin consolidating and beginning to form a definite num- 

 ber of threadlike bodies; centrosomes moving farther from each other; spindle 

 figure (spindle) arising between them; C, prophase (late) with nuclear membrane 

 disappearing; each chromosome, under highest magnification, appearing as a 

 double structure because of its two parallel strands (chromatin elements) in con- 

 tact with each other; D, prophase (later), with nuclear membrane absent, cen- 

 trosomes at opposite ends of the cell, chromosomes distributed on the spindle; 

 E, metaphase, in which the chromosomes are arranged in equatorial plate, and 

 each chromosome splits lengthwise into two similar parts; F, anaphase (early), 

 in which half of each original chromosome moves ov'er the spindle toward o.p- 

 posite centrosomes; G, anaphase (later), in which the two sets of chromosomes 

 continue to travel over the spindle toward each centrosome and aster; H, telophase 

 (early), in which the chromosomes reach their respective centrosomes and there 

 gradually lose their distinctive chromosome characteristics; spindle and asters 

 disappearing; cytoplasm starts to divide; /, telophase (later), in which the cyto- 

 plasm is completely divided by the newly formed cell membrane ; nuclear wall and 

 entire nuclear contents reappear; centrosomes are already dividing in prepara- 

 tion for the next cycle of cell division. Each of the newly formed cells will now 

 grow to normal size and, sooner or later, will undergo the process of mitosis. 



