CHAPTER XI 



MITOSIS 



The necessity of ultimate cell division in the cases of cells which 

 continue to grow has been explained in the previous chapter (Sec. 70). 

 If the cell thus dividing is itself a one-celled animal, then cell division 

 and reproduction occur at the same time. If, however, the cell is only- 

 one of the cells in a many-celled animal, then division does not in most 

 cases result in reproduction, which is the formation of a new individual, 

 but simply in an increase in size of the individual of which the cell is a 

 part. It makes no difference, however, as to the precise manner in which 

 the division is carried out except that in one-celled animals it is a rela- 

 tively simple process and in many-celled animals it is more complex. 



72. Normal Cell Division. — The ordinary way in which a cell divides is 

 by a series of steps (Fig. 12) which do not occur in every case in precisely 

 the same order, but all of which are passed through before the division 

 is complete. These steps may be outlined as follows: First, the central 

 body, if it has not before been visible, comes into view beside the 

 nucleus and both it and its centriole divide into two. These two central 

 bodies begin to separate, and as they do so fibers appear between them 

 which form a spindle-shaped figure. At the same time the chromatin 

 granules in the nucleus, which have been scattered irregularly upon the 

 linin network, begin to collect together into a slender and very much 

 tangled thread, or spire^ne, and the linin network as such begins to dis- 

 appear. This thread shows itself early to be a double thread. Radiating 

 rays, termed astral rays, appear about each central body, forming star- 

 shaped figures known as asters. As the central bodies gradually separate 

 each with an aster about it, the spindle fibers between them seem to 

 press against the nuclear membrane. The chromatin thread shortens 

 and thickens, forming a much less involved tangle, while the linin com- 

 pletely disappears. This shortened, double chromatin thread breaks 

 crosswise, producing a number of pieces which are known as chromosomes 

 and which, because the spireme was double, are in pairs. During these 

 changes, while the central bodies still continue to separate, the nuclear 

 membrane disappears and the spindle swings into the area occupied 

 hitherto by the nucleus. The central bodies come to lie on opposite 

 sides of what was the nuclear area and the spindle stretches across this 

 area from one central body to the other. The chromosomes arrange 

 .themselves in a double row across the center of the spindle, forming in 



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