88 BIOLOGY 



containing two granules, at H; this nucleus is an exact repe- 

 tition of the one with which we started. Meantime a division 

 plane forms, passing through the cell midway between these 

 reconstructed nuclei, and the division of the cell into two 

 parts is now completed. There are thus produced two cells, 

 identical with each other and identical with the original cell, 

 each with similar chromatin material, since each contains 

 half of the original chromosomes. By this process, therefore, 

 the chromatin of the nucleus is continuous from one cell gen- 

 eration to another. 



It will be evident that the essential purpose of this cell 

 division is the splitting of the chromatin material into identical 

 halves. It would seem much simpler for the cell to divide 

 immediately into two parts without this long process; but this 

 might not make the two parts equivalent. In order that they 

 may be equivalent, the cell adopts the complicated process 

 of karyokinesis. In the case described, the two final cells 

 are practically of equal size; but even in instances where the 

 cells finally produced are of very unequal size (Fig. 121), 

 the amount of chromatin in each is the same. Since, therefore, 

 the essential purpose of this process of karyokinesis is the 

 splitting of the chromatin, it is evident that this material 

 must be of extreme significance in the life of the cell. When 

 we combine this knowledge with the fact mentioned in Chapter 

 II, that the cell can carry on its life processes only when it 

 has nuclear material, it becomes manifest that the nucleus, 

 instead of being a negligible part of the cell, is really the cen- 

 tral feature of its life. 



Nuclear Division without Cell Division. As a rule, almost 

 immediately after the nucleus completes its division, the body 

 of the cell divides so that a cell does not contain more than 

 a single nucleus for any length of time. Occasionally, however, 

 the division of the cell body is delayed and the nucleus divides 

 a second time, and perhaps several times, before the cell body 

 divides, the result being one mass of protoplasm containing 



