132 GENERAL ZOOLOGY 



become attached to some of the fibers making up the spindle. 

 For a very short time the chromosomes are arranged mid- 

 way (Fig. 71, D) between the two ends of the spindle. 

 While in this position each of the chromosomes splits 

 lengthwise into two exactly equal parts. Immediately after 

 the splitting of the chromosomes the halves of the chromo- 

 somes begin to move away from each other toward the 

 poles of the spindle (Fig. 71, E). When the chromosomes 

 become grouped around the two poles of the spindle they 

 begin to lose their individual forms (Fig. 71, F). A mem- 

 brane starts to form around each group of chromosomes. 

 When these two membranes are completed there are two 

 nuclei within the cytoplasm of the original cell (Fig. 71, G). 



Division of the Cell. This condition is only temporary, for 

 ordinarily mitosis is followed by a division of the cytoplasm. 

 The mass of the cytoplasm begins to constrict about its 

 middle (Fig. 71, F). As this constriction deepens, the origi- 

 nal mass of cytoplasm is pinched into two pieces, each of 

 which surrounds a nucleus (Fig. 71, G). By this means two 

 new cells take the place of the one cell with which the 

 description started. Each of the two new cells contains but 

 half the material from the mother cell. But it is one of 

 the characteristics of living protoplasm that it can take 

 material furnished to it as food and build up more proto- 

 plasm in the process termed growth. By growth the two 

 small cells increase in size until they are as large as the 

 mother cell which produced them. 



Numbers of Chromosomes. As mentioned in an earlier 

 paragraph the chromatin material is very important in the 

 nucleus. It seems that the whole elaborate process of mito- 

 sis has as its chief aim the exact division of this material. 

 When the chromatin forms into chromosomes there is al- 

 ways a fixed number of chromosomes typical for each species 

 of animal. In some animals there are only two, while others 



