CELLS 



41 



stretch out from them in all directions and in the zone 

 between the centrosomes these form a framework, the 

 spindle. The chromatin has meantime formed a long 

 thread, the spireme, and this later breaks up into a number 

 of compact rods of chromatin material, the chromosomes. 

 The nuclear membrane now breaks down and the chromo- 

 somes arrange themselves around the center of the spindle 

 halfway between the centrosomes. 



3. Metaphase. Each chromosome splits into two equal 

 parts. 



>ndle 



Spin 



chromosomes 



Prophose 



F 

 Mefophase A/iapha^e Te/ophase 



FIG. 25. Mitosis, or indirect cell division. The ehromatiii is very accurately 

 divided because it is the bearer of hereditary qualities. 



4. Anaphase. The new chromosomes formed by split- 

 ting in the last stage migrate toward the nearest centro- 

 some. Thus half go toward each pole of the cell. 



5. Telophase. The chromosomes break up into scattered 

 bits of chromatin and a nuclear membrane is formed about 

 each of the two groups. The centrosomes divide and the 

 new pairs come to rest beside their respective nuclei. A 

 separation takes place in the cytoplasm between the two 

 nuclei, thus completing mitosis and forming two cells. 



It seems strange that the division of a minute bit of liv- 

 ing matter should involve such a complicated mechanism 



