HEREDITY 



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analyses which show it to be composed of nucleo-proteins. Our 

 chief interest here is in its behavior, first, during mitotic cell di- 

 vision. The account given is in general applicable to mitosis in all 

 sorts of animal cells. 



When the description of a typical animal cell is recalled, it 

 will be remembered that the nucleus is surrounded by a thin 

 membrane, outside of which is a small body, the centrosome, that 

 was said to play a part in the division of the cell. Within the 

 nuclear membrane the chromatin is scattered. Seemingly the quan- 



FiG. 197. — Two steps in the prophase stage of mitotic cell division. 



tity of chromatin in a cell not undergoing mitotic division is often 

 less than appears later; either it is not in the chemical form neces- 

 sary to accept the stains that are used in distinguishing it, or it is 

 dissolved, or diffused in bodies too small to be visible. For con- 

 venience in treatment, the whole process through which the chro- 

 matin passes during mitosis is divided into four stages or phases. 

 The Prophase (Fig. 197). Approaching division is first indi- 

 cated by the division of the centrosome into two units. Each ac- 

 quires in the cytoplasm around it a set of radiating fibres, so that 

 each presents a somewhat star-like appearance; these are now known 

 as the ASTERS. The nucleus being essentially a sphere, the asters now 

 move around on arcs toward opposite poles of the sphere and come 

 to rest in positions exactly opposite each other, the nucleus between 



