THE CELL IN GENERAL. 2J 



Finally, regarding the reproduction of cells, in a 

 few instances the act has been directly observed 

 under the microscope, but in the majority of cases 

 our knowledge is derived from the study of a suc- 

 cession of consecutive stages in the process. Every 

 new cell which appears in the animal body, during 

 and subsequent to its development, is derived from a 

 pre-existing cell, and all cells are derivatives of a 

 single original cell, the ovum. New cells are pro- 

 duced by a process of division in older cells. Cell- 

 division seems to occur in a variety of ways, usually 

 commencing in the nucleus. 



At present it is believed that the chief mode of 

 multiplication is by indirect cell-division Karyomi- 

 tosis. In this mode of division the intranuclear net- 

 work undergoes a succession of changes. It first 

 assumes the form of a convoluted thread, then of a 

 wreath or rosette, then of a star or aster, the centre of 

 which is the centre of the nucleus, the rays diverging 

 toward the periphery. At this stage the nucleus 

 assumes an oval form. The rays of the aster next 

 collect around points at each end of the nucleus, the 

 poles, leaving a clear space midway between the 

 poles, the equator, forming the diaster. The nu- 

 cleus then divides at the equator forming daughter- 

 nuclei. 



These nuclei then return to the resting state by a 

 reversal of this process. While these changes are 

 going on in the nucleus, the protoplasm also changes 

 its form. At first a slight constriction appears on a 



