THE CELL AND CELL DIVISION 55 



completed in fifteen minutes, or it may occupy one or even two 

 hours, and in some special cases a much longer period. 



This account of mitosis, although brief and including only 

 some of the essentials, brings out clearly the unity of the 

 nucleus as an organ; it behaves as a more or less separate 

 unit of cell organization throughout all this intricate process. 

 And we see clearly this extremely important fact, that the 

 nucleus of a cell is formed from a preexisting nucleus of 

 the same constitution. Nuclei arise only from preexisting 

 nuclei; there is a nuclear continuity quite parallel with cell 

 continuity. And going one step farther, it is probable that 

 chromosomes are derived only from preexisting chromosomes. 

 This idea of genetic continuity is not completely applic- 

 able to all cell organs, however, for occasionally the centrosomes 

 are not derived from preexisting centrosomes, but may arise 

 de novo, and in the development of the new organism the 

 centrosome is typically derived from the sperm cell alone. 

 Among the plants many of the plastids seem to be genetically 

 related and to be formed by the division of preexisting plastids. 

 The other less living cell structures are usually distributed 

 passively to the daughter cells, and such structures may be 

 formed anew in the new cells. 



The relation between the direction of the plane of cell division 

 and the general morphology of the cell body demands a word. 

 From the preceding account it is obvious that the plane of 

 division is at right angles to the longitudinal axis of the spindle, 

 but the position which the spindle assumes is itself prede- 

 termined. The position of the spindle axis is fixed by the 

 location of the centrosomes. When the single centrosome 

 divides and the daughter centrosomes pass to opposite sides of 

 the nucleus, they usually migrate equal distances from the ori- 

 ginal position of the centrosome; it follows, therefore, that this 

 region falls within the plane of the equator of the spindle and 

 consequently in the plane of the new division. When the 

 centrosome does not alter materially its relative position in the 

 cell, the next division, again being through the plane occupied 

 by the centrosome and the center of the nucleus, will be hi 



