Cell Division and Heredity 59 



of growth and cell division, yet they are also to a 

 large extent independent of one another. The 

 plastids which occur in the cells of the higher 

 plants are often freely movable in the cytoplasm 

 and apparently have no specific structural connec- 

 tion with the other parts of the cell. The central 

 bodies, which occur mostly in animal cells, are 

 also freely movable in the cytoplasm which would 

 not be possible if they would have any specific con- 

 nection therewith, and the fact that they also 

 sometimes occur within the nuclear membrane 

 proves conclusively that they must be structurally 

 independent of the other parts of the cell. The 

 chromosomes must also be structurally separate 

 entities, because if they would have any perma- 

 nent and specific connection with the other parts 

 of the cell they could not become twisted about one 

 another at random so as to effect the segregation 

 of inheritable characteristics in accordance with 

 the laws of probability. 



Although a heterogeneous internal structure is 

 always necessary for spontaneous division, yet 

 when the radical differences in structure and be- 

 havior between plastids, central bodies, and chro- 

 mosomes are taken into consideration, it seems 

 highly improbable that they all undergo division 

 by the same specific method. A central body, for 

 example, consists of a radiating cluster of twisted 



