Cell Division and Heredity 57 



divide. Attempts to explain cell division on the 

 basis of food economy have been due to a confu- 

 sion of the principles of phylogeny with the prin- 

 ciples of ontogeny. It may be true that division 

 of a large cell into smaller cells has its economic 

 advantages and gives the species a better chance 

 to survive, but that does not explain what causes 

 the two halves of a cell to become separated from 

 each other. 



There are many reasons for believing that the 

 forces which cause a cell to divide arise from 

 within and not from without, and that a hetero- 

 geneous internal structure is necessary for this 

 purpose. Objects such as crystals which are in- 

 ternally homogeneous will not generally divide 

 spontaneously if their environment is not changed, 

 and in those exceptional cases where spontaneous 

 division does take place it is probably due to a cer- 

 tain amount of internal heterogeneity resulting 

 from defects in the crystal structure. 



The true cause of cell division seems to be inti- 

 mately connected with the spirazine structure 

 itself. In order to perpetuate the pattern which 

 is characteristic of the species, we must assume 

 that division always takes place in a plane per- 

 pendicular to the lengths of the spirazines ; and to 

 replace by subsequent growth what was lost by 



