THE PROCESS OF FERTILIZATION 311 



division may take place with a double number of chromosomes as 

 well as with the undoubled number; while in the divisions of the 

 mother-egg-cells and the mother-sperm-cells we have proof that 

 a doubled number of chromosomes does not in itself compel to 

 division. 



The exceptional and artificially produced cases of parthenogenesis 

 which we have discussed above are probably to be interpreted thus : 

 tln'ough slight differences in the constitution of the ovum, or through 

 certain mechanical or chemical stimuli, the metabolic processes in the 

 ovum are so altered that the centrosome of the ovum, instead of 

 breaking up, is stimulated to growth, and thus produces the active 

 dividing apparatus which is otherwise only brought into it by the 

 sperm. This is a more exact definition of the interpretation I gave 

 earlier (1891) of the 'chance' parthenogenesis of the silk-moth, which 

 was then the only case known, when I said ' the nucleoplasm of some 

 ova must possess the power of growth in a greater degree than the 

 majority.' 



But we are not yet in a position to go further, or to define more 

 exactly the nature of the processes of metabolism which are involved. 



