STIMULUS TO DEVELOPMENT 147 



surface of the egg of the " fertilization membrane," while the 

 other is supposed to prevent the cytolysis or disintegration of the 

 ovum, induced by the formation of the fertilization membrane, 

 from going too far. It remains to be seen whether this " lysin 

 theory" will stand the test of time. 



What it is that stimulates the unfertilized ovum to develop in 

 normally occurring parthenogenesis we do not yet know. In 

 most cases of this kind the process of maturation seems to differ 

 more or less from that which takes place in eggs which are 

 destined to be fertilized by a spermatozoon, and it may be that 

 these differences have something to do with the power of the egg 

 to develop parthenogenetically, but the discussion of this very 

 difficult problem is altogether beyond the scope of the present 

 work. 



L 2 



