128 OUTLINES OF EVOLUTIONARY BIOLOGY 



of the proper equilibrium. It may be that from the very first 

 the inequalities of fission resulted in the accumulation of more 

 active protoplasm in some cells and a greater amount of reserve 

 material in others, and that this was the starting point of the 

 differentiation into male and female. 



Although fusion of the nuclei (karyogamy) of the two 

 gametes appears now to be the most important feature of con- 

 jugation, we must suppose that it was preceded by plastogamy 1 or 

 fusion of the cytoplasm, which obviously constitutes the natural 

 preliminary to karyogamy. In some of the lower Protozoa such 

 plastogamy has been sometimes observed unaccompanied by 

 karyogamy, and it is possible that in some cases plastogamy 

 alone is sufficient to bring about rejuvenescence and renewed 

 activity in cell-division. 



1 Sometimes called plasmogamy. 



