FERTILIZATION 



197 



may conjugate together isogamically. Here then anisogamy is not 

 obligatory, but facultative or accidental. In this case exogamy is the 

 rule since a single colony forms gametes of one size only, though in 

 isogamy endogamy may occur. 



FIG. 103. Formation of gametes and syngamy in the Sporozoan, Klossia 

 octopiana. From Calkins, "Protozoa," after Siedlecki. The chromatin of the 

 nucleus is distributed throughout the cell, A, B, finally forming nuclei of the 

 future gametes, C, D, E. The mature gametes, s, swim about, and join a macro- 

 gamete, F. The nuclei mingle, G, and then the cleavage nucleus divides repeat- 

 edly by mitosis, to form the spores, H. d", microgametic nucleus. 



In true anisogamy conjugation is practically always exogamous for 

 as a rule a single organism forms gametes of only one size at a time. 

 The essential difference between the gametes is probably that of behav- 

 ior, i.e., degree of activity, associated with which are constant differences 

 in size. The larger gametes or megagametes, are less numerous and less 



