ONTOGENESIS 145 



because of their greater number, would have more 

 chances for meeting and conjugating with one 

 another, assuming, as is probably the case, that their 

 meeting is purely accidental. But this sort of a 

 specialization would also have its drawback, arising 

 from the fact that the reduction in size of the micro- 

 gamete also involves a reduction in the amount 

 of reserve food material stored within the cell. 

 The latter is an important consideration in tiding 

 the zygote over the critical period of encystment, 

 and on this account it is evident that in such a 

 form as the Bodo just described the zygote arising 

 from the accidental fusion of a larger gamete 

 (megagamete) with a microgamete would stand 

 a better chance of surviving and perpetuating the 

 race than the zygote formed of two fused micro- 

 gametes. As a matter of fact, we find that in the 

 great majority of existing forms zygosis l occurs be- 

 tween dissimilar gametes, a condition we have 

 called anisogamy. The distinction is very likely 

 more than that of mere size, and probably involves 

 subtle differences in protoplasmic organization, but 

 our conclusions in that regard are wholly inferential. 

 Even in isogamy, the two conjugating gametes 

 may be physiologically different ; that is, the mor- 

 phological difference may develop secondarily. 



1 The partial or complete fusion of two gametes is more often referred 

 to as "fertilization," but this word, which represents an archaic con- 

 ception of development, is misleading. Zygosis, a word coined by 

 Lankester to denote the fusion of the gametes to form the zygote, is 

 preferable because it describes the phenomenon without interpreting it. 



