200 GENERAL EMBRYOLOGY 



number of cells which may reproduce in this way is limited, and there 

 seems to be a real distinction between soma and germ, much like that 

 of the Metazoan organism, the mother cell which divides to form the 

 multiple spermatozooids, has even been compared with the testis of a 

 Metazoan. 



The principal forms of fertilization and gamete formation are sum- 

 marized in the accompanying table. 



Several facts of prime importance are to be drawn from this account. 

 (a) Among the Protozoa, as well as the Metazoa, the process of fertiliza- 

 tion is widespread. (6) Out of a variety of forms comes that form of 

 fertilization characteristic of the Metazoa, namely, karyogamy. (c) 

 Accompanying this karyogamy is a gradual and finally complete 

 differentiation of gametes, which differ morphologically and physio- 

 logically, both from vegetative cells and from each other, (d} The 

 Metazoa show much less diversity than the Protozoa respecting the 

 process of fertilization and the form of the gametes. 



Such a series of stages as that outlined above, of the gradual differ- 

 entiation and specialization of gametes, cannot fail to suggest the 

 general subject of sex. It does indeed indicate the nature of the 

 original distinction between the sexes. Among the Metazoa the pri- 

 mary and familiar facts upon which the definition of sex is based, are, 

 that spermatozoa-producing individuals are males, ova-producers are 

 females. In all cases of isogamic conjugation no distinction between 

 the gametes, and therefore between sexes, obtains. In those instances 

 transitional between isogamy and anisogamy, we may see the begin- 

 nings of sex distinction, often facultative. True anisogamy involves true 

 sex distinction; at first relatively slight (Pandorina), in such forms as 

 Volvox or Coccidium and many other Sporozoa, the fundamental 

 differentiation of sex seems to be completely established, i.e., the 

 gametes are markedly unlike and conjugation occurs only between two 

 dissimilar cells. 



The essential processes of fertilization are entirely equivalent in 

 isogamy and anisogamy, so that the fundamental distinction of sex is 

 based only upon the external form and behavior of the gametes, not 

 upon any differences in the nature of the conjugation processes in sexual 

 and non-sexual forms, for none exist. 



Most of the colonial Protozoa are monoecious or hermaphroditic, 

 producing gametes of both kinds, but cross-fertilization (exogamy) is 

 the rule here, as it is among the hermaphroditic Metazoa, and frequently 

 for the same reason in both groups, namely, a difference in the times of 

 ripening of the two forms of gametes of a single colony or individual. 

 Many of the Sporozoa are clearly dioecious or unisexual, and in some of 

 these there are also secondary sexual characters, usually size differences, 

 such that macrogamete-forming or female individuals can be distin- 



