126 THE PROTOZOA 



Sexual union of similar gametes is termed isogamy ; of dissimilar, 

 anisogamy. When the gametes are differentiated, then one gamete 

 is generally smaller, more active, often with highly developed 

 motor mechanisms, and without reserve food-material in the 

 cytoplasm ; this is the microgamete, regarded as male. The other 

 gamete, on the contrary, exhibits a tendency, more or less pro- 

 nounced, to be large, inert, without motor mechanisms of any 

 kind, and to store up reserve food-material in the cytoplasm 

 the macrogamete, regarded as female. The differentiation of the 

 gametes is seen to be a specialization of two kinds of cell-individuals, 

 the one rich in motile or kinetic protoplasm but poor in trophic 

 substance, the other rich in trophic protoplasm but poor in kinetic 

 substance. In some cases the sexual differentiation may affect 

 also the mother-cells of the gametes, the gametocytes, or may be 

 thrown back still farther in the series of generations preceding 

 the gametes ; in such cases a number of successive generations of 

 gamonts exhibiting sexual differentiation terminate in a gameto- 

 cyte generation from which the actual gametes arise. 



Thirdly, in the process of sexual union, or syngamy, as it may 

 be termed comprehensively, the bodies of the two gametes do not 

 always fuse completely ; in some cases the two gametes come 

 together and merely interchange portions of their nuclear apparatus, 

 remaining separate and retaining their distinct individuality. The 

 nucleus which remains stationary in the one gamete then fuses 

 with the migratory nucleus derived from the other gamete. 

 Examples of this type of syngamy are seen in the Infusoria (Fig. 77). 

 The type of syngamy in which the two gametes fuse completely 

 is sometimes termed copulation (or total karyogamy) ; that in which 

 they remain separate and exchange nuclear material, is known as 

 conjugation (or partial karyogamy), and the two sexual individuals 

 themselves as conjugants (they should not, perhaps, be termed 

 " gametes," strictly speaking, for reasons explained below) ; but 

 the term " conjugation " is often used quite loosely for either type 

 and lacks precision. 



These differences in the sexual process between Protozoa and 

 the higher organisms enable us to give a wider significance, and 

 at the same time a more precise definition, to the word " syngamy." 

 However varied in detail, syngamy is essentially nothing more 

 than an intermingling of chromatin-substance derived from two 

 distinct cell-individuals. Plus qa change, plus c'est la meme chose. 

 The chromatin that undergoes syngamic union may be in the 

 form either of chromidia or of nuclei ; in the former case the process 

 is termed chromidiogamy, in the second karyogamy. Chromidiogamy , 

 though probably the most primitive type, is known to occur only 

 in a few Sarcodina (Difflugia, p. 230 ; Arcella, p. 148). In the vast 



