PROTOZOA 169 



poda), or there is a disparity in size (sexual dimorphism), in which 

 smaller and consequently more mobile 'males' (microgamctes, zoosporcs) 

 fertilize the larger fixed or slowly moving 'females' (macro gametes, oospores) 

 as in Yorticellidce, most Sporozoa, and flagellates, forming with them 

 a permanent zygote (copulation). The formation of a zygote can also 

 occur by the permanent fusion of two isogametes, but usually the union 

 of isogametes is transitory (conjugation) and lasts only long enough for 

 cross fertilization, gamete A fertilizing B, and in turn bring fertilized by 

 B, after which the two separate. A striking phenomenon is the not very 

 rare 'autogamy' in which the mother animal divides into two daughter 

 animals, which form polar globules and fuse to a zygote, an extreme case 

 of inbreeding. 



Thirty years ago it could be laid down as a universal fact that the Protozoa 

 in contrast to the Metazoa lacked sexuality. Since then observations have so 

 increased that the conclusion is that fertilization occurs in all Protozoa, although 

 the rarity of the process in many species renders the demonstration difficult. 

 Perhaps also in many groups fertilization has been lost through degeneration 

 (similar to the apogamy of plants). 



Still there remain certain interesting differences from the Metazoa. The 

 Protozoa lack special sexual cells eggs and spermatozoa. On the contrary, 

 the whole body functions as a sexual cell. Further, the relations of fertilization 

 to reproduction are not the same as in the Metazoa. (i) Protozoa may increa e 

 in the same way before and after fertilization, indeed somewhat more slowly 

 after (Infusoria). (2) Sometimes fertilization brings nourishment and repro- 

 duction to a standstill, in which case encystment appears (many rhizopods and 

 flagellates). (3) A third case is where division follows fertilization, occurring 

 more rapidly and having another character (sexual reproduction, better 'nutu- 

 gamic division,' 'sporogamy') than the pre-fertilization divisions (asexual repro- 

 duction, 'schizogony,' better, 'metagamic reproduction'). These alternating 

 pro- and metagamic reproductions have been called alternations of generations 

 (most Sporozoa, many rhizopods). 



Analysis of these phenomena leads to the conclusion that we may speak ef 

 fertilization but not of sexual reproduction in the Protozoa. As was said pre- 

 viously (p. 149) these facts have great importance in the explanation of the 

 existence of fertilization, since they show that it has not always the purpose c f 

 stimulating the reproductive processes and thus leading to the formation of a 

 new individual. Fertilization has to accomplish other things for the organism; 

 they must be of great importance, since they are so widely spread; but as yet 

 their significance is not clea 1 ' 



The Protozoa, with small and soft protoplasmic bodies, are but slightly 

 protected against drying up, and therefore they are aquatic. Nmie, 

 like Amoeba terricola, are terrestrial, but these only occur in moist places. 

 Salt and fresh water, of the latter stagnant pools rich in vegetation, are 

 the favorite places. The fresh- water forms are cosmopolitan, so that the 

 species in all lands are very similar. This depends upon certain peculiar- 

 ities. The fresh-water Protozoa can become encysted and in the encysted 



