PROTOZOA. 185 



the nuclei, a process comparable to the formation of the polar glob- 

 ules in the egg, to this extent, that in each of the conjugating 

 individuals the nucleus divides twice and of the products of divi- 

 sion only one, the nucleus intended for caryogamy, persists while 

 the others (polar globules) degenerate. 



These cases of true fertilization permit of great diversity. The 

 conjugating individuals can be equal in size (most Infusoria, many 

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

 which smaller and consequently more mobile ' males ' (microga- 

 metes, zoospores) fertilize the larger fixed or slowly moving 

 ' females ' (macrogametes, oospores) as in Volvox globator, Vorti- 

 cellidae, and many Sporozoa. In conjugation of individuals of 

 equal size there is frequently a mutual fertilization A fertilizes 

 B, and is in turn fertilized by B after which the animals sepa- 

 rate (most Infusoria, Gregarines, Noctiluca). 



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

 Protozoa in contrast to the Metazoa lacked sexuality. In the mean time 

 observations on Protozoa belonging to different classes, even the Rhizop- 

 oda, have so increased that the conclusion is that fertilization occurs in 

 all Protozoa, although the rarity of the process in many species renders 

 the complete demonstration difficult. Still there remain certain interest- 

 ing 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. It does indeed occur (swarm-spore formation 

 in Noctiluca, formation of pseudonavicellee in gregariues) that fertilization 

 precedes rapid division, but much more commonly fertilization is the 

 result of rapid division and a cause of slower reproduction (Infusoria) or 

 even of complete rest (Actinosphcerium, Actinophrys, Volvox). One can 

 therefore only speak of fertilization, not of sexual reproduction, in the 

 Protozoa. These facts are of great importance in the consideration of the 

 nature of impregnation, for they show that it has not only the purpose of 

 stimulating the developmental processes, but that it accomplishes other 

 functions, and that these functions, obscure as they at present are, are 

 the more important since they are the more primitive and the more widely 

 distributed. 



With Noctiluca, many Sporozoa, and perhaps in Rhizopods a period 

 follows impregnation in which the division (* sexual reproduction ') has a 

 special character (swarm-spore formation in Noctiluca, formation of sporo- 

 blasts and sporozoites in the Sporozoa) and differs from the customary 

 ' vegetative ' reproduction. This alternation of methods of reproduction 

 recalls the alternation of generations of the Metazoa and is called by the 

 same name. 



The Protozoa with thin small and soft protoplasmic bodies are 

 but little if at all protected against drying up, and therefore they 



