SEXUAL PHENOMENA OF THE PROTOZOA 221 



the same individual. Maupas's conclusion, therefore, that cross- 

 fertilization is necessary for Infusoria, as for Metazoa, appears to 

 have been somewhat premature, although in view of the extreme 

 care with which his observations and experiments were made, 

 the objections which have been brought against it are not entirely 

 conclusive. 



It is evident from the foregoing review that, with the exception of 

 sex-differentiation, all of the essential features which characterize 

 fertilization are present in those forms of Protozoa where conjugation 

 takes place between similar adult individuals. Here, also, a hint as 

 to the significance of fertilization is seen in the fact that the form of 

 the conjugating individuals is altered, thus indicating some change in 

 the density of the protoplasm. Thus some Mastigophora and Sarco- 

 dina become viscous, and some Infusoria show unmistakable signs 

 of exhaustion. Under these changed conditions the fusion of the 

 cell-body is possible (plastogamy). This fusion may be partial 

 (Cystoflagellidia, Gregarinida, Infusoria), or total (Monadida, Heliozoa, 

 Rhizopoda), and it may or may not be accompanied by nuclear fusion 

 (karyogamy). The same, stages may be conceived for the union of 

 the nuclei as for the union of the cell-bodies, the evidence appearing 

 to show that as plastogamy is the outcome of cytotropy, or positive 

 chemotaxis, so karyogamy is the outcome of karyotropy or nuclear 

 attraction, and is made possible by plastogamy. 



2. The union of similar but different-sized individuals, 



Sexual differentiation is established when the conjugating organ- 

 isms are of different size. No sharp line, however, can be drawn 

 between conjugation in isogamous and anisogamous forms, but a 

 number of instances might be cited in which the union of different- 

 sized individuals is purely facultative, and the same result is accom- 

 plished either by the union of similar or of dissimilar forms. Thus, 

 of two conjugating Bodos (Heteromita\ one, which is formed by 

 transverse division, is motile and becomes attached to a stationary 

 form resulting from a longitudinal division, and anchored by one of 

 its flagella. With the exception of these differences, which certainly 

 indicate some internal difference in the gametes, the conjugants are 

 identical. Here, for the first time, a distinction can be made between 

 the more quiescent and the more motile conjugant, although it is not 

 marked by difference in size. The latter condition, however, exists 

 in Polytoma, where, according to Krassilstschik ('82) and Dallinger 

 and Drysdale, normal free-swimming forms unite with smaller ones. 

 Here, however, the process is purely facultative, for conjugation 

 between similar-sized individuals also takes place. Biitschli does not 



