SEXUAL PHENOMENA IN THE PROTOZOA 219 



Rhumbler reached similar results in the fused Difflngias. The 

 latter, however, calls attention to the fact that two organisms thus 

 united are subject to the interchange of substances through osmosis, 

 and he maintains that such an interchange must take place between 

 them. This interchange may even extend to the substances of the 

 nuclei, which are constantly renewed from, and given off to, the 

 cytoplasm. 



Although fusion of the nuclei of the forms just mentioned does not 

 take place, the stimulus of the cytoplasmic interchange in some simi- 

 lar cases is apparently sufficient to bring about reproduction. Thus, 

 in certain Reticulariida (Patellina corrugata and Discorbina globularis, 

 Schaudinn, '95), two, three, four, or even five individuals may fuse 

 and form embryos without a previous nuclear union. In these cases 

 plastogamy alone is apparently a sufficient stimulus for reproduction. 



It is by no means a fanciful assumption to postulate the union of 

 the two nuclei, or karyogamy, through conditions similar to those 

 which lead to the union of two organisms through cytotropy, i.e. 

 mutual attraction and consequent fusion when the nuclear plasm is 

 in the right condition, a condition defined by Dangeard ('99) as 

 " sexual hunger." 



Almost all cases of karyogamy are complicated by nuclear pro- 

 cesses analogous to maturation in Metazoa, a few doubtful cases 

 among the Mastigophora and Rhizopoda alone indicating that fusion 

 may take place without a preliminary loss of a portion of the nucleus. 

 Thus Jickeli ('84) and Rhumbler ('98) observed two individuals of 

 Difflugia glbbulosa with but a single nucleus in conjugation, and 

 similar observations by Penard ('90) upon a number of different 

 species indicate that the phenomenon is widespread (Fig. 118, A). 

 In D. lobostoma, Rhumbler occasionally found two mouth openings 

 in one shell, and interpreted it as a case of fusion of shells as well 

 as of protoplasm (B, C). Blochmann ('88) observed the fusion of 

 two Euglyphas and the formation of a large double shell. In both 

 cases there was but one functional nucleus observed, although in the 

 latter form the peculiar behavior of the nucleus in one animal was 

 very suggestive of maturation {vide infra). 



The union of nuclei in temporary conjugants among the Flagel- 

 lidia has been observed in at least one case, Noctilnca miliaris 

 (Cienkowsky, '73, an d Ishikawa, '91). Two individuals fuse, their 

 nuclei come together, but do not fuse, and then separately undergo 

 mitosis, which results in four daughter-nuclei. These separate and 

 then fuse in the daughter-cells, two by two ; thus nuclear fusion 

 takes place some time after conjugation. In the simpler flagellates 

 the nuclei fuse before spore-formation (Dallinger and Drysdale, 



'73)- 



