Tin: PUVSIOLOCn of I'l-RlIMZAriON IN CIMATFS 327 



rion of some of the problems of fertilization may he obtained by 

 anah'/ing tiic "partialh' isolated" system as was done for Paramechivt. 

 Many scattered attempts to activate eggs with dead sperm and sperm 

 extracts are recorded in the literature. Althouirh these have been un- 

 successful, a more systematic effort \\'ould now seem in order. Indeed, 

 a beginning in this direction has been made by Hutner and Provasoli 

 (1951) in the alga Clamydomonas. These investigators found that 

 gametes killed by formalin, heat, ultraviolet light, or Streptomycin 

 clumped strongly with gametes of complementary mating type. Like- 

 wise Metz (1945) and Metz and Donovan (1951) showed that sea 

 urchin sperm killed w'lxh. heat {Strongyloceiitrotiis pnrpuratns) or 

 with Bouin's fluid {Arbacia) agglutinated specifically with fertilizin. 

 Evidently the sperm retained its specific organization when killed by 

 these agents, at least so far as one surface group is concerned. As yet 

 attempts to activate eggs with such dead sperm have failed, but fur- 

 ther efforts may yet demonstrate the utility of the method in such 

 material. 



Regardless of the outcome of such attempts with eggs and sperm, 

 the ciliates offer a \\'ealth of material for the investigation of sexual 

 phenomena that should not be neglected. In this material a number 

 of different sexual mechanisms appear to exist, any one of which may 

 serve as a useful model for studies on other forms. In Paramecium 

 distinct mating types occur, and sexual reactions depend upon inter- 

 action of surface substances. In Eiiplotes "mating types" of a different 

 sort are found, and diffusible substances appear to mediate conjuga- 

 tion. Vorticella produces macro- and microconjugants by a single 

 division, and these may then copulate (Finley, 1939, 1952). The pos- 

 sible relation of sexual cycles to hormonal activity of the host in 

 parasitic forms such as Nyctotherus (Wichterman, 1937) presents 

 new opportunities for the investigation of the role of defined sub- 

 stances in sexual control. As a final attribute mutant stocks deficient 

 in their sexual mechanisms occasionally become available in this mate- 

 rial. As Metz and Foley (1949) have shown, analysis of such defec- 

 tive stocks can provide valuable information concerning fertilization. 

 xApart from such examples as those cited here, great numbers of cili- 

 ates remain to be studied by modern experimental methods. A thor- 

 ough examination of this reservoir of material certainly should extend 

 our general understanding of the physiology of fertilization, 



