THE PHYSIOLOGY OF FERTILIZATION IN CILIATES 325 



intimate union of conjugants is required for micronuclear changes 

 in E. J.wpih Ne\ertheless, some physiological change does occur in 

 E. harpa even before permanent union is established, for pairs sepa- 

 rated at this early stage did not reunite or conjugate with other ani- 

 mals. AMiether this is a delayed effect of a fluid agent or a result of 

 initial union remains to be determined. Indeed, fluid agents have yet 

 to be demonstrated in E. harpa. 



The possible relationship between conjugation-inducing agents 

 from fluids and the responsive animals presented above can be for- 

 maUzed in terms of specific interacting substances. As Kimball (1943) 

 is aware, this relationship forms a system resembling the blood groups 

 in man. However, further speculation on Euplotes is pointless because 

 Kimball's stocks appear to have been lost and there is little prospect 

 of further investigation in the immediate future. 



Downs (1952) has described five mating types in another hypo- 

 trich, Stylouichia piitrina. These five types constitute a variety since 

 conjugation occurs when any two of them are mixed. Apparently 

 Downs has not yet tested animal-free fluids for conjugation-inducing 

 action. Therefore any comparison with the related Euplotes would 

 be premature. 



CONCLUSIONS REGARDING METAZOAN FERTILIZATION 



Thirty years ago the major problems in metazoan fertilization 

 seemed on the threshold of solution, for Glaser (1921) confidently 

 wrote "whatever transformations our views on the initiation of devel- 

 opment may undergo within the next few years, the zone within 

 which we seek for understanding is now marked off by the reaction 

 capacities of perfectly definite physiological compounds." Unfor- 

 tunately the present outlook is not so optimistic, for the factors 

 responsible for initial adhesion, specificity, activation, and most other 

 aspects of sperm-egg interaction remain obscure. 



Of the various "sex substances" that have been obtained from 

 metazoan gametes only the tg^ membrane lysins from sperm have 

 well-understood action in fertilization (for recent discussions see 

 Krauss, 1950; Berg, 1950; Swyer, 1951.) These ^o^g membrane lysins 

 remove mechanical barriers that would otherwise block the approach 

 of the fertilizing sperm. In certain cases at least, these agents supply 

 a specificity factor in fertilization. 



