318 SEX IN MICROORGANISMS 



substances can be blocked by antiserum. However, this action of anti- 

 serum was not mating-type specific. In fact, several lines of evidence 

 suggest that the reactive groups of the mating-type substances do 

 not combine directly with the inhibiting antibody. Thus, inhibiting 

 antisera can be prepared from both reactive and unreactive para- 

 mecia; both can absorb out the inhibiting antibody; antisera which 

 inhibit the reactivity of one race and mating type fail to inhibit ani- 

 mals of this same mating type but of another race; finally, animals 

 of a single race and mating type may gain or lose the ability to give 

 the blocking effect. Granting a chemical constancy of mating sub- 

 stances within a type, these results must mean that the inhibiting 

 antibodies react with antigens other than the mating-type substances. 

 Therefore it is concluded (Metz and Fusco, 1948) that the inhibiting 

 antibody combines with some "neighboring" antigen and prevents 

 intimate contact in the mating reaction by a mechanical masking or 

 steric hindrance of the mating-type substances. 



No direct attempt has yet been made to relate the mating- 

 reaction-inhibiting antigens to the immobilizing antigens of Sonne- 

 born, but in some cases, at least, the two appear to differ. Thus 

 cultures were obtained by selection (P. anrelia) in which the mating 

 reaction was inhibited by a given antiserum but was not immobilized 

 by the same serum. 



It is evident from this study that the mating-type substances are 

 associated with antigenic material, but until specific anti-mating sub- 

 stance antibodies are obtained, this approach is not likely to be re- 

 warding. Perhaps such antibodies can be prepared by immunizing 

 animals other than the rabbit. 



Parthenogenesis 



The eggs of many metazoa can develop without the mediation 

 of a spermatozoon. The developmental machinery may be set in 

 motion spontaneously (natural parthenogenesis) or through the ex- 

 perimental application of various physical or chemical agents (arti- 

 ficial parthenogenesis). In view of the many other parallels that exist 

 between fertilization in Faramecimn and metazoa, it is not surprising 

 that phenomena comparable to natural and artificial parthenogenesis 

 also occur in Paramecium. 



The protozoan counterpart of natural parthenogenesis is natural 



