326 SEX IN MICROORGANISMS 



Among other agents the sperm and egg isoagglutinins, fertihzin 

 and antifertiHzin (obtained from eggs and sperm respectively), have 

 been studied most extensively. The highly specific nature of their 

 interaction and the dramatic character of the agglutinating action 

 suggest that these substances are essential factors in metazoan fertili- 

 zation. This may prove to be the case, but as yet substantial exper- 

 imental evidence to support this view is wanting. Initial adhesion 

 of gametes may involve a fertilizin-antifertilizin reaction (Tyler, 

 1948), and a modest case may be made for a role in specificity 

 determination for these agents, but again experimental support for 

 these possibilities is lacking. A clear demonstration of the function 

 or functions of these extraordinary substances is much needed and 

 may well prove to be the turning point in the analysis of fertilization 

 in metazoa. Unfortunately, we have no more positive information 

 concerning the role of specific substances in the attachment of sperm 

 to egg, specificity determination, and activation initiation in metazoan 

 fertilization. For detailed accounts of the present status of these 

 problems, the reader should consult the reviews listed on page 284. 



For Parmnecium our understanding of the mechanism of fertili- 

 zation has progressed to a somewhat more satisfactory position. The 

 analysis of fertilization in Paraiiieckim clearly demonstrates the pri- 

 mary role of the cell surface in fertilization. It appears that initial 

 adhesion, fertilization specificity, and initiation of activation depend 

 upon the interaction of complementary surface substances, the 

 mating-type substances. No diffusible agents of any kind appear to 

 operate. Faraviechnu then exemplifies the classical concept of a 

 "trigger reaction" in fertilization. As many workers have pointed out, 

 a similar interaction of surface substances may function in metazoan 

 fertilization. It is certainly clear from the many observations on 

 normal fertilization in the sea urchin (see Dan, 1950, for a recent 

 account). Nereis (Lillie, 1911, 1912) and the starfish (Fol, 1879; 

 Chambers, 1930; Horstadius, 1939) that activation of the egg results 

 from a rather superficial sperm-egg contact. Apparently this reaction 

 does not even require a firm union between sperm and egg, for 

 Goodrich (1920) was able to remove the activating sperm intact (?) 

 from the surface of the Nereis egg by microdissection. Similarly, 

 Reverberi (cited by Runnstrom, 1949) found, in certain interspecific 

 tunicate crosses, that the activating sperm not only failed to penetrate 

 but actually fell from the egg surface in some cases. Perhaps clarifica- 



