404 



SCIENCE 



[N. 6. Vol. LI. No. 1321 



TABLE I 



'Doric" Ahsidia 



giuin may produce mycelia, neutral toward 

 each other, though reacting with the plus and 

 minus parent stocks. Here again the neutral- 

 ity or self-sterility is only temporary, since 

 after a few spore generations their ability to 

 take part in active zygospore formation inter 

 se is completely established. 



In nature the number of neutral strains hp- 

 pears to be large and many si)ecies have been 

 studied, the races of which have never been 

 induced to react, either inter se or with 

 strong testers of other species. There is no 

 evidence that even in these cases, the neutral- 

 ity is absolute and the races completely de- 

 void of sexual tendencies. Their apparent 

 neutrality may mean merely that we have not 

 yet happened to expose these forms to the 

 peculiar environmental conditions necessary 



for an expression of the sex which is actually 

 present. 



SEXUAL DIMORPHISM 



Similar tabulations of the sexual reactions 

 between their races have been made for 

 several species beside the " dark " Absidia and 

 with similar residts (Table II). All the races 

 which were able to assist in zygospore forma- 

 tion appear to be sexually dimorphic, consist- 

 ently either plus or minus. A few investiga- 

 tors have believed they have found evidence 

 in certain species that would militate against 

 sexual dimorphism in the diecious mueors. 

 In a specific instance (8), however, where it 

 was possible to retest the material upon which 

 such conclusions were based, it was found that 

 the dimorphism was in fact present but, for 



