306 PROCEEDINGS OF THE AMERICAN ACADEMY. 



species was in all cases similarly connected with their sexual differen- 

 tiation, there was no satisfactory basis for such a belief. It is now, 

 however, hardly to he doubted that in the (+) and (— ) columns are 

 represented the two opposite sexes. 



In testing the hybridization between two different species the four 

 possible contrasts have in all cases been made. Generally the strauis 

 of one species have been successively inoculated in two stender cultures 

 between the opposite strains of the other, but with spreading forms like 

 Rhizopus and Absidia, in order to avoid the possibility of conjugation 

 between the strains of the same species, it has been necessary to oppose 

 but two strains in a single dish, and consequently in hybridizing such 

 forms four stender cultures have been made. Although the presence of 

 hybrids at the line of contact between (+) and (— ) strains of different 

 species is often indicated by a more or less distinct white line, this is 

 not always the case, and with such forms as Rhizopus and Absidia no 

 line is apparent, although a microscopic examination may show that 

 hybridization has occurred. In every instance, whether a line has been 

 visible or not, the growths at all four lines of contact have been micro- 

 scopically examined before the position of the strains has been listed in 

 the table. 



In tests made between certain species no hybrids were obtained, but 

 had the external conditions been altered it is possible that hybridization 

 would have occurred. By changing the nutriment, hybrids were ob- 

 tained, for example, between Phycomyces and Mucor Mucedo, but if, as 

 sometimes happens, it is impossible to satisfy oil the same substratum the 

 conditions which are necessary in each of the two species for the forma- 

 tion of zygospores, no hybridization is to be expected when their strains 

 are contrasted. 



The difference in the abundance of hybrids and in the distinctness of 

 the lines resulting between the sexually opposite strains of two species 

 contrasted frequently shows a difference in the sexual stimuli exerted by 

 the (+) and (— ) strains of the individual species. In the most marked 

 instance of this nature observed, a characteristic white line was formed be- 

 tween the (+) strain of Mucor n and the (— ) strain of Mucor iii, while 

 between the (— ) strain of Mucor n and the (+) strain of Mucor in 

 on the same substratum, no liybrids were found, even though a micro- 

 scopic examination was made of tlie region of contact where they might 

 have been expected. 



In the case of some species hybridization is more active with the (+) 

 strain of Mucor v, while in other species the hybridization is more 



