312 PROCEEDINGS OF THE AMERICAN ACADEMY. 



related with the difference previously observed in the vegetative vigor 

 of the sti'ains tested. If such be the case it will further emphasize the 

 conclusion that the two opposite sexes are represented in the series con- 

 taining respectively the vegetatively more and less vigorous strains. 



By the facts presented in tlie foregoing section, it has been shown 

 that a process of imperfect hybridization will occur between unlike 

 strains of different heterothallic species in the same or even in different 

 genera or between a homothallic form and both strains of a heterothallic 

 species, and that, by taking advantage of this character, it has been 

 possible to group together in two opposite series the strains of all the 

 heterothallic species under cultivation. It has been seen, moreover, that 

 when thus grouped the (— ) or less luxuriant strains of all species 

 known to show a vegetative difference between their opposite strains 

 will be in one series, while the (+) or more luxuriant strains will be in 

 the other. It has also been shown that when a contrast is made between 

 the opposite strains of two different heterothallic species or between a 

 homothallic form and both (+) and ( — ) strains of a heterothallic species 

 the activity exhibited by these strains in the formation of hybrids at 

 their lines of contact is generally unequal. 



Hybridization may be assumed to indicate that the formation of 

 progametes is determined by the stimulus of contact between the (+) 

 and ( — ) hyphae whether of the same or of different species, and that 

 the formation of zygospores is dependent on the union of gametes of 

 the same species. From the facts observed it will be concluded that 

 the formation of zygospores is a sexual process, that the mycelium of a 

 homothallic species is bisexual, while the mycelium of a heterothallic 

 species is unisexual, that differences in the hybridizing activity observed 

 in opposite strains is correlated with the differences in vegetative 

 activity which they exhibit, and further that in the (+) and (— ) series 

 of the heterothallic species are represented the two opposite sexes. 



SUMMARY. 



An outline of the general conclusions which have been reached as a 

 result of the investigations embodied in the present paper has already 

 been presented in the Introduction. A more extended discussion of the 

 relations of the problems which have been suggested by the work of 

 the past year the writer reserves until he has accumulated a greater 

 body of facts on the subject. It now remains to give merely a brief 

 recapitulation of the principal results considered in some detail in the 

 foregoing pages. 



