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ALBERT FRANCIS BLAKESLEE. 



in reality the two sexes are represented in the (+) and ( ) 

 groups is shown by the sexual reaction which may occur not 

 only when the ( + ) and ( ) races of the same species are grown 

 together and perfect zygospores are produced, but also by the 

 sexual reaction which may occur when ( + ) and ( ) races be- 

 longing to different species are grown together. This reaction 

 between the opposite races of different species has been called 

 "imperfect hybridization" since it does not lead to the formation 

 of perfect hybrid zygospores, but usually stops short with the 

 formation of progametes, though occasionally gametes are pro- 

 duced which, however, never unite. 



A sexual race of a dioecious species if grown between the ( + ) 

 and ( ) races of another test species used as a standard, will 

 show a line of imperfect hybridization on one side only. Some 



Petri dish culture showing dark line of zygospores between the (+) and ( ) 

 strains of a species of Choanephora (MC) and white lines of "imperfect hybridiza- 

 tion" between the strains of this species and the opposite strains of Mucor V. (MV). 



of the hermaphroditic species, on the other hand, when similarly 

 grown, show a response to both ( + ) and ( ) test races and 

 produce therefore two lines of imperfect hybridization. When 



