BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 277 



various external conditions have a secondary and variable effect in infiu- 

 eucing zygospore formation. 



Se7-i(il Cultures. 



In investigating the persistence of the (+) and ( — ) characters in 

 the individual strains, the writer has begun a number of serial cultures, 

 and while they are for the most part incomplete and far from satisfactory, 

 the resuLs already obtained are of sufficient interest, it is believed, to 

 warrant a presentation of them even in their present fragmentary con- 

 dition. In describing the series which were carried on as pure cultures 

 ill test-tubes, it will be convenient to represent the series by letters of 

 the alphabet, and the individual cultures in each by the letter of the 

 series followed by the sign of its sexual strain and a numerical subscript 

 indicating the generation which the culture represents. 



It has been touud that zygospores do not form in the warm oven at 

 26° -28° C, and in order to test the effect of this unfavorable tempei-a- 

 ture on the (-[-) and ( — ) strains a warm oven series was started Febru- 

 ary 20, in the tubes A(-f-)i, and A(— )i. On March 21, the (+) and 

 ( — ) strains from this series were mutually contrasted. Between A(-f-)2 

 and A(— )2, a normal zygosporic line was developed; between A(-f)3 

 and A(— )3, a few zygospores only were found in the centre of their line 

 of contact ; while from the contrast between the remaining strains up to 

 Ay, the last generations, no zygospores resulted. It was uncertain from 

 this experiment whether this loss in the power of conjugation, which 

 had been evidently induced by the unfavorable temperature, was equally 

 shared by the two opposite strains, and in order to decide the question 

 contrasts were made from all the (+) and (— ) tubes in the series ao-ainst 

 normally active (— ) and (+) strains. Transfers from tubes A (+).> to 

 A(+)5 were able to take part in the formation of a decreasing number of 

 zygospores, but contrasts against transfers from tubes A(-|-)g to A(-l-)g 

 produced no zygospores. With transfers made from the (— ) tubes it 

 was possible to obtain zygospores only in the case of tube A(— ).,. More- 

 over, when this latter tube was contrasted between A(-|-)3 and A(+)^, 

 pooily developed white tufted lines occurred at the areas of contact and 

 were found to be made up of arrested stages of conjugation suggestive of 

 the imperfect hybrids discussed in a later section. The line on the 

 side of A(+)4 was slightly better developed and showed its superiority 

 over the other in possessing a single perfect zygospore. A similar con- 

 tact of A(+)2 between A(— )3 and A(— )^ resulted in neither zj'gospores 

 nor arrested stages of development and showed, as the contrasts with 



