BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 303 



HYBBIDIZATION. 



In the Introduction it has been already stated in some detail that 

 there is a sexual response not only between the(-(-) and ( — ) strains of 

 the same species but also between (+) and (— ) strains of dill'erent 

 species, and that no interaction ever occurs between strains with like 

 signs. Although the process has not been observed to go further than 

 a cutting oft" of the two gametes, and more frequently stops at the pro- 

 duction of the two progametes or the formation of a gamete on but one 

 side, the word " hybridization " has for convenience been applied to 

 these imperfect attempts at conjugation. Just as, when the hyphae of 

 two sexually opposite strains of the same species are allowed to come in 

 contact, a dark line may be apparent by the accumulation of zygospores 

 there formed (Plate IV), in a similar manner, when hyphae of sexually 

 opposite strains of two different species are allowed to come in contact 

 a white line may be apparent, which results from the accumulation of 

 these attempts at hybridization above mentioned. Plate IV, Figure 57, 

 is a photograph of a culture made to illustrate these lines of z^^gospores 

 and hybrids. Where the (+) and (— ) strains of Mucor v have come 

 in contact, a very wide, dark line resulting from the zygospores there 

 formed is apparent, while between the (-[-) growth of Mucor v and the 

 ( — ) growth of a different heterothallic species, Mucor n, as well as be- 

 tween the (— ) growth of Mucor v and the (+) growth of Mucor n, a 

 white line resulting from the imperfect hybrids there formed is distinctly 

 evident. The absence of such a white line between the growths with 

 like signs results from the fact that no hybridization has occurred at 

 their areas of contact. 



The first suggestion that an interaction is possible between sexual 

 strains of different species arose during the investigation of the natural 

 occurrence of sexual strains of Mucor Mucedo. A pure sporangial trans- 

 fer from an unbranched Mucor growing on rabbit dung was inoculated 

 into a stender dish together with inoculations of (4-) and (— ) strains of 

 M. Mucedo so arranged that the mycelia in developing would come in 

 contact with one another. In two days an unusually broad light line, 

 apparently of young zygospores, resulted between the growth from 

 this pure transfer and the {-\-) strain off M. Mucedo, and a similar 

 line of less width occurred between the (-f) and (— ) strains of M. 

 Mucedo. The day following the zygospores of M. Mucedo had become 

 black in maturing, but the line between the unknown Mucor and 

 the (+) strain of M. Mucedo remained white, and the conjugations 



