BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 307 



active with its (— ) strain. A difference in the sexual activity of the 

 (+) and (— ) strains of Mucor v thus seems apparent. If, however, 

 instead of assuming, as has been done in the previous sentence, that the 

 strains of all the other species are mutually equal in the strength of the 

 sexual stimulus which they exert, we consider the strains of Mucor v as 

 equal in this respect, it will be found that with some species the (+) strains 

 and with others the ( — ) take the more active part in hybridization. 

 This inequality in the sexual activity of the opposite strains in contrast 

 between two given species has been constant on the limited variety of 

 the substrata tested, but wliat significance should be attributed to it is 

 not at present entirely clear. As will be later pointed out in the dis- 

 cussion of hybridization with homothallic forms (p. 311), the inequality 

 in the lines of hybrids may be correlated with the sexual differentiation 

 of the ( + ) and ( — ) strains. 



The difference which has just been under consideration is one between 

 the individual strains of a single species, and can be seen only when 

 hybridization is incited between two forms. Individual species, how- 

 ever, show varying intensities of sexual vigor, and while some can be 

 induced to form zygospores only under special conditions, others will 

 produce them under every condition to which they have as yet been 

 subjected whenever their two strains are allowed to grow in contact. 

 This condition is well illustrated by the fact that on certain substrata 

 the production of the zygospores by (+) and (— ) strains of Mucor iv 

 does not occur, while distinct lines of hybrids are produced between 

 strains of this species and of Mucor v respectively which are dis- 

 tinguished by unlike signs. Under somewhat different conditions of 

 temperature and food supply contrasts of (+) and (— ) strains of this 

 form yield zygospores, but it is certainly remarkable that under any con- 

 ditions the sexual response between them should be less intense than 

 when they are contrasted against strains of a different species. 



Mucor Macedo var. A is the only form in which the process of hybri- 

 dization has been followed on slide cultures. The formation of progam- 

 etes with (-f-) strains of M. 2Iucedo appears to be essentially similar to the 

 process where the contrast is made between the (+) and (— ) strains of 

 the latter species. In only a few instances have terminal cells been 

 observed to have been cut off by septa in both progametes (Plate I, Fig- 

 ure 22) . It frequently happens, however, that the gamete on one side 

 only is distinguished (Figure 21), and a careful search in a number of 

 slide cultures shows that, in the material examined, when only one 

 gamete was formed, it invariably has been derived from the mycelium of 



