BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 281 



is a difference in vigor between the two opposite strains which, as is 

 shown in the section on hybridization, is correlated with their sexual 

 differentiation. 



Phycomycks nitens. 



The zygospores of this heterothallic species are extremely rare, having 

 been reported, in so far as is known, by but two observers, although the 

 sporangial condition is not uncommon in spontaneous cultures, especially 

 ou horse dung. After the secret of zygospore formation was discovered 

 in M. Mucedo, the writer endeavored to apply the principle involved to 

 obtaining zygospores of other forms then at his disposal, and contrasts 

 were therefore made between the different cultures of a number of species 

 then grow'ing in the laboratory. By a fortunate chance, the first two 

 cultures contrasted of Phycomyces contained the opposite strains which 

 exist in this species, and consequently a black line of its huge thorny 

 zygospores was produced at the region of contact of the two mycelia. 



The zygospores are peculiar, not only on account of the dichotomously 

 branched outgrowths, which arise from either suspensor, but also from 

 the method in which the progametes are developed. Van Tieghem and 

 Bainier, the two investigators who have done the most work on the 

 group here considered and the only ones who have described the zygo- 

 spores of Phycomyces, have figured stages, showing both suspensors iu 

 connection with the same mycelial hypha. Though such an account is 

 incorrect, the condition seemed, a priori, not impro1)able at the time the 

 observations were made, and the fact that the lower portions of the 

 progametes are intricately convoluted and closely adherent renders a 

 determination in every case of their basal connections a task of unusual 

 difficulty, especially when one is dependent on stages obtained from 

 spontaneous cultures. "With the two sexual strains in hand, however, 

 it has been comparatively easy to control the production of zygospores 

 and to follow the main steps in the process of their development. 



The stages figured in Plate III, Figures 45 to 50, were taken from 

 van Tieghem cell cultures, from the regions of contact of the (+) and 

 (— ) strains. In these cultures, no doubt because of the small amount 

 of nutriment afforded in the thin layer of agar used, zygospores have 

 not been formed in abundance, but their development has been none the 

 less easily observed. 



When a (+) mycelial hypha is met by one from the (— ) mycelium, 

 stout much-lobed branches arise from both at their places of meeting, 

 and become adherent with interlocking of their lobes (Figures 45 to 47), 

 and from the development of two branches thus in contact the pro- 



