1907] BRIEFER ARTICLES 417 
fact that out of 46 cultures on bread, 6 (over 1 3 per cent.) were destroyed by 
bacteria and 13 more were probably also infected by bacteria, since they 
produced only a scanty mycelium devoid of fructifications, shows that 
NAMYsLOWSKI’s cultures were far from pure, and renders it probable that, 
in'the 14 instances in which zygospores were obtained out of the 46 cul- 
tures, the (+) and (—) strains had become present through infection; 
while in the 13 cultures in which only sporangia were obtained, whatever 
infection occurred was of the same strain as the spores used in the inocula- 
tion. The rg cultures on bread from the same sporangium would give 
zygospores if the inoculating material became infected by spores of the 
opposite race before sowing, even though subsequent infection did not occur. 
That zygospores may be obtained by accidental infection of a favorable 
substratum with both (+) and (—) spores has been repeatedly demon- 
strated by the writer’s own experience in the Harvard laboratory. Pro- 
fessor CAMPBELL, moreover, has written that for ten years he has gotten 
zygospores by spontaneous infection of suitable substrata, and an investi- 
gation showed that in zygosporic material sent from his laboratory only the 
two (+) and (—) strains were present which alone were incapable of 
zygospore formation. 
Since the above was written, Dr. NamMysLowskI has kindly sent the 
writer zygosporic material of the Rhizopus nigricans which he used in his - 
experiments. Inoculations were at once made into sterilized nutrient and 
a large number of the young zygospores resulting were isolated and laid on 
cleared nutrient agar in Petri dishes. Paired cultures X & XX and Y & 
YY were obtained by making mycelial transfers from the outgrowths from 
the suspensors of two zygospores both of whose suspensors had germinated. 
On the assumption that the species is heterothallic, each pair should contain 
the two opposite strains, and each strain when grown alone should be incap- 
able of zygospore formation. In addition, cultures Z & ZZ were obtained 
from suspensors that could not be traced with certainty to the same zygo- 
spore, and cultures A-D were obtained from the germination of single 
isolated sporangiospores. By opposing inoculations from these cultures 
against one another and against standard (+) and (—) test strains, it was 
seen that X and Y are (—) and XX, YY, Z, ZZ, and A-Dare(+). The 
contrasts numbered over 40 and were made for the most part on sterilized 
flour paste in stender dishes, but were controlled by cultures on nutrient 
agar in test tubes. None of the strains produced zygospores on substrata 
favorable to zygospore formation when sown alone in pure cultures, nor 
when sown together with the same strain from a different source. They 
have always formed them, however, whenever the opposite strains are grown 
