1921] BLAKESLEE, CARTLEDGE, & WELCH—CUNNINGHAMELLA 217 
extreme of a continuously graded series of sexual vigor, and the 
term as applied undoubtedly includes both plus and minus races. 
It is doubtful whether much significance can be attributed to 
the proportion of plus and minus races in the collections of the 
different species of Cunninghamella as indicative of their relative 
distribution in nature. In C. bertholletiae the minus sex seems to 
greatly predominate over the plus. In C. elegans the condition is 
reversed. The first species was obtained from Brazil nuts bought 
in different stores, mostly in or around New York City. Many of 
the gross cultures, therefore, may have originated from the same 
wholesale shipments. The races may be representative of the 
shipments from which they came rather than of the locality where 
they were grown. Experience with Rhizopus (4) indicates that in 
a mixed culture which is producing zygospores in abundance, one 
is likely to isolate almost exclusively one or the other of the two 
sexes. The cargo carriers from which the nuts originated may 
have been infected chiefly with minus strains. That there is 
considerable diversity in sexual vigor of these strains, however, is 
- seen from the tables. C. elegans was obtained from different types 
of soil around Cold Spring Harbor, and it is possible that collections 
from other regions would show a predominance of the opposite sex. 
The clearest result from the study of Cunninghamella is the 
fact that in 2091 contrasts (2250 including contrasts between 
different species of Cunninghamella) made between 202 races from 
four different species (see footnote 4) there were none which, if they 
showed any sexual response at all, reacted otherwise than as either 
a plus or a minus. 
Miss Atice M. Pricket, Miss MARGARET CONOVER, and Miss 
Mary E. Drummonp have assisted in the progress of the investi- 
gation which is here reported. 
Summary 
1. The terms heterothallic and homothallic are distinguished 
as applied to gametophytic sexual differentiation in the mucors. 
2. Types of the evidence in support of sex intergrades in hetero- 
thallic mucors are given and criticized. 
