1921] BLAKESLEE, WELCH, & CARTLEDGE—MUCORS 165 
the individual species demanded. Two per cent agar with the addi- 
tion of different nutrients was used as a substratum in all cases. 
_ The. formula varied with the different species tested. No. 230 
standard stock nutrient (already described) was used in the tests 
when possible. Before starting an extensive series of tests of a 
given species, however, the effects of a number of different nutrients 
were tested, and the one chosen which was able to support a relatively 
abundant production of zygospores. For ‘‘imperfect hybridiza- 
tion’’ reactions no better nutrient was found than no. 362, which 
is a milk whey agar consisting of 2 per cent agar, 1 per cent dextrose, 
and 2 per cent dry milk whey powder. Some species form zygo- 
spores at laboratory temperatures, while others require a higher 
temperature for sexual reproduction. The species of Cunning- 
hamella investigated belong to the latter class, and accordingly 
tests of this genus have been grown in the incubating oven at 
temperatures between 24° C. and 28° C. 
CULTURE DISHES.—A suitable culture dish is a matter of some 
importance, especially when large numbers of cultures are handled 
together. It should be relatively small in order to economize 
space, and yet should provide sufficient surface of the nutrient 
medium to support vigorous growth of the two contrasted mycelia. 
The danger of infection must be reduced to a minimum, and yet 
the dishes should be such as to be manipulated easily in being 
filled with nutrient, inoculated with the races to be tested, stored 
during incubation, and examined under the microscope. Test 
tubes, although fitted for holding material in stock cultures, 
require considerable time to be plugged, filled, and sterilized, and 
moreover cannot easily be handled for observation under the 
microscope. Petri dishes, while in many ways superior to test 
tubes, are expensive, do not stack well, and are furnished with a 
loosely fitting rimmed cover. The Syracuse watch glass with 
ground rim for pencil labeling has been found to satisfy all the 
requirements of a safe and convenient culture dish, and has been 
used almost exclusively in recent years in testing contrasts between 
different races. They are conveniently handled in stacks of five, 
four dishes for cultures and an empty one for the top cover. Each 
dish, except the bottom one, serves thus as a cover for the one 
