PETRI DISH CULTURE PHOTOGRAPHED BY TRANSMITTED LIGHT 
The small black dots are sexual spores (Zygospores); the large dark areas are places where innocu- 
lations were made. The two left hand colonies are the normal race with numerous hermaphroditic 
zygospores’ The central row of three colonies are mutant ‘‘X’’ with large zygospores; the two 
right hand colonies are mutant ‘‘D”’ with only an occasional zygospore or none. Mutant “X” 
has a plus sexual tendency and hence forms lines of zygospores in contact with mutant ‘‘D”’ and 
the normal race, both of which have a minus tendency. (Fig. 27.) 
CULTURE PHOTOGRAPHED BY REFLECTED LIGHT 
The same arrangement of races as in Fig. 27 above. (Fig. 28.) 
