282 
this species, primarily on account of the 
light appearance of growth due to the 
lack of zygospores. The ability to 
form hermaphroditic zygospores did 
not seem to have been entirely lost in 
1913, however, since on more suitable 
nutrient than is available in the thin 
layer in an isolation tube, zygospores 
were occasionally produced, although 
in very small numbers. At the present 
writing (February, 1920), it fails to 
produce zygospores on the nutrients 
tested. 
Tests made in 1913 of the A3 genera- 
tion showed that the mutant had a 
minus sexual tendency since it gave 
good reactions with the plus races 
of two different dioecious species. In 
addition, it formed a line of zygospores 
with the ‘““X”’ mutant known to have a 
plus tendency. This will be discussed 
in a later paragraph. 
That it was not entirely lacking in 
the plus sex was further shown by its 
reaction, although weak, with a minus 
race of one of the dioecious test species. 
The mutant “A” therefore cannot 
be considered an example of complete 
transformation from a hermaphroditic 
into a dioecious species although it may 
show a tendency in this direction. It 
may be added that the “A” mutant 
has recently given rise to a striking new 
form ‘‘F”’ characterized by a lcw, white, 
felted, aerial growth and a scanty 
production of zygospores. It has 
been carried to only a few generations 
but so far has remained constant. 
The “Dwarf” and Mutant “A” are 
the only examples of true-breeding 
mutations so far investigated in the 
species. Further study may show that 
even these have the power of reverting 
at times. Those discussed in the 
following paragraphs are examples of 
the more common type of reverting 
mutations. 
MUTANTS WHICH HAVE NOT REMAINED 
CONSTANT 
In Figs. 27 and 28 are shown two mu- 
tant races, ‘“X’’ and “D,”’ which have 
not remained constant under cultiva- 
tion. The two colonies at the left in 
The Journal 
of Heredity 
each figure are the normal stock; the 
three colonies in the central row are the 
“X”’ mutant and the two colonies at the 
right are the ‘“‘D”’ mutant. The photo- 
graph shown in Fig. 27 was taken by 
transmitted light and shows the her- 
maphroditic zygospores as small black 
dots; the large dark areas are the places 
where the inoculations were made. 
The photograph in Fig. 28 was taken by 
reflected light and shows better than 
does Fig. 27 the differences in habit of 
growth between the three races. 
Mutant “X”’ has a lower, whiter 
growth than the normal race. Its 
sporangia, as well as its zygospores, are 
less abund nt and the latter are some- 
what larger than normal and tend to 
be arranged in groups, which often 
form dark sectors radiating from the 
point of inoculation. Its greatest 
interest lies in the fact that it forms a 
line of zygospores with the normal race 
on its left as well as with the “D” 
mutant on the right, as shown in Fig. 27 
and less well in Fig. 26. It was this 
ability to form lines of zygospores with 
adjacent colonies that attracted our 
attention to its first appearance in an 
isolation culture of a strongly zygo- 
sporic mutant consisting of 41 colonies. 
Ordinarily, as mentioned under the 
Dwarf mutant, colonies exercise some 
inhibitory action toward one another 
which retards their growth on adjacent 
sides and prevents their meeting when 
the nutrient is thin, as in an isolation 
culture. The inhibitory action is 
absent and the colonies meet when 
they are of opposite sexual tendencies. 
This seems to be the case with mutant 
“X” and its parent race. The normal 
race (called ‘‘Y’’) gives a strong reac- 
tion with plus test races of dioecious 
species and is therefore a hermaphro- 
dite with a minus tendency, while 
mutant ‘“X”’ gives a strong reaction 
with minus test races and is therefore 
a hermaphrodite with a plus tendency. 
In a similar way mutant “A” and 
mutant ‘‘D"’ have been shown to be 
hermaphrodites with a minus tendency. 
Mutant ““D” formed at first a yellowish 
dense growth almost entirely devoid of 
zygospores. By continued cultivation 
