1906] BLAKESLEE—DIFFERENTIATION OF SEX 163 
as yet it has not been possible to substitute the terms male and female 
for (+) and (—), or vice versa, does not in the least detract from the 
conclusion, however, that the differentiation is a sexual one. 
Forms characterized by gametes equal in size have been commonly 
classified as isogamous. The term, it need hardly be pointed out, 
can have only a morphological application among the mucors. Sex- 
ually the two gametes which unite have diametrically opposite char- 
acters. The mutual indifference of two mycelia of the same sex, 
and the active sexual reaction between mycelia of opposite sex which 
leads to the formation of zygospores when the mycelia are of the same 
species, and to the formation of imperfect hybrids when they are of 
two different species, indicate that the isogamy is by no means 
physiological. The classical researches of BERTHOLD (1) have shown 
that among the morphologically equivalent motile gametes of certain 
species of Ectocarpus there is a physiological differentiation into 
gametes which are attractive and those which are attracted, and a 
similar condition is met with among the Conjugatae. In the mucors 
the sexes seem to be equally attractive. If in other zygophytic forms 
the gametes are ever physiologically equivalent, their union can 
scarcely be considered a sexual process in the usual acceptation of 
the term. 
The physiological differences which exist between the sexually 
opposite thalli of heterothallic mucors reaches morphological expres- 
sion in those instances in which the (+) in comparison with the (—) 
strain is characterized by a greater vegetative luxuriance. Although 
the heterothallic forms are morphologically all isogamous, the sexual 
differentiation which they exhibit into two distinct races cannot be 
considered a lower grade of sexuality than the differentiation shown 
in the morphologically unequal gametes of the heterogamic species. 
Heterogamic forms are found only in the homothallic group. It 
Would seem most reasonable to suppose that the isogamous homo- 
thallic forms were the more primitive, and had given rise on the one 
hand to heterogamic forms by a differentiation of the individual 
gametes, and on the other hand to heterothallic forms by a differen- 
tiation of the individual thalli. The partial transformation of the 
heterothallic species Phycomyces into a homothallic form which has 
been accomplished might, however, suggest the possibility of a deri- 
