2 I 8 
The “ Sexuality ” o/ the Mucorinece. 
seen in various groups of alga?, we find first syngametic unions 
in which sex has not appeared, i.e., in the process of syngamesis 
the fusing cells are exactly alike, are in fact isogametes. In the 
next stage of evolution we find individuals producing two kinds of 
gametes, showing, in varying degrees, a larger size, a loss of activity 
(female), and a smaller size, and greater activity (male). The next 
step appears to he the production of individuals which form only 
one kind of gametes, either male or female. The term sex as 
applied to organisms thus means that there is a binary differen¬ 
tiation of individuals which are distinguished primarily by the fact 
that they bear morphologically and physiologically different types of 
gametes. 
In the Mucorinece, however, we have in the heterothallic forms 
a new type of binary differentiation, in which individuals (mycelia) 
are characterized, not by the formation of morphologically different 
gametes (for the fusing cells, at least in the majority of cases, are 
indistinguishable), but by a purely physiological differentiation, such 
that the gametes which lead to the production of zygospores are 
only formed when different mycelia come in contact. 1 In this 
group there is thus a purely physiological binary differentiation of 
individuals in relation to syngamety, a condition hitherto unknown 
in biology. For this reason, until further light has been thrown 
on the matter it would seem better to use merely the very apt 
terms, homothallic, heterothallic, ( + ) and (—) strains, and to avoid 
the use of the term “sexual ” in relation to the mycelia in this group. 
It is also interesting to note that while ordinary sexual 
differentiation can be related teleologically to an advantageous 
division of labour between the gametes, later transferred to the 
individuals bearing the gametes ; yet the differentiation exhibited 
in the Mucorinece appears to have no advantage, or, at least, no 
advantage which could not be gained by simple exogamy. 
In a later paper- Blakeslee has made further interesting 
observations which, however, do not help to bring the heterothallic 
Mucorinece into line with the ordinary sexual forms. He has 
shown that in the germination of the zygospore of Mucor Mucedo 
a ll the spores in the germ sporangium are alike, either ( + ) or (—) ; 
’ It must be remembered that the gametes of the Mucorinece are 
eoenogamctes, representing, no doubt, gametangia ; it is 
therefore possible, though not probable, that a difference in 
the behaviour of the two coenogametes or a difference in the 
structure of the constituent gametes may be observed when 
the zygospores can be more closely studied cytologically. 
1 Zygospore Germinations in the Mucorinece. Annales Mycologiei 
IV., 1906, p. 1. 
