442 THURSTON: SEX IN THE CONJUGATAE 
and in these cases there is often a morphological differentiation 
of the gametangia.”” The question certainly arises whether we 
ever find cases of reproduction by cell fusion as contrasted with 
reproduction by cell division in which the process is not. to be 
considered as involving the essential features at least of sexual 
reproduction. We have obviously come to think of sexuality as 
involving some fundamental difference between the cells that fuse 
or conjugate, and consequently when we can find no. visible 
difference between the gametes and can not in any way tell the 
male from the female, we find ourselves inclined to doubt the 
so-called sexual nature of such a conjugation. This is the case 
for all species of filamentous Conjugatae that form their spores 
midway between the two conjugating cells (TABLEs II and IV) 
and the common doubt as to the sexual nature of this conjugation 
is expressed by the above quotation from West. Naturally among 
low plant forms of this kind we should expect to find the most 
primitive kind of sexuality in its most fundamental expression. 
Many of our fundamental ideas regarding the nature of sex pro- 
cesses we owe to the study of these very algal forms. Schmitz 
in 1879 saw two nuclei in a zygospore of Spirogyra ‘approach 
each other and fuse to a single nucleus.’ This was the first 
description of the nuclear behavior in the sexual process in the 
plant kingdom and was made before any such process was known 
to occur in any of the higher plants. It is certainly possible that 
by the intensive study of these primitive forms we may further 
clarify our ideas as to the nature and origin of sex. 
Wittrock (8) has defined a zygospore as ‘‘a spore formed by an 
act of fecundation in which two or more cells of the same kind, 
not sexually different, have participated.” Can we accept such 
a definition, and at the same time accept conjugation in these 
forms as being essentially a fecundating reproduction? If we 
can, the formation of gametes can involve only the difference - 
between somatic cells as such and gametes, as such, rather than 
any essential difference between the two gametes themselves. 
Perhaps, after all, cells fuse rather because they are alike than 
because of an inherent difference between them. Certainly both 
of a pair of fusing cells in Spirogyra differ in some way from the | 
somatic cells from which they arose. Such differences between 

