Embryology. 115 
while nothing resembling it has ever been observed in 
any of the Protozoa. 
We must now consider these several points of 
difference serzatim. 
First, with regard to sexual propagation, we have 
already seen that this is by no means the only method 
of propagation among the multicellular organisms ; 
and it now remains to add that, on the other hand, 
there is, to say the least, a suggestive foreshadowing 
of sexual propagation among the unicellular organisms. 
For although simple binary fission is here the more 
usual mode of multiplication, very frequently two 
(rarely three or more) Protozoa of the same species 
come together, fuse into a single mass, and thus 
become very literally “one flesh.” This process of 
“conjugation” is usually (though by no means invari- 
ably) followed by a period of quiescent “encystation” ; 
after which the contents of the cyst escape in the form 
of a number of minute particles. or “spores,” and these 
severally develope into the parent type. Obviously 
this process of conjugation, when it is thus a pre- 
liminary to multiplication, appears to be in its essence 
the same as fertilization. And if it be objected that 
encystation and spcre-formation in the Protozoa are 
not always preceded by conjugation. the answer would 
be that neither is oviparous propagation in the Metazoa 
invariably preceded by fertilization. 
Nevertheless, that there are great distinctions 
between true sexual propagation and this fore- 
shadowing of it in conjugation I do not deny. The 
question, however, is whether they be so great as to 
justify any argument against an historical continuity 
between them. What, then, are these remaining 
I 2 
