Embryology. 1 1 5 



while nothing resembling it has ever been observed in 

 any of the Protozoa. 



We must now consider these several points of 

 difference seriatim. 



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 spore-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 



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