208 GENERAL EMBRYOLOGY 



ing the properly effective form of stimulus, which might other- 

 wise be lacking in the environment of the egg. 



Turning now to the evidence which the Protozoa offer 

 regarding the relation of fertilization and reproduction, we may 

 approach more closely the problem of the fundamental signifi- 

 cance of syngamy. Nearly all the known life histories of Pro- 

 tozoa are cyclic in character. The process of reproduction by 

 simple fission may proceed uninterruptedly for a longer or 

 shorter period, but finally this is interrupted by some form of 

 syngamy. Formerly this seemed obviously to mean that the 

 ordinary reproductive processes depended ultimately upon a 

 process of conjugation or fertilization, and that the life cycle in 

 the Protozoa was essentially similar to that of the Metazoa, the 

 divisions of the somatic cells of the latter being equivalent to the 

 simple fissions of the former, and the fertilization processes of 

 both being essentially similar (homologous). It was overlooked 

 at first that the process of fertilization might just as well be 

 considered the result of vegetative divisions as the cause of 

 them; to this phase of the relation we shall return shortly. 



It is true that in some Protozoa, e.g., Noctiluca, Trichosphcer- 

 ium, some Gregarines, fertilization is really followed by a 

 marked increase in reproductive activity. And it is often true 

 that multiple fission tends to follow conjugation. But in 

 other cases reproductive activity seems not to be affected by 

 fertilization. And in many, probably most Protozoa, fertiliza- 

 tion tends to inhibit reproduction. In many Rhizopods, 

 Flagellates, and Ciliates, a pause in the succession of fissions 

 may be quite marked after conjugation. In many of the Sporo- 

 zoa a period of encystment follows, and the same is true of 

 many Algae. In such cases, therefore, fertilization seems 

 opposed to reproduction, or at least to any immediately ensuing 

 processes of multiplication; it may still be true that the ultimate 

 effect of fertilization may be increased rate or duration of fission. 

 Conjugation may occur without reproduction; reproduction 

 may occur without conjugation. And that conjugation is 

 frequently to be regarded as determined by external rather than 

 internal conditions is indicated by the occurrence of so-called 



