THE MOSAIC THEORY OF DEVELOPMENT. 7 



specific form of idioplasm, a portion of unmodified idioplasm 

 afforded by purely quantitative division. This unmodified 

 idioplasm (" accessory idioplasm" of Weismann, or in some 

 cases "germ-plasm"; "post-generation or regeneration idio- 

 plasm " of Roux) remains latent in normal development which 

 is controlled by the active specific idioplasm. Injury to the 

 ovum e. g., mechanical separation of the blastomeres acts 

 as a stimulus to the latent idioplasm, which thereupon becomes 

 active, and causes a repetition of the original development. 

 By assuming a variable latent period following the stimulus, 

 Roux is able to explain the fact that regeneration takes place 

 at different periods in different animals. 



Considered as a purely formal explanation this subsidiary 

 hypothesis is perfectly logical and complete. A little reflection 

 will show, however, that it really abandons the entire mosaic 

 position, by rendering the assumption of qualitative division 

 superfluous; and, aside from this, its forced and artificial char- 

 acter, places a strain upon the mosaic theory under which it 

 breaks down. Both of the two fundamental postulates of the 

 modified theory viz., qualitative nuclear division, and accessory 

 latent idioplasm are purely imaginary. They are complicated 

 assumptions in regard to phenomena of which we are really 

 quite ignorant, and they lie at present beyond the reach of 

 investigation. The " explanation " is, therefore, unreal; it 

 carries no conviction, and no real explanation will be possible 

 until we possess more certain knowledge regarding the seat of 

 the idioplasm (which is entirely an open question), and its 

 internal composition and mode of action (which is wholly 

 unknown). In the meantime we certainly are not bound to 

 accept an artificial explanation like that of Roux, however 

 logical and complete, unless it can be shown that the phenomena 

 are not conceivable in any other way. 



We turn now to a brief consideration of opposing views, 

 among which I ask attention especially to those of Driesch and 

 Hertwig. In common with Kolliker and many other eminent 

 authorities, these authors insist that cell -division is not quali- 

 tative but quantitative only, and hence is not, per sc, a cause 



