294 THE BIOLOGY OF STENTOR 



completing itself even in the absence of the nucleus. This 

 chronology suggests that clumping of the nucleus has nothing to 

 do with primordium formation ; and in reference to fission (Johnson 

 (1893)) it indicates that the cytostome leads the nucleus — rather 

 than the reverse, as in the case of mitosis in cleaving eggs and 

 dividing tissue cells. We can readily suppose that the primordium 

 can go through its entire development in regeneration and reorgani- 

 zation without the act of nuclear clumping, and indeed this occurs 

 in experimental animals in which only one bead is left. Again, 

 since the products of artificially divided (transected) stentors 

 behave normally there seems to be no obvious reason why a 

 dividing stentor could not simply pinch the nuclear chain in two 

 where it originally lies. Perhaps we may put it this way, that if the 

 macronucleus is to clump, this has to occur before the cell divides, 

 if one daughter is not to be left without a nucleus. 



Questions concerning control of nuclear behavior include the 

 following: What "tells" the nucleus when to fuse? Is the macro- 

 nucleus capable of autonomous division, or is it merely pinched in 

 two by the division furrow? Does the rod-shaped nucleus have to 

 be ''told" to renodulate? Less anthropomorphically, does the 

 macronucleus time its own phases or is it guided by the cytoplasm, 

 and if so, by what part of the cytoplasm? 



Weisz (1951b) suggested that coalescence of the nucleus is 

 stimulated by primordium formation, for if the early division 

 primordium is removed or caused to be resorbed there is no division 

 and no compacting of the nucleus. Yet the division anlagen may be 

 removed at stage 4 or even stage 3 and fission can still be completed, 

 with the nucleus clumping normally just before furrow formation. 

 This implies either that it is not the primordium which gives the 

 stimulus for clumping or that the response of the nucleus to this 

 stimulus is much delayed. 



Even if the primordium is not implicated, one could still 

 maintain as Weisz (1954) said, that ''Evidently, nuclear kinetics 

 depend on direct stimulus from the ectoplasm". It has already 

 been suggested that during primordium formation the cytoplasm, 

 and perhaps especially or exclusively the ectoplasm, is in a state 

 of activation. The ripening state of activation, or particularly when 

 this stage is changing over to that of inhibition at stage 5 or 6, 

 could therefore provide the stimulus or the means for coalescence 



