298 THE BIOLOGY OF STENTOR 



primordium can generally continue one or two stages further in its 

 development after enucleation. Stage-2 primordia could continue 

 to stage 4, and stage-4 anlagen to stage 6, etc. After going one or 

 two steps further, early primordia through stage 4 were then 

 resorbed. In incipient regenerators, enucleated, the primordium 

 could put in its initial appearance and then disappear, i.e., develop- 

 ment was possible from stage o to stage i. Animals with stage-6 

 anlagen, or even at stage 5 when there is still no sign of a gullet and 

 oral pouch, were able to complete oral differentiation and move 

 the new structures into their definitive position (see Fig. 8ob). 

 These performances are explainable on the assumptions, first, that 

 there is a nuclear contribution to cytoplasmic differentiation which 

 does persist for a short while or is present in small quantity at any 

 one time; and second, that by stage 5 the anlage has completed 

 most of its synthesis of new material in the form of oral cilia, etc., 

 and needs only to invaginate and shift the parts already formed to 

 complete the elaboration of the feeding organelles. 



Formation of the fission furrow after enucleation of mid-stage 

 dividers demonstrates its independence from the presence of the 

 nucleus. 



The experiments on stentors with late-stage anlagen clearly 

 confirm many earlier observations on the completion of regenera- 

 tion and division of stentors in the absence of the nucleus, begin- 

 ning with Gruber (1883, 1885a, b). He was much impressed by 

 the continued normal behavior of enucleated ActinophrySj a 

 heliozoan, and of Stentor. For the former, he claimed " regenera- 

 tion" (however this may be manifest in a rhizopod) in the absence 

 of the nucleus, but requiring cytoplasmic chromatin of nuclear 

 origin. In S. coeruleus he found that cells, after removal of the 

 compacted macronucleus, could complete division with separation 

 of daughters and full development of the primordium in the 

 opisthe. He therefore supposed that enucleated stentors might 

 regenerate "under conditions not yet devised". This remark is 

 not very different from Morgan's (1901a) conjecture that, if the 

 nuclear contribution could be supplied in some other way, then 

 the presence of the nucleus as such should not be necessary for 

 regeneration in stentors. I therefore feel that Gruber has been 

 somewhat maligned in reviews of this subject as saying that the 

 nucleus is not necessary for regeneration and having to correct 



