84 THE BIOLOGY OF STENTOR 



same result and frequently when dividers were grafted to non- 

 dividing animals the complex then simply reorganized doubly, 

 the single division primordium first resorbing, to be followed by 

 two anlagen which served only to replace the original feeding 

 organelles and no furrow formation occurred (see Figs. 38D and c). 

 In only one case did I obtain what appeared like a prompt and 

 indubitable induction : a stage-2 divider was grafted to a small non- 

 divider, the division primordium was not resorbed and another 

 was induced in the smaller component, whereupon simultaneous 

 division occurred; and yet the same result was obtained in one 

 case when a stage- 1 regenerator was used in place of the divider, 

 though fission then proceeded more slowly. Further studies, 

 however, may firmly establish a phenomenon of induced division, 

 and if so, this would afford great potentialities for causal analysis 

 of division in Stentor. 



5. Persistence of division 



Clues to the nature of the fission process may be sought in its 

 persistence in spite of often drastic operations. Long ago, Balbiani 

 (1891c) reported a case in which a longitudinal half of a stage-6 

 divider completed fission without either a nucleus or the division 

 primordium. In other instances, whether of aboral or adoral 

 halves, the division products did not separate though it was clear 

 that the cortical striping had been divided into two systems 

 because double cell shapes resulted. This was also the experience 

 of Stevens (1903) who obtained division without separation in 

 aboral halves and even in one enucleated oral half, which correlated 

 with her studies of the year preceding showing that enucleated 

 halves of sea urchin eggs are still capable of division. Much later 

 Schwartz (1935) described one instance of complete division into 

 two daughter cells after removal of the macronucleus from what 

 was, to judge from his drawings, a stage-6 divider. This has been 

 confirmed by de Terra (1959) and myself. Yagiu (1951) found the 

 same in Condylostoma, and Suzuki (1957) in Blepharisma. These 

 cases show that there is some ''momentum" in the processes of 

 division, or that after the primordium is well formed the final 

 shifts in the disposition of the anlage as well as the cutting of the 

 cortical striping into two systems and even their total separation 

 can be effected. 



