ACTIVATION AND INHIBITION: ORAL PRIMORDIUM 155 



these instances the stentors divided instead (Fig. 39). One may 

 suppose that for some obscure reason the primordium which 

 developed for the purpose of reorganization was used instead for 

 the division, nevertheless, of animals considerably smaller than 

 the maximum size. 



It is also possible, but again not indubitably demonstrated, 

 that regenerators can be converted into dividers. In experiments 

 already described, all of the membranellar band but none of the 

 mouthparts or all of the mouthparts but none of the membranellar 

 band were removed from larger animals, with the result that 

 division almost invariably occurred, thus representing cases in 

 which an operation which would ordinarily be expected to incite 

 regeneration led to fission instead. The primordia did appear at 

 first exactly like those of regenerators, but subsequently the 

 anterior ends of the anlagen bent to the right as division was 

 accomplished in the typical manner (b). 



Regenerating stentors can easily be converted into reorganizers. 

 When a complete head is grafted to a regenerator to replace the 

 one which was excised and the regeneration anlage is not resorbed, 

 then this primordium finds attachment to the intact membranellar 

 band and the preexisting mouthparts are resorbed as they are 

 replaced (see Fig. 37). 



Dividers are frequently converted into reorganizers by many 

 types of operation which permit the continued development of 

 the division primordium but somehow block furrow formation. 

 Causin reported that mere transfer of dividing stentors onto a 

 slide often resulted in their undergoing reorganization instead, 



B. Regenerator becomes a divider, a: If either membranellar 

 band (as shown) or the mouthparts only are excised, a regenera- 

 tion primordium is called for and appears {b) but serves for 

 division instead {c) even though ablation decreased cell volume. 



d: Proter regenerates proportionate oral structures later. 



C. Divider becomes reorganizer. a: Stage-3 divider grafted 

 to oral half of a small, non-differentiating stentor. b: No 

 induced primordium in small partner ; anlage of divider develops 

 to stage 6 without commencing division or reorganization, c: 

 Anlage finally used to reorganize larger animal ; later the mouth- 

 parts of the partner were also resorbed though having no 

 reorganization primordium from which to replace them. Next 

 day the specimen performed double regeneration-reorganization. 



