98 THE BIOLOGY OF STENTOR 



by cutting and shifting does act as a stimulus to primordium 

 formation although no portion of the feeding organelles is excised. 

 Even when heads were rotated 180" in place and primordium 

 formation did not occur at once, a remarkable readjustment of the 

 cell pattern took place (Tartar, 1959b) as shown in Fig. 23. The 

 mouthparts, including the oral pouch and its membranellar margin 

 were autonomously severed and thrust in towards the center of 

 the frontal field while the membranellar band closed together. In 

 the primodium site there appeared a "reorganization" anlage 

 which eventually broke into the oral ring and provided a new set 

 of mouthparts, now in the correct location. In this performance 

 we see that the very structures which are resorbed in the normal 

 course of reorganization can in fact be " autotomized ". 



3. Stimulus to reorganization and the significance of this 

 process 



The seemingly adventitious occurrence of reorganization in 

 stentors, which appear to be the same as their non-reorganizing 

 fellows, gives the impression of a quite unnecessary act which 

 leaves the animal just as it was before. Hence the enigmatic 

 character of reorganization. Yet we naturally assume that ciliates 

 would not go through this complicated process without good 

 reason, and several hypotheses have been advanced in the case 

 of Stentor, though there is none w'hich has not left its residue of 

 paradoxes. 



(a) To REPLACE DEFECTIVE MOUTHPARTS? 



On discovering reorganization, Balbiani (1891a) suggested that 

 the process is for replacement of worn-out ingestive organelles. 

 The act would therefore be essentially the same as regeneration 

 which is evoked by removal of these parts. Having well observed 

 that the entire membranellar band is not replaced, Balbiani 

 (1891a) assumed that the mouthparts are the most "used" and 

 therefore the most subject to deterioration; but there was also a 

 hint in his initial report that aging alone might result in these parts 

 eventually becoming defective. Among modern students of Stentor, 

 Weisz (1954) accepted this interpretation of the raison d'etre of 

 reorganization and emphasized (1951b) that injury or defect 

 might be either structural or functional, justifying the term physio- 



