202 THE BIOLOGY OF STENTOR 



small tail pieces since both have holdfasts, but I found the 

 regeneration of the latter to be always slower. Slower regeneration 

 of oral parts in tail pieces may be the consequence of another 

 aspect of polarity which will now be discussed. 



3. Gradients in head and tail formation 



Popoff (1909) had found in abortive fissions of Stentor that even 

 though the daughters did not separate, a new tail pole with hold- 

 fast, projecting laterally, was produced for the anterior cell because 

 the lateral striping had been severed by the fission line. Weisz 

 (1951b) then showed that foot formation could be brought about 

 by excisions of post-oral striping, but the nearer the anterior end 

 the more incomplete and temporary was the pedal diff"erentiation 

 (see Fig. 26a). Notice that the holdfast forms not merely where 

 the stripes come together (at the posterior pole) but also differen- 

 tially along the whole side of the cell, wherever ablation creates a 

 new terminus of polarization. 



Uhlig (1959) confirmed that there is a gradient in tail formation, 

 highest at the posterior end and diminishing anteriorly. This 

 gradient is strongest on the ventral side where the oral primordium 

 is also formed, as shown by the appearance of a secondary tail 

 projection in this region when anterior halves are rotated on the 

 posterior (see Fig. 26b). The polar pedal gradient is therefore 

 involved with the circumferential gradient in stripe widths, since 

 it is on the ventral side that the locus of stripe width contrast 

 determines both the location of the oral anlage and the side on 

 which the new tail-pole will appear. 



Now, the polar gradient in foot formation is also coincident with 

 that responsible for the induction of mouthparts formation. When- 

 ever, but only when, an end of the oral primordium lies near a part 

 of the posterior end, or its entirety, are mouthparts produced. 

 This inductive relationship has already been discussed (p. 202) 

 but should now be considered further within the context of polar 

 gradients. 



Following the implications of double oral differentiation in 

 reversed primordium sectors (see Fig. 44A), Uhlig (1959) has 

 explored this matter thoroughly and concluded that the inductive 

 action is strongest just beyond the posterior pole, diminishing 

 anteriorly. Because his detailed report is not yet available, I have 



