148 THE BIOLOGY OF STENTOR 



inferred inhibiting effect upon the primordium site. They probably 

 do not act directly, because the primordium site and anlage are 

 at some distance from these structures. Moreover, in tandem 

 grafts the head of the anterior cell effectively inhibits regeneration 

 in the posterior partner, the head of which has been excised, 

 though the distance between ingestive organelles and the posterior 

 primordium site is then abnormally great. Nor do these organelles 

 give off some '' inhibitory substance ", since regeneration will occur 

 if the mouthparts are merely cut and separated or the intact head 

 rotated in place. Not the materials of the organelles but their 

 proper pattern and relationship to the whole is essential to their 

 inhibitory effect. Moreover, non-differentiating stentors from 

 which the mouthparts have just been excised still can induce 

 resorption of early regeneration primordia grafted to them. The 

 tendency of the normal primordium site to form anlagen is appar- 

 ently stronger than that of other loci in the lateral ectoplasm, and 

 therefore requires a stronger inhibition. This is indicated by the 

 finding that fusions of six aboral halves promptly regenerate, 

 whereas anlagen formation in these grafts without normal primor- 

 dium sites is long delayed if one set of intact feeding organelles is 

 present (Tartar, 1956a). In contrast, when one set of feeding 

 organelles is removed from a doublet stentor, the remaining set is 

 insufficient to prevent, or often even to delay, regeneration in the 

 " unsaturated " primordium site left on the cut side. 



As a working hypothesis it is suggested that formed oral struc- 

 tures act upon the lateral stripe pattern, with which they are 

 connected, in such a way as to render this pattern inhibitive of 

 primordium development. The entire cell-body ectoplasm would 

 be involved in this inhibition, as indicated by the fact that the 

 larger the volume of cytoplasm the greater the inhibition exerted. 

 This state of inhibition could then be transmitted across the 

 borders of a grafted sector, rendering the included striping in the 

 patch also inhibitory and producing resorption of the primordium ; 

 or the state of inhibition could be transmitted in a similar way over 

 the ectoplasm of an adjoining cell. Conversely, when the head is 

 excised or the mouthparts removed, oral inhibition is dis- 

 continued and the pattern of the body striping gradually trans- 

 forms, with the aid of the nucleus, from a state of inhibition to one 

 of activation which is to be characterized in the same way. The 



