174 



THE BIOLOGY OF STENTOR 



posterior end of the primordium act upon it to cause formation of 

 mouthparts and that when the anlage is shifted away from this 

 environment such induction is missing. Later primordia in stage 4 

 or 5 have already received this influence and are then semi- 

 autonomous systems capable of complete self-differentiation 

 regardless of their surroundings. 



Fig. 43. Oral induction associated with the posterior pole. 



A. Stage-2 primordium of regenerator shifted to the anterior 

 pole develops no mouthparts but only membranellar band. 



B. Stage-3 regeneration primordium implanted heteropolar 

 on a stage-3 regenerator. Primordium patch slips to posterior 

 end where it forms good mouthparts and an extra oral pouch (y) 



and gullet (x) in addition. 



4. Induction of mouthparts formation 



As just indicated, there is accumulating evidence that the 

 posterior end of the cell in Stentor has an inductive action on the 

 end of the developing membranellar band which causes it to 

 invaginate and form mouthparts. Perhaps the first indication of 

 this relationship was in experiments in which a sector bearing the 

 primordium site was reversed in situ and the original mouthparts 

 excised to initiate regeneration (Tartar, 1956b). An oral primor- 

 dium then appeared in the reversed patch but mouthparts now 

 were formed at both ends (Fig. 44A). Formation of the normal 

 mouthparts at the original posterior end of the anlage may be 

 regarded as due to the influence of its own surroundings of 

 "posterior" ectoplasm, and that of the additional formation at the 

 other end as being produced by an influence of the adjacent tail- 

 pole, passing across the graft and affecting the originally anterior 

 end of the anlage. This experiment has been confirmed by Uhlig 



