146 THE BIOLOGY OF STENTOR 



(Weisz, 1956), for we have seen that regeneration primordia can 

 also be caused to be resorbed. The difference lies rather in the 

 simple fact that dividers have an intact set of feeding organelles 

 but regenerators do not. Thus if the injurious cut through a 

 divider is such as to remove the entire feeding organelles or the 

 mouthparts, then, as in Prowazek's original experiment, the 

 division primordium is not resorbed. And therefore the simplest 

 interpretation is that presence of intact organelles is the cause of 

 resorption. Then, as Weisz himself suggested, in division (as in 

 reorganization) the primordium site is somehow enabled to escape 

 the inhibitive action of the existing feeding organelles and to 

 produce an oral primordium in spite of their presence; and I 

 would add that cutting injuries in some manner nullify this 

 delicate escapement, thus enabling the formed parts to re-exert 

 their full inhibitive force. 



Inhibition by the intact feeding organelles would also explain 

 why dividers do not produce a new primordium at once after 

 anlage excision, as do regenerators. 



The situation in dividers can be duplicated in regenerators by 

 grafting a new head in place of the one that was removed (Tartar, 

 1958c). If the regenerator had not yet produced a primordium, it 

 was prevented from doing so ; or if it already had an early primor- 

 dium, this was then resorbed (Fig. 37). When tails were grafted 

 instead of heads, primordia were not resorbed. This is evidence 

 that the formed feeding organelles exert an inhibitive action on 

 primordium formation and development. With regenerators 

 which had progressed beyond stage 2 the effect was not as marked 

 and only partial resorption or merely arrested development 

 occurred. But when the primordium was completely resorbed in 

 recapped regenerators the majority of the specimens later re- 

 organized. This suggests that complete healing may not have 

 occurred, with complete union of lateral striping, and thus set the 

 stage for later escapement of the primordium site in re- 

 organization. 



Similarly, it may be that in division, as in reorganization, there 

 is some temporary and invisible severance of connection between 

 the lateral body striping and the feeding organelles sufficient to 

 break the path of oral inhibition and permit the formation of a 

 primordium although intact feeding organelles are present. 



