ACTIVATION AND INHIBITION: ORAL PRIMORDIUM 145 



relationship is common to regeneration in general, whether of 

 plants or multicellular animals; for it is a general rule in re- 

 generation and embryological studies that formed parts prevent 

 neo-formations of their like and so allow the organism to attain 

 stability and unity of form (see Child, 1941; and Rose, 1957). 

 That there is specific inhibition between formed and potential 

 structures can be demonstrated on the cell level in Stentor where 

 it presents special problems as well as unusual opportunities for 

 analysis. 



The first exploratory experiment in this direction was performed 

 by Prowazek (1904) when he cut dividing coeruleus in two trans- 

 versely. If the animals were in an early stage of fission, the half of 

 the primordium remaining in the anterior fragment was resorbed, 

 but not in the posterior piece ; yet he was not aware of the full 

 significance of this simple test. Today we can say that the portion 

 of the anlage in the anterior fragment was resorbed because of 

 the presence of the intact feeding organelles, and conversely, that 

 their absence in the posterior piece permitted the maintenance and 

 continued development of its section of the primordium. 



Weisz (1956) later found that it was sufficient merely to sHce 

 into an early divider to cause total resorption of the entire primor- 

 dium. I have also found that a single slice into the cell, merely 

 removing the tail tip (Tartar, 1958c), or even a too long exposure 

 to the quieting agent, methyl cellulose, may cause stage i and 2 

 dividers to resorb the primordium. Even at stage 4 the anlage 

 may be completely resorbed in the adoral half of dividers cut in 

 two longitudinally. Early primordium sectors cut from these 

 dividers, including the mouthparts but not much of the membran- 

 ellar band, also resorb the anlage when isolated but not if the 

 original mouthparts are also excised from the piece. The response 

 of regenerators to cutting Weisz found to be entirely different, 

 for the primordium is then never resorbed because of injuries. This 

 point has also been adequately confirmed; following a standard 

 maximal disturbance in which the regenerator was cut into three 

 sections and spread out widely, the anlagen were never resorbed 

 (Tartar, 1958c). 



A simple explanation for this difference between dividers and 

 regenerators is at once apparent. It is not because the division 

 primordium is uniquely subject to reversal of its development 



