254 ^^^ BIOLOGY OF STENTOR 



oral cilia had not yet grown out to their definitive length, the anlage 

 was notably resistant and resembled in this regard the general 

 ectoplasm from which it presumably arises. Stage 4 is the time 

 when the primordium becomes susceptible to sloughing. It may 

 be inferred that development involves the elaboration of a certain 

 type of organization which is peculiarly sensitive to these external 

 agents. 



We do not know how this shedding of the membranellar band 

 is brought about but we can at least exclude some possibilities. 

 Osmotic pressure probably plays no part because even very weak 

 solutions of Na2C03 produce sloughing, and sugars do likewise 

 even long before the cell begins to collapse. The action is not ionic 

 because it is shown by neutral substances like sugars. There seems 

 to be no relation to valency, for both NaCl and CaCl2 produce 

 like results. Hydrogen ion effects are ruled out by the efficacy of 

 neutral substances. Nor does the result appear to be due to injury 

 as such, since the most drastic operations with a glass needle do 

 not produce it. The great variety of substances producing the 

 effect itself poses a difficulty to analysis. 



12. Morphogenetic effects 



After encountering such striking and specific effects on the 

 stentor cell of relatively simple compounds in lethal concentrations, 

 I prepared sub-lethal solutions which obviously affected the animals 

 but allowed their indefinite survival (Tartar, 1957a). When stentors 

 which had shed their membranellar band in sucrose were not 

 washed before replacing in normal medium regeneration was 

 delayed from i to 2 days, doubtless because of the carry-over of 

 some of the sugar. Otherwise the animals were entirely normal in 

 their behavior. Hence sugar is in itself an effective inhibitor of 

 primordium formation. 



Diluted sea water also gave reversible inhibition of oral regenera- 

 tion. Often there were graded effects, depending upon the con- 

 centration in the solution and the susceptibility of the individual. 

 Sometimes regeneration was merely delayed. In other individuals 

 or at different concentrations there was formation of a complete 

 membranellar band but with inadequate development of the 

 mouthparts, only a small pit being produced. A further influence 

 was shown when the anlage was arrested in mid-development at 



