248 C. p. RHOADS 



In examining a number of varieties of killer animals, Sonneborn 

 found, in variety 4, that if grown rapidly with fission occurring quickly, 

 after several generations some of the animals become sensitive. Subse- 

 quently, however, these sensitive animals would revert to killer when 

 grown slowly. If he continued to grow them rapidly more and more of 

 them were converted to permanently sensitive animals and fewer of 

 them would revert to killer under the effect of a slower rate of multipli- 

 cation. In other words, in this case the organisms grow more rapidly 

 than particles of Kappa are elaborated. Sensitive animals result when 

 the concentration of Kappa in the cytoplasm falls below a certain critical 

 level. In the animals which remain permanently sensitive, after the rapid 

 growing of many generations, Kappa becomes entirely diluted out, and 

 even though the genie make-up was K, the animals remained sensitive 

 due to the inability of K to initiate the production of Kappa. 



From these data, Sonneborn was able to calculate the number of par- 

 ticles of Kappa in the normal killer animal, getting an estimate of be- 

 tween one and three hundred particles by two different methods of 

 calculation. He showed that a single particle of Kappa in the cytoplasm, 

 while not enough to manifest itself, was enough to enable the animal 

 ultimately to revert to killer provided the nucleus contained a K. His 

 data indicates that Kappa may have a limited power of independent 

 self -reproduction in the cytoplasm in the absence of i^ but that the major 

 factor controlling its elaboration in the cytoplasm is K. The number of 

 particles of Kappa necessary to cause the animal to be a killer ranges 

 perhaps from fifty to sixty. 



These data indicate that a substance elaborated into the cytoplasm 

 controls the cell phenotype and may be to some extent independent of 

 the genotype. Thus an animal recessive {kk) for killer may, in effect, be 

 a killer phenotypically due to the presence of Kappa in the cytoplasm. It 

 indicates also that Kappa is elaborated at a rate independent of the gen- 

 eral rate of growth and multiplication. Thus sensitive animals with the 

 genotype KK or Kk and containing a sub-critical amount of Kappa in 

 the cytoplasm might become killers following autogamy because autog- 

 amy prevents fission for many hours and this allows the independent 

 production of Kappa in the cytoplasm to a point at which animals mani- 

 fest the killer characteristic. Killer animals held for twelve to thirty- 

 six hours at temperatures higher than normal (37°-38°) become sensi- 

 tive but revert subsequently to killers. This may be due to inhibition of 

 the production of Kappa at higher temperatures, or to its breakdown at 

 these temperatures. 



