NEOPLASTIC ABNORMAL GROWTH 247 



actually determines the phenotype of the organism as to whether it is a 

 killer or a sensitive. Paramecium aureli-a divides by simple fission, and 

 two sexual processes may intervene. The first of these is conjugation in 

 which two animals come together to form a cytoplasmic bond and ex- 

 change, reciprocally, one micro-nucleus. This is preceded by two reduc- 

 tion divisions in each animal. The micro-nuclei, which migrate, fuse 

 with corresponding micro-nuclei in each animal to form a zygote nu- 

 cleus. There may be, during conjugation, considerable exchange of 

 cytoplasm, a fact which is significant. The second process is autogamy, 

 in which the micro-nuclei in a single animal go through two reduction 

 divisions. Two daughter nuclei fuse to form a zygote nucleus and the 

 others degenerate. Sonneborn found that animals containing the gene 

 K and having Kappa present in their cytoplasm are invariably killers. 

 Animals of kk genotype which do not have Kappa in the cytoplasm are 

 invariably sensitive. However, the distribution of Kappa in the cyto- 

 plasm means that, due to the presence of the cytoplasmic bridge, it can 

 be distributed independently of the micro-nuclei on conjugation. Ani- 

 mals possessing the genotype Kk undergoing autogamy might change 

 in genotype from Kk to kk because of the two reduction divisions and 

 consequent independent genie segregation of these two alleles K and k. 

 It was possible, therefore, to obtain animals of the following make-up: 



NUCLEUS CYTOPLASM 



1. KK Kappa 



2. Kk Kappa 



3. kk Kappa 



4. KK — 



5. Kk — 



6. kk — 



Consequently, animals i, 2 and 3 were killers regardless of their geno- 

 types ; animals 4, 5 and 6 were sensitives even though 4 and 5 contain 

 K gene. 



It was demonstrated that K can maintain and increase the amount of 

 Kappa present in the cytoplasm but cannot initiate its production. Once 

 an animal of composition KK or Kk is completely deprived of Kappa 

 it remains a sensitive animal regardless. Conversely, an animal of geno- 

 type kk containing Kappa in the cytoplasm will be a killer although it 

 will revert to sensitive within four to six generations due to dilution of 

 the substance Kappa below the point at which it can manifest the killer 

 character. These animals subsequently will be sensitive. 



