Heredity in Protozoa 521 



and regenerated KK macronuclei, or kk micronuclei and regenerated Kk 

 macronuclei, since the production of kappa is inhibited at temperatures 

 above 33.5° (62, 83). 



It has been suggested that the macronucleus controls mating types in 

 group A of P. aurelia (76, 81). This assumption does not explain the 

 origin of two mating types from one ciliate after autogamy, a phenome- 

 non implying formation of two genotypically different macronuclei from 

 one zygotic nucleus. The least improbable explanation, according to 

 Sonneborn (81), involves macronuclear mutation, an assumption which 

 cannot be tested experimentally at present. 



THE CYTOPLASM IN 

 INHERITANCE 



There is a growing tendency to relate inheritance of certain traits 

 in Paramecium aurelia to cytoplasmic factors which may, in different 

 cases, be dependent upon or independent of nuclear genes. These char- 

 acteristics include the killer trait, mating types, and antigenic varieties. 

 Cytoplasmic inheritance is, in a rather real sense, a phenomenon familiar 

 to all protozoologists in the self-perpetuation of blepharoplasts in flagel- 

 lates and basal granules in ciliates. Therefore, there is nothing very 

 startling in the possibility that self-perpetuating cytoplasmic particles 

 may induce the appearance of specific substances which show physio- 

 logical activity without assuming the concrete form of new organelles. 

 The current investigations on ciliates are being followed with much in- 

 terest and with the hope that future developments may furnish logical 

 explanations for some of the unsolved puzzles in protozoan genetics. 



The killer trait in Paramecium aurelia 



A killer strain of P. aurelia, as described in variety 4, gives off into 

 the culture fluid a substance, paramecin, which is lethal to sensitive 

 strains but without effect on killers (74, 77, 78, 79). As an antibiotic sub- 

 stance, paramecin is interesting in that it shows differential effects within 

 a single variety. Different strains of P. aurelia seem to produce different 

 quantities and different kinds of paramecin (13, 62, 74, 77). A single 

 particle of paramamecin (stock 51, variety 4), produced by a killer about 

 once every five hours, may be enough to kill a sensitive ciliate (2), whereas 

 Sonneborn (84) has found that as many as half of the sensitive ciliates 

 may survive when exposed singly or in small numbers to 10,000 or more 

 particles of paramecin. Paramecin seems to be a desoxyribonucleoprotein 

 which is inactivated by pepsin, chymotrypsin, and desoxyribonucleases 

 and shows a sensitivity to high temperatures comparable to that of 

 various enzymes (95, 96, 97). 



The ability to produce paramecin is said to depend upon the presence 



