T. M. SONNEBORN 179 



(1956). At present, only variety 4 has been carefully studied in 

 respect to chromosome variation (Dippell, 1954). I have reason 

 to suspect that very different results might be obtained with 

 varieties 15 and 16, and perhaps certain others. 



In sum, three groups of varieties may be readily distinguished 

 on the basis of size, and the largest varieties may be further sub- 

 divided on the basis of the number of nuclei just after fertiliza- 

 tion. Variety 12 regularly has two micronuclei and two macro- 

 nuclear anlagen at this stage whereas the other large animal 

 varieties regularly have four of each. All the smaller varieties 

 regularly have two of each. 2,3 



Killers and Sensitives. Some strains of P. aurelia liberate a 

 poison to which they are resistant, but which kills other strains 

 of paramecia. Only certain varieties of P. aurelia include strains 

 of killers. They have been reported in variety 2 (Sonneborn, 

 1939) and variety 4 (Sonneborn, 1943). A different kind of killer, 

 which liberates no poison but kills by prolonged contact during 

 mating, was discovered in variety 8 (Siegel, 1954). I have re- 

 cently found a killer in variety 8, which kills without mating, and 

 also killers in variety 6. Thus killers of one kind or another are 

 now known in varieties 2, 4, 6, and 8, but not in any other va- 

 riety. 4 



As a rule, all nonkiller strains of all varieties are sensitive to 

 all killers. Only strains that can conjugate with mate-killers can 

 be tested for sensitivity, but this restriction does not apply to 

 other killers. Sensitive strains of variety 2 alone show heightened 

 resistance to killers; the degree of resistance varies from strain to 

 strain, but it is sometimes very great (Preer, 1950). The signifi- 

 cance of this variation in resistance is somewhat complicated by 

 Austin's (1951) discovery that variations in resistance occur 

 within a single sensitive strain of varieties 4 and 8 in correlation 

 with the serotype they happen to be expressing at the time (see 

 under "Serotypes" ) . Nevertheless, the commonly greater resistance 

 of variety 2 may well be independent of this. 



For purposes of identifying strains, the preceding information 

 is of some value. If one has a killer strain, it probably belongs 



