THE PHYLUM PROTOZOA 153 



Table 2. BR.EEDIXG RELATIONS IN PARAMECIUM BURSARIA 



A plus sign indicates the possibility of mating, and a minus sign indicates that mating 

 does not occur. No mating type will mate with another individual of its own type, but it 

 will mate with any other mating type of its variety. Three varieties are found in the 

 United States, and no mating type of one variety will mate with any mating type of any 

 other variety. (From Sonneborn, 1947.) 



and secrete into the medium small killer particles which, if they come 

 in contact with a "sensitive" individual, cause death. All individuals that 

 are unable to produce such particles are sensitive, whereas all the in- 

 dividuals that do produce killer particles are resistant to their effect and 

 are not killed. These particles are manufactured in the cytoplasm from 

 granules called kappa particles, present, of course, in killer animals 

 but absent in sensitive ones. A killer may contain some 800 kappa 

 particles, and secrete one killer particle every five hours. Kappa par- 

 ticles are self-reproducing, multiplying in the cytoplasm independently 

 of the division of the cell. When the paramecium divides, the kappa 

 particles are divided randomly between the daughter cells. So long as 

 each daughter cell receives at least one kappa particle, it remains a 

 killer. Under certain culture conditions the paramecia divide more 

 rapidly than the kappa particles, and the number of particles per cell 

 slowly decreases; ultimately, some cells are produced that lack particles. 

 Such animals become sensitive to killer particles, and are no longer 

 able to produce either kappa or killer particles. Thus, an inherited 

 characteristic may be lost. Occasionally, however, a sensitive animal may 

 mate with a killer before it is killed through chance encounter with a 

 killer particle, and during the mating process kappa particles may be 

 transferred into the sensitive cell. These will subsequently survive and 

 transform the cell into a killer. Furthermore, this trait will be trans- 



