REPRODUCTION 193 



large masses break down into small masses (c) and still later, con- 

 jugants appear in pairs (d). The only other ciliate in which mating 

 types are definitely known to occur is Euplotes patella in which, ac- 

 cording to Kimball (1939), there occurs no agglutination mating re- 

 action. 



How widely mating types occur is not known at present. But as 

 was pointed out by Jennings, the mating types may be of general oc- 

 currence among ciliates; for example, Maupas (1889) observed that 

 in Lionotus (Loxophyllum) fasciola, Leucophrys patula, Stylonychia 

 pustulata, and Onychodromus grandis, conjugation took place be- 

 tween the members of two clones of different origin, and not among 

 the members of a single clone. Precise information on the occurrence 

 of mating types among different ciliates depends on future research. 



In Paramecium aurelia, Sonneborn distinguishes seven varieties 

 which possess the same morphological characteristics of the species, 

 but which differ in addition to mating types, also in size, division 

 rate, conditions of temperature and light under which mating reac- 

 tion may occur, etc. (Sonneborn, 1947). There occurs ordinarily no 

 conjugation between the clones of different varieties. Within each of 

 six varieties, there are two mating types, while there is only one type 

 in the seventh variety. Animals belonging to the same variety, but 

 to different mating types, only conjugate when put together (Table G). 



Under optimum breeding conditions two mating types of the same 

 variety give 95 per cent immediate agglutination and conjugation. 

 But exceptions occur. Sonneborn and Dipell (1946) place the 7 va- 

 rieties of aurelia under two groups: A (varieties 1, 3, 5 and 7) and B 

 (varieties 2, 4 and 6) on the basis of their conjugational reactions. 

 Mating types in group A do not conjugate with those of group B; no 

 mating type of group B is known to conjugate with any type of other 

 varieties in this group; but a number of combinations of mating 

 types belonging to different varieties of group A conjugate with each 

 other. For example, varieties 1 and 5 conjugate (namely, type I with 

 type X and type II with type IX); however these interparietal mat- 

 ing reactions are (1) always less intense than intra varietal reaction, 

 (2) dependent upon the degree of reactivity of the culture, and (3) 

 different from the intravarietal reaction with respect to the condi- 

 tions for optimum reaction. Furthermore in most cases, the progeny 

 of intervarietal matings are not viable. In the varieties of group A, 

 the mating types appear to be of a more general sort. Therefore, 

 Sonneborn (1947) designated even- and odd-numbered types as + 

 and — respectively. 



