496 H. S. Jennings and George T. Hargitt. 



Inteoduction 



In an earlier paper (Jennings, '08) the senior author showed that 

 there exist in Paramecium (aurelia or caudatum) many races, 

 differing in size. On account of the extreme importance of this 

 fact in all sorts of systematic and experimental work with such 

 animals, and particularly in work on heredity and variation, it 

 seems well to give an account of the structural and physiologi- 

 cal characteristics of these races. Some of the races have now 

 been under observation in the laboratory for more than three 

 years, so that there has been opportunity for a careful study. 



In the paper of 1908, Jennings distinguished eight different 

 races, permanently differentiated in mean size. Each race con- 

 tained individuals of varying size, owing to growth and environ- 

 mental influences, but under identical conditions each race showed 

 a characteristic average size, differing from that of the others. 



Do these races differ in other respects as well as in size? In the 

 paper of 1908 the only other differences set forth were the follow- 

 ing: (1) the smaller races are on the average a little thicker in 

 proportion to the length than the larger (pp. 500-503) ; (2) one of 

 the larger races, D, had a single micro-nucleus; one of the smaller, 

 c, had two (p. 500). The relation between these races and the 

 supposed distinction between Paramecium aurelia and P. cauda- 

 tum was briefly discussed, without reaching a definite conclusion. 



The present paper is divided into two parts. Part 1, by Geo. 

 T. Hargitt, deals with the number and structure of the micro- 

 nuclei and the relation of the races to the two supposed species, 

 Paramecium aurelia and P. caudatum. Part 2, by H. S. Jennings, 

 takes up (1) the permanence of the differences in size; (2) the 

 similarities and differences among the various races in the form of 

 the body and in other structural features; (3) differences with 

 respect to conjugation; (4) differences in rate of fission; (5) differ- 

 ences in cultural requirements, and other physiological differences ; 

 (6) the relation of these races to the various described species of 

 Paramecium; (7) the occurrence of such races in other Protozoa, 

 and (8) in higher organisms; (9) certain theories of the origin 

 of these races; (10) a list of the various races and lines studied, 



