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



A point of some interest, though not directly connected with 

 the matter of distinctness of the two groups, was raised by Simp- 

 son ('01) who was not able to determine whether in aurelia forms 

 ''the two halves of the same micro-nucleus go to one daughter, or 

 whether it is a half of each of the micro-nuclei that go to form the 

 daughter micro-nuclei in any one of the offspring." Fig. 4 is a 

 drawing of a section of an individual of an ''aurelia" race. The 

 section though somewhat oblique is fortunately cut exactly in the 

 plane of the dividing micro-nuclei and shows all But a very small 

 fraction of the entire division figures in a single section. Other 

 cases were not obtained in which the entire process was visible in 

 a single section, but it was not difficult to find many instances in 

 which the connections of the micro-nuclei could be accurately 

 determined and followed through the two or three sections bearing 

 them. It is thus perfectly clear that in the "aureha" forms a 

 half of each micro-nucleus goes into each of the daughter 

 cells. 



Thus a cytological study shows that there are two groups of 

 races of these elongated Paramecia, differing absolutely in the 

 structure of the micro-nuclei, and differing typically (though with 

 some rare exceptional individuals) in the number of the micro- 

 nuclei. The larger races have typically one micro-nucleus, of 

 the structure and size shown in fig. 8; theses races have commonly 

 been grouped as Paramecium caudatum Ehr. The smaller races 

 have two micro-nuclei, of the structure shown in fig. 10; these have 

 commonly been grouped as Paramecium aurelia Mtiller. Even 

 in the rare individuals where the number of micro-nuclei is not 

 the typical, the structure of the micro-nucleus is always that which 

 is typical for the race; with an alteration in the number of micro- 

 nuclei there is no alteration in the structure. Thus there is no 

 overlapping of the two forms, no transformation from one to 

 another, so far as the structure of the micro-nuclei is concerned. 

 Thus there is cytological warrant for distinguishing caudatum 

 races from aurelia races, and it seems probable that it will continue 

 to be convenient to designate these as two species. No useful 

 purpose appears to be served by adopting the circumlocution 

 "caudatum form of Paramecium aurelia" in place of "Paramecium 



