PARAMAECIUM AURELIA AND PARAMAECIUM CAUDATUM 229 
staining relations of the micronuclei, in ways that correspond 
to the descriptions of Hertwig and Maupas. But 7 rare cases 
specimens of the caudatum races have two micronuclei, those 
of aurelia races but one, thus confirming the observation of 
Calkins on this point.” 
In accordance with the conclusions of Calkins, I have used the 
specific name aurelia to include both the aurelia and caudatum 
forms; but my extended study of Paramaecia cultures has demon- 
strated that these two forms are remarkably constant, and I am 
inclined to the view that they are distinct species, in the sense 
in which this term is generally used in biological work. The data 
on which I base this conclusion are chiefly as follows: the pedi- 
gree culture of P. aurelia which I have had under daily observa- 
tion for (so far) more than three and one half years, during which 
time more than 2100 generations have been attained, has bred 
practically true to the aurelia type as described by Maupas in 
the passage quoted. The pedigree culture of P. caudatum which 
I have carried for nearly seven months, and which has attained 
more then 300 generations up to the present time, has bred prac- 
tically true to the caudatum type as described by that author. 
The pedigree culture of P. aurelia was started on May 1, 1907, 
with & ‘wild’ individual which was found in a laboratory aquar- 
ium, and was carried on at Williams College during May and June, 
1907; at the Woods Hole Marine Biological Laboratory during 
parts of the summers of 1907 through 1910; and at Yale Univer- 
sity during the academic years from 1907 to the present time, 
November 30, 1910. The pedigree culture of P. caudatum was 
started on May 14, 1910, with a ‘wild’ individual collected from 
a pond at New Haven, Conn., and was carried on at Yale Univer- 
sity except for a period of a few weeks in the summer when it 
was taken to the Woods Hole Laboratory. 
The original specimen of each culture was placed in about five 
drops of culture fluid on a glass slide having a central ground 
concavity, and when the animal had divided twice, producing 
four individuals, each of these was isolated on a separate slide to 
form the four lines of the respective cultures. The pedigree 
cultures have been maintained by the isolation of a specimen from 
