REINVIGORATION PRODUCED BY CROSS FERTILIZATION 339 
produced in some minor experiments which had been performed 
at about this time in February. In this way the race was pre- 
served and used in later experiments in connection with the 
problem of in- and cross-breeding. ‘The parthenogenetic race A 
is alive at the present time in the 503rd generation, but is in a very 
exhausted condition. 
During the whole period in which the two races were con- 
ducted in parallel generations the external factors or environment 
were as identical as possible. The individuals of each generation 
were isolated at the same time, put into the same kind of dishes, 
with approximately the same amount of tap water to which was 
added the same kind and amount of food from the same food 
culture jars. They were always kept side by side in the stacked 
watch glasses, at the same room temperature, and in the same 
illumination. Some of the time they were kept in a dark room 
and some of the time in a well lighted room. The greater part 
of the time they werein Middletown, Connecticut, but during a few 
weeks of the summer of 1909 they were at Cold Spring Harbor, 
New York, and also in Vermont. The summer of 1910 they were 
only out of Middletown two or three weeks when in Vermont. 
In the summer of 1911 race A was in Woods Hole, Massachusetts, 
and in Vermont for a few weeks. 
The criterion selected for deciding whether the races were strong 
or weak was the rate of parthenogenetic reproduction. This was 
selected because of simplicity of observation together with its fun- 
damental importance in connection with growth and metabolism. 
In order to determine the comparative vigor of the two races 
A and B their rates of parthenogenetic reproduction were obtained 
and compared with the'rates of reproduction of two other parthe- 
nogenetic races Cand D. Race C was started about nine months 
later than races A and B, from a parthenogenetic egg of a wild 
individual which was isolated from a general wild culture of roti- 
fers supposed to have started a few months previously from a 
fertilized egg. Race D was started at the time of the experiments 
from a fertilized egg of another wild unpedigreed general culture 
of rotifers which was begun in October of 1908. 
