338 DAVID DAY WHITNEY 
Although considerable work on the problem of rejuvenescence 
by fertilization has been done on plants, nevertheless experiments 
and observations on the multicellular animals in connection with 
the reinvigoration of the race by fertilization are as yet very few 
and inconclusive. The purpose of this present paper is to demon- 
strate that a great amount of rejuvenescence occurs when two 
weak races are cross bred and that only a small amount of re- 
juvenescence takes place when each weak race is inbred with 
itself. 
On October 6, 1908, a fertilized egg from a wild culture of the 
rotifer, Hydatina senta, was put into some fresh culture water 
and on October 12, 1908, a young female hatched from the egg. - 
A pedigreed parthenogenetic culture or race was started from this 
female and was called race A. In the 59th generation of this race 
A, on February 24, 1909, two parthenogenetic sisters were iso- 
lated. One became the mother of what has been called the 60th 
. generation of race A and the other became the mother of what has 
been called the 60th generation of race B. In other words at the 
59th generation the race was split into two sister races. One was 
still called race A and the other was called race B. These two 
sister parthenogenetic races A and B were kept in syracuse watch 
glasses. Usually once in forty-eight hours ten daughter-females 
of each race were isolated, each daughter-female being placed in 
a separate watch glass. They produced the young females of the 
succeeding generation. Both races were always fed from the 
same food culture jars made from a culture of horse manure and 
water inoculated with bacteria and protozoa. During the first 
fifteen months, until January, 1910, these food cultures contained 
a miscellaneous assortment of protozoa but in January, 1910, pure 
food cultures of the flagellate, Polytoma, in horse manure solu- 
tions were started and proved so successful that they have been 
continued to the present time. The special method of making 
these cultures has been described in a former paper. 
The two pedigreed sister parthenogenetic races were continued 
up to March 3, 1911, at which time race B apparently from exhaus- 
tion died out in the 384th parthenogenetic generation. How- 
ever, some fertilized eggs of this race were saved which had been 
