LIFE CYCLE OF HYDATINA SENTA 291 
expected to show a correspondence between ‘vigor’ and the pro- 
portion of male-producers. 
Effect of inbreeding on the proportion of male-producers 
In the second of these studies (Shull, 711 a) I described experi- 
ments in which two distinct lines of rotifers, yielding different 
proportions of male-producers, were crossed, the zygotes giving 
rise to lines in some crosses yielding more male-producers than 
either parent line, in other crosses a proportion of male-producers 
intermediate between those of the parent lines. In order to 
explain these phenomena, an attempt was made to inbreed the 
same lines that were used in the earlier crossing experiments. 
Females were paired with males of the same line, and a large 
number of fertilized eggs was secured. These eggs, however, 
did not hatch before it was necessary to discontinue the experi- 
ments. I have now, however, in other lines, succeeded in obtain- 
ing viable offspring from females paired with their own nephews 
or cousins, and give the results in the following experiments. 
Experiment 1. Inbreeding. Some winter eggs were collected 
in the spring and kept in an ice-chest or in cold running water 
until September. The eggs were then brought to room tempera- 
ture and began to hatch in five days. From one of the females 
thus obtained was reared the line of rotifers used in this and the 
following inbreeding experiments. 
Between October 7 and October 20, many females of the parthe- 
nogenetic line just mentioned were paired with males of the same 
line. Of 1099 eggs obtained, one hatched October 22, and from 
her another parthenogenetic line was bred. The number of male- 
and female-producers in this inbred line is compared, in table 2, 
with the corresponding data from that part of the original line 
which was reared at the same time. 
It appears from the table that the inbred line yielded 16.7 per 
cent of male-producers, the original line only 10.6 per cent. In 
such an experiment, however, the original line is necessarily 
further removed from the fertilized egg than is the inbred line. 
We have learned above that there may be a progressive decrease 
