88 A. FRANKLIN SHULL 



within a period of' two weeks; in another hne, the hatching was 

 spread nearly uniformly over a period of four weeks or more. 

 This variability of the duration of the egg stage may be measured 

 by the standard deviation. In the first of the two lines just 

 mentioned, the standard deviation of the hatching time was low; 

 in the second, the standard deviation was high. 



The variability of the duration of the egg stage is inherited. 

 When a line whose inbred eggs showed great variability (high 

 standard deviation) of the hatching time was reciprocally crossed 

 ■iwith a line showing small variability (low standard deviation) 

 of the hatching time, the reciprocal hybrid lots of eggs were inter- 

 mediate between the parents in variability, but were unequal. Each 

 lot was more nearly like the inbred eggs of its maternal line. 



The same pair of reciprocal hybrids were unequal both in 

 viability and in variability of hatching time. This inequality 

 was not permanent, however; that is, it lasted only during the 

 egg stage. For when a parthenogenetic line was derived from 

 each lot of eggs, and these two 'reciprocal' lines were inbred and 

 reciprocally crossed, the inequality observed in the former lots of 

 eggs did not appear in the eggs of the reciprocal lines. In the 

 . latter lots of eggs, the inbred lots were approximately equal, and 

 the reciprocal hybrids were approximately equal. 



The inequality of the reciprocal hybrids in the egg stage can 

 not be interpreted as due to the influence of the cytoplasm in 

 heredity. The cytoplasm must rather be regarded as part of 

 the environment. 



In the inheritance of viability of the eggs of Hydatina, segre- 

 gation occurs in the production of the gametes. For this reason, 

 selection for high and low viability within a hybrid (heterozygous) 

 lot of eggs was plainly effective. But the whole effect was 

 obtained by the first selection, this effect not being increased 

 by subsequent selections. I conclude from the rapidity with 

 which the maximum effect of selection was reached, that the 

 number of segregating units concerned with viability of eggs is 

 small. 



After several successive inbreedings from a line that was plainly 

 heterozygous with respect to viability, selection for high and 



