50 A. FRANKLIN SHULL 



The same condition was found among inbred eggs. Females 

 fertilized by males of their own parthenogenetic line laid eggs 

 which, in some lines, did not hatch at all, though the eggs were 

 kept for many weeks, and though some of them were dried, 

 others frozen — treatment generally supposed to favor the hatch- 

 ing of such eggs. Of other lots of inbred eggs, 50 to 70 per cent 

 were viable. 



In the autumn of 1911 I obtained particularly favorable mate- 

 rial with which to test the possible inheritance of this viability 

 of the fertilized eggs. Two lines were found, one of which pro- 

 duced highly viable fertilized eggs by inbreeding, the other only 

 slightly viable. The experiments about to be described show 

 this viability to be a rather definitely inherited character. Its 

 behavior in inbreeding, in crosses, and in selection have been 

 tested, and some results of theoretical interest beyond the limits 

 of this particular problem obtained. 



I take pleasure in acknowledging the invaluable assistance of 

 Miss Frances J. Dunbar in carrying the experiments over criti- 

 cal periods during my enforced absence. 



FACTORS DETERMINING VIABILITY OF EGGS 



The following are some of the factors that seem to me to 

 govern the proportion of eggs that hatch: 



1. A solvent which reduces the thickness of the chitinous shell 

 of the egg is apparently produced. The effect of this solvent 

 is perceptible in parthenogenetic eggs shortly before hatching, 

 when the shell, thinner at some points than at others, yields to 

 pressure from within, and is raised in visible humps. When the 

 embryo has emerged, the shell is plainly very much thinner than 

 the shell of a freshly laid egg from which the egg itself has been 

 crushed by pressure. In fertilized eggs this solvent does not 

 usually produce any visible effect, for the young rotifer breaks 

 out while the shell is still quite thick. It is ordinarily quite 

 difficult to distinguish an empty shell from an unhatched fer- 

 tilized egg, unless the break in the chitin can be seen. Some- 

 times, however, the embryo does not break from the shell until 

 the latter is quite thin and transparent. These occasional cases 

 show that such a solvent exists. Variations in the production 



