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For the Oregon strain ofDrosophila melanogaster Meig. here studied, approximately 40% of the eggs laid during the first 10 days of adult life fail to hatch under optimum conditions. The level of egg viability is highest during the third to fifth days. Thereafter it falls irregu­larly. Eggs laid on the second day, and to a small extent those laid on the third day, are formed from food reserves accumulated during the larval stage and are less viable than those laid during the next few days. Under optimum feeding conditions, changes of egg viability as the flies age are associated with physiological changes in the female and are not due to direct effects on the male. Egg viability is considerably influenced by the quality of the yeast fed to the adults. Qualitatively different yeast foods presumably differ with respect to specific substances necessary for maintaining egg viability. Some of these sub­stances can be destroyed by heat and extracting with water. The lower viability of the eggs laid by flies fed on a qualitatively inadequate diet is probably due to inhibition of development at some point prior to hatching. Alterations of fecundity due to diet always accompany similar alterations of egg viability, but egg viability is more sensitive to nutri­tional differences than is fecundity. Substantially, flies require the same type of food for maintaining both fecundity and egg viability. Under uniform conditions the upper limit of egg viability is probably under genetic control. Within any strain individual egg viability differences are due to differences of larval environment and adult nutrition. These findings are discussed in the light of environmental changes which occur in a normalDrosophila culture, and it is suggested that changes in the composition of the yeast flora therein may affect the viability of the eggs laid. The extent of this effect is probably insufficient to influence population growth significantly.

The ecological determinants of population growth in a Drosophila culture. II. Circumstances affecting egg viability