With 3 Figures.
Frequently in the literature concerning Cladocera are recorded the appearance or hatching of sexual eggs of various species, but opinions or information regarding causative factors are quite diverse and often even contradictory. When it became desirable t o undertake an extensive genetical study in biparental inheritance in Daphnia longispina ( B a n t a , Snider and Wood, 1926; B a n t a and Wood, 1928) no satisfactory account was found of a successful procedure for obtaining and hatching in the laboratory sexual eggs of any cladoceran form. Accordingly, it was necessary, previous to and coincident with carrying out this genetical study, to work out such a technique.
The present paper is a record of observations made (at room temperature, 19"--82" C., unless otherwise stated) during the development of the technique which is now used to induce the production and hatching of the sexual eggs of Daphnia longispina. During the seven years the work has been in progress some 13,308 eggs have been obtained. Of these 2,590 have been hatched. The specialization of this technique for Daphnia longispina must be emphasized inasmuch as Moina rnacrocopa as well as Daphnia pulexother forms studied in this laboratory -require somewhat different treatment in order to induce sexual egg production.
Daphnia longispina females ordinarily produce only diploid psrthenogenetics eggs which usually develop into females but which under appropriate environmental conditions are male-determined and develop into males; or these same females may produce the sexual (hap-