Karyologic observations on the maturation of the summer and winter eggs of Daphnia pulex and Daphnia middendorffiana

The cladoceran Daphnia middendorffiana is an arctic species able to produce winter eggs which develop parthenogenetically. The cytological study of the maturation of summer and winter eggs of an italian population of this species has shown that both types of eggs undergo only one maturation division of equational type with the expulsion of only one diploid polar body, the same maturation process as had been found in the pseudosexual eggs of a race of Daphnia pulex (Schrader, 1925).—The chromosomes behaviour during the maturation of summer and winter eggs of D. middendorffiana has been compared with that of a population of D. pulex which shows a normal heterogonic cycle.