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It has been well known' since the time of Si eb old (1836) that in Viviparus there are two kinds of spermatozoa—the one a normal flagellate type having head and tail distinct (typical spermatozoa9, the other an abnormal worm-shaped type having a small quantity of chromatic substance in its head (atypical speamatozoa) (Fig. 1. t and a). v. Br unn (1844) thought the atypical spermatozoa to be functionless and presented the hypothesis that the atypical spermatozoa is rather homologous to the egg of the hermaphroditic Prosobranchiata. M e v es (1903), in his excellent work on the' spermatogenesis of the animal, found that the atypical spermatozoa is rather normal spermatozoa produced through two maturation divisions, although the maturation divisions are different from those of the typical spermatozoa in that most of the chromatic substance is ejected from the nucleus into the cytoplasm. So he protest-

Numerical Ratio of the Atypical to the Typical Spermatozoa in Viviparus and Several Other Prosobranchiata