DEVELOPMENT OF THE PRIMARY GONAD AND SEXUAL PHASES IN VENUS MERCENARIA LINNAIUS

¹ From the U. S. Bureau of Fisheries and the Osborn Zoölogical Laboratory, Yale University

1. Examination of the developing gonads of young clams from Long Island Sound, at two-week intervals during the first year of life, shows that a primary bisexual gonad is formed in each individual when the animal reaches the size of 4-6 mm.

2. The primary gonads form from associations of germinal cells in the connective tissue, not immediately beneath the body walls but at some distance from them, at the level of the heart or slightly below it. A lumen soon appears in each gonad follicle and the germinal cells begin to proliferate rapidly.

3. The primary gonad contains the antecedent cells of both sexes but the protandric nature of the primary gonad becomes manifested by the rapid proliferation of the spermatogenic cells.

4. This species is not strictly protandric because few individuals develop into females without passing through a functional male phase.

5. Functional sperniatozoa have been observed in the gonads of young clams, collected in October and November when only 5-7 mm. long. In some cases discharge of spermatozoa occurs at that time.

6. In September of the second year, after the animals have spawned as males, two types of individuals become distinguishable as definitive males and females. In the males a second period of spermatogenesis begins in the autumn and continues at a reduced rate throughout the winter. The bisexual character of the definite sexual gland is usually retained to at least sonic extent because of the presence of many small ovocytes.

7. In animals destined to become females, after the completion of the initial male phase, the gonads may remain virtually empty but in a distended state throughout the winter. There is little indication of ovogenic activities until the following spring when the water temperature approaches 15° C. From then on the proliferation and growth of ova proceeds very rapidly until the animal reaches the functional female phase.

8. In V. mercenaria the adults are, with few exceptions, of separate sexes. Among six hundred and fifty mature clams studied by means of serial sections only three cases of true hermaphroditism were observed.

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