DEVELOPMENT OF THE PRIMARY GONADS AND DIFFERENTIATION OF SEXUALITY IN TEREDO NAVALIS AND OTHER PELECYPOD MOLLUSKS

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Abstract
Two more or less distinct types of primary gonads are found in bivalve mollusks. The simplest type occurs in several families of the order Prionodesmacea, where the profusely branching follicles on each side of the body are composed mainly of gametogenic cells and each follicle is nourished by the surrounding mesenchyme or vesicular connective tissue. In the second type, found mainly in the order Teleodesmacea, the branching follicles of the primary gonads are composed principally of large, vacuolated follicle cells of a nutritive nature, the primary gonia being scattered along the central axis or on the periphery. These nutritive cells are cytolyzed during gametogenesis. In some bivalves intermediate conditions are found, with associated gametogenetic follicles and nutritive tissues. In the ambisexual or hermaphroditic species, as Teredo navalis, the primary gonia are early differentiated into the two sexual types. Of these the spermatogonia usually, but not invariably, proliferate and complete gametogenesis in advance of the ovogonia, giving each follicle the appearance of a spermary with a layer of ovocytes on the periphery. As a general rule the ovocytes do not become fully mature until after the discharge of the spermatozoa. There are many variations in the relative proportion and time of spawning of the two types of gametes, however, and in exceptional cases the gonad is almost exclusively of the male or of the female type. Most individuals function first in the male phase, then change to the female phase, while some individuals experience in addition a second sequence of male and female phases. Less frequently a primary female phase precedes the first male phase. Individual differences in the conbinations of the modifying hereditary sex factors are presumably responsible for most of these variations. Each species studied, except those that are strictly unisexual, shows some variations in the sequence of male and female or functionally hermaphroditic phases.