[The individualization and organogenesis of the embryonic gonad in the gonochoric prosobranch molluscViviparus viviparus L.]

The origin and evolution of the embryonic gonad ofViviparus viviparus were studied ultrastructurally. The gonad is formed by migration and multiplication of pericardial cells.There are several successive stages during gonad organogenesis: during the sexually undifferentiated stage with one cellular type, the gonad is made up of primordial cells which seem to be identical. The sexually undifferentiated stage with two cell types is characterized by the differentiation of germinal and non-germinal cells. The non-germinal cells are linked to one another by septate junctions and their long extensions completely surround the germinal cells which remain isolated and do not form junctions with their neighbours. The gonad sexualisation does not appear until after organogenesis, towards the end of embryonic development, or even after birth.