Organiser role of the stem cell of the mesoderm in prototroch patterning in Patella vulgata (Mollusca, Gastropoda).

During early development of the gastropod mollusc Patella vulgata, the stem cell of the mesoderm (3D-macromere) is induced. As a result of this induction, the embryo becomes dorsoventrally organised. At about the same time in development, ciliated cells, so-called trochoblasts, are formed. Later in development, some trochoblasts deciliate and, together with the ciliated trochoblast, form the dorsoventrally organised prototroch, the locomotory organ of the larva. In order to study the role of the 3D-macromere in the specification of trochoblasts and in the induction of the dorsoventral organisation of the prototroch, induction of 3D has been prevented in various ways. it is shown that preventing 3D-induction results in the formation of a radially symmetrical prototroch. The trochoblasts of all four quadrants developed like corresponding trochoblasts of the A-quadrant. Somewhere between 30 and 120 min after fifth cleavage the 3D-macromere induces the formation of specific trochoblasts and organises the dorsoventral pattern of the prototroch. Besides a role of the 3D-macromere, a role of other cells has been demonstrated in the conditionally specified deciliation of trochoblasts.