The relation of the two blastomeres to the polar lobe in Dentalium.

Experiments on the influence of the polar lobe on the development of molluscs have shown that after removal of the polar lobe cleavage is radially symmetrical and indications of bilateral symmetry do not appear (Wilson, 1904: Dentalium; Clement, 1952: Ilyanassa). In normal development the polar lobe fuses with one of the two cells of the trefoil stage, and this cell becomes the posterior side of the embryo. The question now arises whether the polar lobe fuses in an arbitrary way with one of the blastomeres at first cleavage, which then becomes the CD cell. Another possibility is that one of the two blastomeres at the trefoil stage is already predetermined to become the CD cell, with which the polar lobe always fuses. In the first case dorsoventrality is determined epigenetically; in the second case it is preformed. Morgan (1936) tried to solve this question by removing one of the blastomeres at the trefoil stage in Ilyanassa. Unfortunately, the other blastomere nearly always cytolysed. Only six eggs out of several hundreds survived. In four eggs the polar lobe fused with the remaining cell; in two it did not. Eggs of Dentalium appeared to be suitable material for this kind of experiment, as they hardly ever cytolyse. An attempt was therefore made to solve this problem by using eggs of Dentalium.