Asymmetries during molluscan embryogenesis

In some molluscan species the unfertilized egg is symmetrical around its centre. The maturation divisions provide the egg with an axial symmetry with an animal-vegetal asymmetry. During the first two cleavages the egg loses its axial symmetry by the formation of unequal quadrants. The size differences may be very pronounced in species where the first two cleavages are accompanied by the formation of a polar lobe or where the first two cleavages are very unequal. There are some molluscan species in which at first glance the four quadrants appear equal. Exact measurements of the relative volumes have shown that the spiral character of the cleavages gives rise to minor differences between the quadrants. During further division this difference is limited to the vegetal macromeres; other corresponding blastomeres in the four quadrants are mutually equal. Therefore the absolute difference between the macromeres increases after each division. The size difference between the macromeres predisposes the biggest macromere to attain a central position and to become induced to develop the stem cell of the mesoderm. The bilateral symmetry is later lost by the counterclockwise rotation through 180 degrees of the visceral mass in relation to the head and foot.