Oocyte and egg organization in the patellogastropod Lottia and its bearing on axial specification during early embryogenesis

In the basal gastropod Lottia, the apical region of the oocyte is normally the site where the meiotic apparatus attaches and polar body formation occurs following fertilization. This site marks the animal–vegetal axis of the egg. A stereotypical cleavage pattern is organized, and the segregation of developmental potential occurs along this axis during early development. The segregation of developmental potential is a relatively late event and probably does not start until after cleavage begins. By compressing oocytes during the process of germinal vesicle breakdown, the position where the meiotic apparatus attaches to the cell membrane can be altered so that it no longer corresponds to the apical end of the oocyte. This new site of polar body formation sets up a new animal–vegetal axis that organizes cleavage and the segregation of developmental potential. The timing of animal–vegetal axis specification in Lottia is much later than it is in derived gastropods with a precocious specification of the D quadrant.