Fig. 1

Unequal spiral cleavage. Schematics of unequally cleaving spiralian embryos during early development. All schematics are in animal view. a The first cleavage gives rise to two unequally sized cells (left). At second cleavage, four macromeres A (green), B (purple), C (pink) and a larger cell, D (blue) are born (center). At third cleavage, the micromeres of the 1st quartet are born (1a, 1b, 1c, 1d) (right). b The embryonic organizer in spiralians is typically derived from the D quadrant (cells in blue). Blue cells with asterisks mark cells that have been identified as embryonic organizers in various spiralian species (see text). Cells in green, purple, pink and blue are descendants from the A, B, C and D quadrants, respectively. c C. pergamentaceus cleavage pattern. Cropped DIC image of live C. pergamentaceus embryo (left). At 2nd quartet stage, there are 16 cells in the embryo, and micromeres 1d¹ and 2d are larger than the corresponding micromeres in the other quadrants (center schematic). At 3rd quartet stage, there are 32 cells in the embryo (right schematic)