Fig. 2

Owenia fusiformis undergoes equal spiral cleavage. Confocal laser scanning microscopy (CLSM) images of early embryonic stages from a oocyte to p blastula stage (approx. 64-cell). a The oocyte activates naturally in sea water and after the germinal vesicle breaks down. b After fertilization, the embryo undergoes equal cleavage resulting in two and later four equal blastomeres (c–f). Open arrowheads in (d) and (e) points to the spiral deformation of the actin cytoskeleton in preparation for the dextral cleavage. f–k The spiral cleavage initiates with the formation of four oblique micromeres (1q) that are larger than the macromeres (1Q). l–o Cleavage continues with the stereotypical alternation of the spindles back and forward from counterclockwise to clockwise to give rise to the second (2q) (l), third (3q) (m, n) and fourth (4q) quartet of micromeres (o, p), until the formatin of the coeloblastula (o). bl: blastocoel; gv: germinal vesicle; pb: polar bodies; vf: vegetal cross furrow. Scale bar is 50 µm