First cleavages during early development of Maritigrella crozieri . Fertilised eggs devoid of an eggshell were collected using the method of Boyer , then coverslipped and observed under a microscope in a room at 27°C. (Ai) through (Avi) show extended focus snapshots of animal views of the 4 four first cleavage stages from a 4D time-lapse movie using Helicon Focus software (HeliconSoft, Kharkov, Ukraine). (Ai) Two-cell stage. (Aii) Four-cell stage. (Aiii) Late four-cell stage showing dextral torsion of the four blastomeres. (Aiv) Eight-cell stage after division of the first quartet micromeres. (Av) Late eight-cell stage showing sinistral torsion of the blastomeres. (Avi) Sixteen-cell stage after division of the second-quartet micromeres. (B) Diagram corresponding to the cleavage stages shown in the part (A) images. Blastomere nomenclature according to Surface , with quadrants arbitrarily assigned in species with equal cleavage. Scale bars represent 50 μm.