Stages 1-3 of C. salei. All scale bars 200 μm. Sytox staining, a-c, e, h-j; light micrograph, d; SEMs, f, g. a-c, Stage 1, Early cleavages. a: The egg is spherical in shape, about 1.2 mm in diameter. Due to the large amount of yolk, the nuclei in the egg centre are not visible. b: Same egg as in a, broken apart. Nuclei (white arrows) are visible within the broken yolky mass (Y). c, More developed egg as in a with about 30 nuclei visible, still embedded in yolky mass. d-e, Stage 2, Blastoderm. d: In a live egg (egg number 2 from Additional file 2 at movie frame 76), the nuclei are visible as dark spots within the whitish periplasm. The periplasm is the primordial cytoplasm surrounding the nuclei and is lobate in shape (white lines) at the surface of the egg. e: Apart from some nuclei (white arrows) still surrounded by yolk only, most of the nuclei are enclosed by cell membranes, forming a cellular blastoderm. f: An egg (comparable to the egg in e) with a cellular blastoderm evident as a knobby surface texture. g: Parts of a broken egg at the same stage as the eggs in e and f. The yolky mass is now organized into large pyramid-shaped compartments (white dotted line). h-j, Stage 3, blastopore. h: A number of blastodermic cells aggregate and form the early blastopore (arrow). i: There are increasing numbers of cells in the blastopore (Bp) as it becomes more prominent. j: Slightly later, the blastopore (Bp) has a pore-like appearance.