Fig. 1

Early developmental stages of a Parasteatoda tepidariorum embryo in a side view. After fertilisation, energid cleavages (nuclei with attached perinuclear cytoplasm) occur in the centre of the egg (not shown). Cellularization takes place around the 16 nuclei stage and the nuclei with attached perinuclear cytoplasm reach the periphery of the yolk at the end of stage 1 (a) and a blastoderm is formed at stage 2 (b). The embryo contracts (c) and the perivitelline space is visible at late stage 2 (the upper part of the vitelline membrane is indicated by the dotted line in c). At the end of stage 2 and the beginning of stage 3 some cells cluster to form the primary thickening in the centre of the germ-disc (arrowhead in d and e). A dense germ-disc has formed at stage 4 (f). All pictures are stills taken from Additional file 1: Movie 1