Variable arrangement of S1 descendants. Three-dimensional models of embryonic cell nuclei. With respect to the spatial arrangement of S1 blastomeres, four different cleavage types can be defined in the 8-cell stage (R, S1 blastomeres on the right side are positioned posterior to their left counterparts (a); L, S1 blastomeres on the left side are positioned posterior to their right counterparts (s); M1/M2, with respect to anterior/posterior S1 blastomeres on both sides occupy equivalent positions (g/m), but with respect to dorsal/ventral they occupy opposite positions (g*, m*). With the division of 16 S1 cells, cleavage types R and L each reproducibly split into two further spatial variants (d + d’; v + v’) due to shifts of left and right S1 descendants relative to each other. To better visualize that in the 64 S1 cell stage, specific positions on the left and right sides (e-w’) and at the anterior pole (f-x’) of the embryo are occupied by blastomeres from variable lineage origin, selected cells are marked with asterisks. Color code: members of the eight S1 descendants are marked as shown in Figure 1. Non-S1 cells are shown in light grey.