Cell contacts of S2a cells. Three-dimensional models of embryonic cell nuclei. In the 12-cell stage in C. elegans, S2a (MS) contacts and induces two of the four S1a descendants ((a); red arrows). In P. sambesii in the three different spatial variants, S2a (MS; orange) contacts all four S1a (ABa) descendants ((b-d); white arrows). In the 26-cell stage in C. elegans, MSap contacts and induces a single AB descendant ((e); red arrow). In P. sambesii either both ((f, g); white arrows) or none (h) of S1 (AB) sister cells contacts the corresponding S2a (MSa) cell. Black bars connect selected sister cells. For nomenclature of spatial variants, see Figure 4.