Figure 1. Founder cell formation in Caenorhabditis elegans. (a) During the first four division cleavages the fertilized zygote gives rise to the somatic founder cells in asymmetric divisions. The major tissues produced by the founder cells are indicated. MS and C also produce some neurons. (b) Schematic representation of the first divisions of the embryo. The first three inductions specifying the AB lineage are indicated. Two later inductions specify minor left–right asymmetries in the AB lineage. These inductions specify blastomere identities corresponding to regions later in the larva (see Schnabel and Priess, 1997).

Figure 2. Regional organization of the Caenorhabditis elegans embryo. (a) Twelve-cell stage embryo. The lineage descent of cells is colour coded as shown at the bottom of the figure. (b) Dorsal, ventral left and right view of an embryo at the premorphogenetic stage (380 cells). The descendants of the early blastomeres populate coherent regions. No mixing of the descendants of blastomeres of the 12-cell stage occurs. Cells form the regions by cell migrations. (c) Three views of an embryo created in a computer simulation (M. Gumpel, unpublished). The blastomeres of the 12-cell embryo shown in (a) were divided only in the anterior–posterior direction. The descendants intermingle and no coherent regions form. The specific arrangement of regions in the C. elegans embryo therefore requires active sorting processes. (d) Arrangement of cells within a region at the premorphogenetic stage. Descendants of ABplpaa (red), ABplpap (yellow), ABplppa (green) and ABplpppp (blue) are shown. The anterior descendants of the next cleavage are shown with a lighter, the posterior ones with a darker tint. As indicated for the most anterior lineages cells are not strictly arranged according to their lineage descent within a region.

Figure 3. The lineage produces cell fates according to the positional requirements. The figure shows the anterior part of a premorphogenetic embryo. Yellow nuclei (spheres), nervous system; blue, pharyngeal cells; green, intestine; grey, nonpharyngeal MS-derived nuclei. The pharynx precursor is also outlined. The MSaa derived cells (colour coded from red to orange) span through the anterior of the embryo. The most anterior cell present at the premorphogenetic stage lies outside of the pharynx and later produces two neurons which contribute to the part of the nervous system formed in this area from ABala. The two most posterior cells are placed, like the most anterior, outside the pharynx and produce body wall muscle. As indicated at the top of the figure the pharyngeal cells acquire the fates needed in the specific positions to produce a functional pharynx, for example at the margin of the intestine valve cells are made which connect pharynx and intestine. The 28 cells derived from MSaa present at this stage produce seven different tissues. The fates must be specified primarily by positional criteria. The complex differentiation pattern indicates that no grouping according to cell fate occurs in the lineage.