Fig. 4. Cell ingression. (A) Schematic lineage diagram of early blastomeres indicating AB and P1 descendants. (B) Lateral view of a three-dimensional model of nuclei at the 26-cell stage; anterior is left. The subset of descendants of the early blastomeres that ingress, or give rise to ingressing cells, are indicated by the color scheme in A; non-ingressing cells (a subset of AB descendants) are shown in grey. (C) Two diagrams of the ventral surfaces of embryos outlining the regions where various cells ingress and listing the periods of cell ingressions in minutes from the 2-cell stage. The left diagram indicates ingression of the E daughters and the MS ‘wishbone’ descendants (red), and the right diagram indicates later ingressions including the MS ‘central’ descendants (red oval). (D-G) Nomarski light micrographs of representative stages of gastrulation; times as indicated. (D) 28-cell embryo; lateral view as in B. An MS descendant (red arrowhead; MSap) and P4 (cyan arrowhead) are shown spreading across the apical surfaces of the E daughters (yellow asterisks). (E) Ventral view showing MS ‘central’ descendants (red asterisks) and a subset of C descendants (green asterisks) prior to ingression. The arrow indicates the cleft created by ingression of the D descendants. (F) Ventral view showing part of the cleft created by ingression of the MS descendants; neighboring cells (arrows) will eventually move to cover the cleft. (G) Ventral view showing epiboly of the skin cells (arrows) to cover remaining cells on the ventral surface.