Fig. 9

Our new model of spider development showing origin of deep cells and fate of cumulus. a Venn diagram representing superficial and deep cells: key to color-coding in b–g. Before gastrulation, the embryo’s cell types are all on the surface: red represents cumulus before internalization; yellow represents prospective mesendoderm; blue represents ectoderm. During gastrulation, multiple layers form, which are shown as overlays of the colors representing the superficial and deep layers. b Pre-gastrula. Cells nearest the central blastopore are fated to become the cumulus (red). c Gastrulation begins. Cumulus cells internalize as a group (and so appear orange, i.e. red under yellow). d Mid gastrula. Other presumptive deep cells have internalized through the central blastopore to form the primitive plate mesendoderm (green, i.e., yellow under blue). e Late gastrula. Cumulus has migrated beneath the superficial layer and additional mesendoderm cells internalize at extra-blastoporal sites (individual green cells). f Dorsal field (extraembryonic epithelium) begins to expand in posterior. Cumulus cells disperse within the deep layer. g Germ band has formed. Most cumulus cells have a posterior fate