Fig. 2

Proposed revisions to the canonical model of spider gastrulation. Diagrammatic midsagittal sections. Ectoderm is blue. Top row: Canonical model. A mixed population of cumulus cells and deep layer mesendoderm cells (orange) enters through the central blastopore. The cumulus (red) then differentiates and migrates towards the prospective dorsal side as the primitive plate (yellow) enlarges. Gastrulation ends when a deep layer has formed under the entire germ disc and signaling from the cumulus has broken radial symmetry. Bottom row: Our working model. The cumulus mesenchyme cells (red) internalize first, as a group, through the central blastopore. As the cumulus migrates, additional mesendoderm (yellow) cells internalize at two distinct locations: through the central blastopore and at the rim of the germ disc or, in species lacking a germ disc, a region several cells away from the central blastopore