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Infolding of a sheet (epithelium) of Sea urchin endoderm, cells, mush like the indention of a, soft rubber ball when it is poked., , , , , , , Invagination, , , , , , , , , , , , , , Inward movement of an expanding, outer layer so that it spreads over, the internal surface of the, remaining external cells, , involution Amphibian mesoderm, , , , , , , , , , , , , , , , , , Sea urchin mesoderm,, Drosophila, neuroblasts, , Migration of individual cells from, the surface into the embryo’s, interior. Individual cells become, mesenchymal (i.e. separate from, one another) and migrate, independently., , Ingression, , , , , , , , , , , , , , , , , Hypoblast formation, , Splitting of one cellular sheet into, in birds and mammals, , two more or less parallel sheets., While on a cellular basis it, resembles ingression, the result is, the formation of new (additional), epithelial sheet of cells., Epiboly Movement of epithelial si, (usually ectodermal cells) sprea ing, as a unit (rather than individually), to enclose deeper layers of the, embryo. Can occur by eve, dividing, by cells lee pce, shape, or by several layers : ie, ‘ntercalating into fewer ave 8;, often, all three mechanisms are, , used., , , , , , , , , , , , , , , , , , , , sheets Ectoderm formation, , in sea urchins,, tunicates, and, amphibians, , , , , , , , , ', ’