This diagram (courtesy of Dr. Ray Keller, Univ. of Virginia) depicts the acquisition of oriented lamellipodial protrusions on the medial and lateral tips of intercalating deep cells in the dorsal involutiong marginal zone, based on analysis of motility in open faced sandwich explants. As cells converge, these protrusions are thought to aid the intercalation of the cells, presumably by facilitating the "wriggling" of cells between one another. Thes result is the dramatic extension of the marginal zone on the dorsal side of the embryo.