I have rarely observed movement of submarginal cells to the margin to ingress solely there. All submarginal cells move about a bit but do not move consistently toward or to the margin prior to their ingression. The ingression of cells farther away from the margin is highly variable. For example, in one series of 26 cells, anywhere from 3-6 cells distant from the margin only 1 cell moved to the margin to undergo ingression there. Thus, one can conclude that in Fundulus there is no involution.
Cells undergo ingression in the marginal region of the blastoderm, but mainly away from the margin.
This may provide an explanation for observations based on random labeling of cells that have led many of us to believe that involution is a normal feature of teleost gastrulation. Prior to the beginning of epiboly, Fundulus deep cells do not undergo ingression. Ingression definitely tapers off as the germ ring forms and convergence toward the embryonic shield begins. In sum, ingression of deep cells of Fundulus takes place mainly during the period between the onset of the epiboly and the appearance of the germ ring. Perhaps the commencement of epiboly provides a stimulus.