Ingression during early gastrulation of fundulus.

This study demonstrates that involution does not occur during early gastrulation of Fundulus heteroclitus, prior to and during germ ring formation. This conclusion has been reached by following the motile behavior of large numbers of individual cells. Instead of involution, superficial deep cells of the marginal region of the blastoderm undergo ingression. They do not leave the surface as members of a flowing cohesive sheet, but sink beneath rather haphazardly as individuals. Indeed, during much of ingression, many marginal cells are so loosely arranged that they move about freely on the yolk syncytial layer. A small proportion of the cells initially at the blastoderm margin undergo ingression there, but most recede from the margin and ingress supramarginally one to three cell diameters from the margin. Cells that are initially supramarginal ingress mainly there, sometimes quite far from the margin. Only a small number moves to the margin and ingresses there. Interestingly, although most ingression takes place supramarginally, much occurs close to the margin-up to one to four cells away. Ingression begins immediately after the onset of epiboly and is most active before appearance of the germ ring; it ceases quite soon thereafter. It is also more active dorsally than ventrally, correlating with the earlier formation of the germ ring dorsally. Ingression constitutes the first invasive cellular activity of development. Significantly, it proceeds by blebbing locomotion, a noncontact inhibiting mode of cell movement. The possible broader import of these discoveries is given appropriate attention.