A current view of secondary invagination involves autonomous extension of the archenteron in the early phase of elongation, followed by secondary mesenchyme dependent pulling in the second phase.
If such a model is true, then we would predict that pulling by secondary mesenchyme cells would be more substantial late in gastrulation. In fact, the archenteron displays "necking" late in gastrulation, a phenomenon well known to engineers as a characteristic of plastic deformation (sort of like pulling salt water taffy). Click on the picture below to see a time-lapse movie that demonstrates this necking behavior.
Size of movie: ??? K