So here's the model of Saccoglossus gastrulation as it stands:
Archenteron formation is by invagination of the "vegetal plate", blastopore closure is by contraction of the ring of "marginal zone" tissue surrounding the vegetal plate. The vegetal plate can buckle more or less autonomously but can not close the blastopore on its own. The marginal zone will contract circumferentially on itself, even if reduced to roughly 1/4 (perhaps less) of its full circumference. This circumferential contraction or convergence probably helps the vegetal plate invaginate. It may also be involved in helping the embryo extend along the animal-vegetal axis; again a partial marginal zone is adequate, as a lateral hemisphere will extend.
A distinct "crease" or some kind of morphological boundary forms between
the vegetal plate and the marginal zone as the vegetal plate begins to buckle
inward. These two tissues show very different and fairly independent subsequent behaviors. Interestingly, the marginal tissue seems to be able to shear with respect to the vegetal plate tissue, leaving the latter behind as it converges to one side.
Interesting questions and how they might be answered:
1) Is there really no involution? (vital dye stain, ideally fluor, or anything else that shows up reasonably well in the microscope, i.e. high contrast, such that I can follow individual cells in a movie, either in intact embryos or in explant.)
2) Is there any intercalation, either within the vegetal plate or in the marginal zone? And if not, how do they accomplish their respective tissue shape changes? (vital dye, movies)
3) How small a bit of vegetal plate will buckle autonomously, and how much will a vegetal plate buckle with and without some marginal zone tissue around it? Does marginal zone tissue have any tendency to bend or curl along the animal vegetal axis? I.e. is the vegetal plate entirely responsible for its invagination, or does any of the bending inward come from the marginal zone directly (rather than as a result of circumferential contraction)?
4) Will a bit of marginal zone tissue contract circumferentially all on its own, or does it need to be in contact with vegetal plate? Can vegetal plate induce animal cap tissue to do anything interesting/different? What if you take a marginal zone and cut the vegetal plate out of the middle; will it converge on the center? (supposing the microsurgery is possible; not so clear ) (explants and movies)
5) How does the vegetal plate buckle inward, if not by cell rearrangement? Cell shape change? (histology / SEM) The old papers describing morphological changes at this stage may be adequate to answer these questions, or at least will certainly give some clues.
6) What is the epithelial / mesenchymal character of the embryo during gastrulation? Any sign of adherens or tight junctions? (broad spectrum antibodies against AJ/TJ components) Other apical / basal markers? (Par3/par6 / aPKC homologs)