The inner layer will form both endoderm and mesoderm . Endoderm will give rise to the epithelial parts of the gut tube, including the pharynx, and as well to the epithelium of gut-associated glands including the liver and pancreas. Mesoderm develops into a huge diversity of cell types, including the notochord, muscle, kidneys, the heart, vasculature, blood, and most of the connective tissue of the body. Two other movements begin as gastrulation and epiboly continue. Convergence narrows the embryo, along the dorsal side.
Extension lengthens the embryo, again along the dorsal side.
In the flipbook movie, you will see that shortly after involution begins one side of the embryo is getting quite distinctively thicker than the opposite side. The anterior pole of the pharyngula develops from cells located at or near the animal pole of the egg. But the location of the posterior pole changes markedly with the lengthening of the body axis. The region really corresponding to the dorsal margin at the beginning of gastrulation extends as the end of the tail. It was made by Nathalia Glickman, using a special vital dye to outline the cells, and a confocal microscope to record the movements.