Explaining Drosophila development just after the cellularisation process to a student can be tough. I tried to and ended up searching youtube for animations. It's mentally difficult to picture the invagination of the mesoderm and then the migration of blastoderm cells to initiate germ band formation from worded descriptions. I am quoting this segment from the wikipedia on germ band extension "During this process the ventral germ-band extends around the posterior end of the embryo, effectively folding over onto the dorsal side of the egg. Multiple individual cells intercalating mediolateral to the anterior-posterior axis drive the resulting global elongation of the embryo." How does one get a handle on that?
Developmental Biology was one of my favorite subjects in NUS, taught by the enthusiastic and nurturing Prof Lim Tit Meng (now director of the Singapore Science Centre). Those days we had plasticine models to play with to help us figure out gastrulation. If you didn't have good spatial cognitive skills, good luck to you. So I am highlighting this video from the Garland Science channel that immediately solves the problem. Two thumbs up. It gets five popcorns from me.