Archenteron

This buckles inwards towards the blastocoel in a process called invagination.

This pouch narrows and lengthens to become the archenteron, a process driven by convergent extension.

The filopodia—thin fibers formed by the mesenchyme cells, found in late gastrulation—contract to drag the tip of the archenteron across the blastocoel.

The endoderm of the archenteron will fuse with the ectoderm of the blastocoel wall.

At this point gastrulation is complete, and the embryo has a functional digestive tube.