The ingression of primary mesenchyme cells comes about through changes in their adhesion to other cells and to the extracellular matrices that surround them. During this time, the archenteron extends dramatically. In the process of extension, the wide, short gut rudiment is transformed into a long, thin tube. This extension is accomplished by the rearrangement of the cells of archenteron by migrating over one another and by flattening themselves. The initial migration of the primary mesenchyme cells is directed by the blastocoel wall and by the parallel fibrils of the extracellular matrix in the blastocoel.
Cells intercalate to narrow the tissue and at the same time move it forward.
The third phase of archenteron elongation is initiated by the tension provided by secondary mesenchyme cells, which form at the tip of the archenteron. Filopodia are extended from these cells through the blastocoel fluid to contact the inner surface of the blastocoel wall. There is a specific ''target'' site for the filopodia that differs from other regions of the animal hemisphere. Where the archenteron contactsthe wall, a mouth is eventually formed. The blastopore marks the position of the anus.