Many cells migrate as one group in a single direction. Cells at the front lead, and the group coordinates so cells do not pile up (contact inhibition of locomotion). Neural crest cells do this to build the face.
Those migrating cells built the facial prominences. Now the tissue itself moves. The upper-lip prominences grow toward the midline, meet, and their surface seam dissolves so they join into one continuous lip.
A cleft lip is a fusion that did not finish. Read each cause as if this, then this. Tap a card to see it.
The whole arc: the same migration machinery that builds the face also drives cancer spread; those cells assemble tissue that then moves and fuses; and when a step fails, the lip does not close. Schematic, grounded in Notebooks 3, 6, and 7. Sits alongside the palate-fusion and cell-mechanics tools.