A ring of ~56 cells (an epithelial blastula wall in cross-section) is seeded with three fates by arc position around the vegetal pole: a small endoderm cap, a flanking mesoderm band, and the ectoderm covering the rest of the sphere — roughly the real proportions in an early gastrula.
Every pair of cells feels short-range repulsion (no overlap) plus differential adhesion: same-fate contacts are far stickier than cross-fate ones (endoderm > mesoderm > ectoderm). Steinberg's differential adhesion hypothesis predicts that alone is enough to sort a mixed tissue so the most cohesive type ends up engulfed at the centre — the same physics that positions real germ layers.
Pressing Begin invagination adds an active inward migration force to the endoderm/mesoderm cells, starting at the vegetal pole and sweeping outward — a simplified stand-in for apical constriction at the blastopore lip. Ectoderm cells are only held to the outer shell and spread to close the gaps left behind (epiboly). Cell motion is over-damped (velocity ∝ force, no inertia), the correct regime for crawling cells at low Reynolds number.
- Migration speed — overall cell motility (mobility multiplier on every force).
- Cell adhesion — strength of the same-fate attraction driving sorting.
- Invagination force — strength and speed of the active inward pull once triggered.