After fertilisation the zygote undergoes repeated cleavage divisions that split the cell mass without net growth, so the cell count doubles on a roughly fixed cycle:
n(t) = min(N_MAX, 2 ^ floor(hours(t) / cycleHours))
Once a critical mass is reached the ball of cells (morula) hollows out into a fluid-filled blastocyst ring. During gastrulation cells sort by fate into three concentric germ-layer rings (ectoderm outer, mesoderm middle, endoderm inner), and during neurulation the embryo elongates along a head-to-tail body axis, seen here as a stretched capsule with the same three layers running lengthwise.
- Developmental time: drag to jump anywhere in the 0–21 day timeline; dragging pauses playback.
- Cell cycle length: how long each doubling takes; shorter cycles reach high cell counts sooner.
- Play / Reset: auto-advances or restarts the timeline; playback speed sets days simulated per second.
- Germ layers: toggles between a single lineage colour and the ectoderm / mesoderm / endoderm colour code once layers exist (day ≥ 9).
This three-layer, axis-forming body plan is the same template that later gives rise to skin and nerves (ectoderm), muscle and bone (mesoderm), and gut lining (endoderm) in every vertebrate embryo.