After fertilisation the zygote undergoes repeated cleavage divisions that split the cell mass without net growth, so cell count doubles on a roughly fixed cycle. Once a critical mass is reached the ball of cells (morula) hollows out into a fluid-filled blastocyst. During gastrulation cells sort by fate into three concentric germ layers, and during neurulation the embryo elongates along a head-to-tail body axis.
N(t) = min( N_max, 2^floor( 24t / T_c ) )
t = developmental time, days
T_c = cell-cycle length, hours (10-30 h)
N_max = 128 (render cap)
Shape(t) blends across keyframes:
0-5 d solid ball (cleavage -> morula)
5-9 d ball -> hollow shell (cavitation)
9-14 d hollow -> 3 germ-layer shells (gastrulation)
14-21 d shells -> elongated tube (axis formation)
- Developmental time: scrubs through the 21-day timeline; recomputes cell number and morphology.
- 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.
- Colour by germ layer: toggles between a single lineage colour and the ectoderm / mesoderm / endoderm colour code (outer, middle, inner) once layers exist.
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.