This is a genuine 2D reaction-diffusion simulation of the fertilisation calcium wave, not a scripted animation. Free calcium [Ca²⁺] = u(x,y,t) and a recovery variable v(x,y,t) obey an excitable-medium PDE (the Barkley model, the standard textbook system for calcium-induced calcium release and other trigger waves) integrated on a grid across the egg's cross-section:
∂u/∂t = D∇²u + (1/ε)·u(1−u)(u − (v+b)/a)
∂v/∂t = u − v
Clicking the egg deposits a local super-threshold patch of u — exactly like the IP₃-triggered Ca²⁺ release at a real sperm fusion pore — and the diffusion term D∇²u carries it outward from that exact point at a finite, computed speed (no scripted radius, no timer: the front position each frame is read directly off the live u/v grid). Every grid cell that crosses the firing threshold flips its cortical-granule "block" flag permanently — that flag is the actual block to polyspermy.
A second sperm's fate is decided by sampling the block flag at its own arrival coordinates and the current simulation clock — nothing is precomputed. Click early or far from the first fusion point and the wave hasn't arrived yet, so it fuses (polyspermy). Click late or nearby and the same spot is already blocked. Run the built-in "Cortex covered" readout and you'll see success only ever shrinks as time passes, because it is measured off the same expanding front, not assumed.
- Sperm arrival rate — how often an ambient sperm starts swimming in.
- Swim speed — how fast ambient sperm cross the perivitelline space.
- Calcium wave speed — how much simulated PDE time elapses per real second (a genuine speed control on D·t, not a cosmetic multiplier).
- Polyspermy block toggle — turn it off and every sperm that reaches the membrane fuses, extra chromosome sets and all.
Clinically, this is why a failed or delayed block (seen in some aged oocytes and a fraction of IVF fertilisations) produces triploid embryos that fail to implant or miscarry early — one of the reasons IVF labs count pronuclei carefully the morning after insemination.