System for generating spots proposed by Turing:

$\bigtriangleup a_{ij} = s(16 - a_{ij}b_{ij}) + D_a(a_{i+1 j} + a_{i-1 j} + a_{i j+1} + a_{i j-1} - 4a_{ij})$
$\bigtriangleup b_{ij} = s(a_{ij}b_{ij} - b_{ij} - \beta _{ij}) + D_b(b_{i+1 j} + b_{i-1 j} + b_{i j+1} + b_{i j-1} - 4b_{ij})$

where s is the source density constant.
Small values of s result in a slower reaction relative to diffusion which gives us larger spots.
Larger values give us smaller spots.
Beta is the parameter which is used to introduce the small amount of variation into the system making it unstable.
The more beta varies, the more irregular in shape the spots become.