A self-organising chemical system that spontaneously generates spatial patterns of concentration of some of their chemical components.

Must have two chemicals that diffuse and react with one another.

The activator stimulates the production of an inhibitor.

The inhibitor diffuses and inhibits the autocatalysis of the activator (the activator stimulating itself). Therefore only a small region of the activator is concentrated enough to show it's affects.

The zebrafish epidermal pigmentation patterns.

Computers have been able to use the pigmentation patterns in young fish and predict the pattern formation in adults.

What phyla show repeating units of segmentation, what does this mean?

What are some minor differences between phyla?

Chordates, annelids and arthropods. It is highly conserved as it is an importaint mechanism in development. 

Some species produce the same number of fragments, some produce variable fragments.