Blaschko's lines

This picture shows Blaschko's lines, which result from the migration of cells in an embryo.

Although they are normally invisible, they become visible in some people with skin disorders, as in this photo:

I learned this from Blaschko's Lines on Pharyngula, which has some more pictures and a very clear explanation of how this phenomenon works. The top image is from Dynamics of Cancer by Steven A. Frank; the bottom image is from Wikimedia Commons.