Blaschko's Lines (Wikipedia)
* "Blaschko's lines, also called the Lines of Blaschko, are skin lines invisible under normal conditions. They become apparent when some diseases of the skin or mucosa manifest themselves according to these patterns. They follow a "V" shape over the back, "S" shaped whorls over the chest, stomach, and sides, and wavy shapes on the head."
Blaschko's Lines (Pharyngula)
* "One of the subjects developmental biologists are interested in is the development of pattern. There are the obvious externally visible patterns — the stripes of a zebra, leopard spots, the ordered ranks of your teeth, etc., etc., etc. — and in fact, just about everything about most multicellular organisms is about pattern. Without it, you'd be an amorphous blob."
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