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It seems that human embryos develop mammary tissue before they bother to check on whether they're going to be male or female and start modifying the basic plan with surges of this or that hormone. After only a few weeks, milk ridges form -- two stripes of tissue that start in the armpits, curve out over the chest, go straight down the stomach and then veer in toward the groin, ending somewhere high on each thigh. Later the milk ridges regress to some extent, usually leaving us with just two nipples.
from salon.com
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