Officially: "The line of union of any two continguous, bilaterally symmetrical structures." In the context of the male genitalia, the Raphe runs down the underneath of the penis in one continous line from the tip of the foreskin, down the shaft, down the center of the scrotum and between the legs to the anus.
So why do men have this seam between their legs? Remember that, for the first seven or eight weeks of a pregnancy, the foetus is physically gender-indeterminate. In terms of genes, the commitment to being a boy has existed from conception. That’s due to mum’s egg having been fertilised by a "Y" sperm rather than an "X" sperm, which would have produced a girl.
Only when production of the hormone testosterone kicks in does the developing baby abandon the default female form and start becoming a male. A bit like a boy’s nipples, the raphe is residual evidence of female physical characteristics that existed early in the pregnancy. It is the seam closing what would have become the vulval cleft in the absence of the "Y" gene and the testosterone.
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