BBC - Countdown to Life - Episode 2
(presented by Michael Mosley)http://www.telegraph.co.uk/news/science/science-news/11874983/Dr-Michael-Mosely-growing-a-penis-at-12.html
At the moment of conception, you were, of course, male or female based on your genes. In males, the Y-chromosome doesn't kick in right away. In fact, the first 5 to 6 weeks of embryonic development are attributed to the X-chromosome alone and females grow from embryo to fully developed through the influence of only the X-chromosome. So what does the Y-chromosome do?
Once activated, one of the genes on Y-chromosome, known as the SRY-gene, has two functions:
♦ to inhibit certain features of the X-chromosome, and
♦ to impose through dominance, the physiological traits that a male must have.
The Ovaries descend and become the Testes, and the Labia fuses to create the Scrotum. If you are male, take a look. You'll likely notice a line or ridge of tissue on the scrotum which is called the Scrotal Raphe. The skin of the scrotum and penis are zipped up like a zipper as the layers of cells develop in the embryo.
GIF via ASAP Sciencehttps://www.youtube.com/watch?v=z1Kdoja3hlk