Snakes and lizards have a double penis. Why not the human species and other mammals?
Because, at the embryonic stage, the penis of snakes grows from two different cell populations: those which, in mammals, will form the tail and those from which emerge, in squamates (scaly reptiles), the two hind limbs. This is the explanation that researchers have given to this beautiful enigma, after having examined numerous embryos of mice, chickens, snakes or lizards.
Snakes: leg stumps?
By coloring, in these embryos, the cell groups destined to develop the legs and the tail, they observed that, in snakes and lizards, the same group of cells gives birth, on each side of the abdomen, to the two external genital organs and to the two hind limbs: the legs in lizards, and the stumps of legs that will never develop in certain snakes like pythons.
On the other hand, in chickens and mice, which are closer to human beings, the cells that form the genitalia are located at the end of the embryo, where the tail eventually grows!
Science & life