Sperm or Egg - a simple switch?

Germ cells are identical in males and females. So what is the mechanism that leads germ cells in males to become sperm, and in females to become eggs? In a report "One big biology question solved" on a recent paper that will be published in Science, we see that the key factor seems to be the timing of when germ cell meiosis begins - before birth in females, after birth in males. And the trigger for meiosis is retinoic acid - a derivative of vitamin A. Male embryos show expression of a particular enzyme that obliterates retinoic acid, thereby suppressing meiosis until after birth.