Most of the time, an embryo growing in the womb fully develops into either a male or a female with all the appropriate body parts and, many scientists believe, a mindset programmed according to its gender. What determines its gender—in most cases—are its sex chromosomes: two X chromosomes in the nucleus of its original egg cell and it will become a female; a Y and an X chromosome and it will become a male. But exactly what happens in the womb to make a boy or a girl? This interactive feature illustrates the astonishing changes that occur during the first 16 weeks of development.
Flash is a plug-in that allows for increased interactivity. If you can see
the animated boxes at left, the plugin is already installed. If you do not see
the boxes, you can install the Flash plugin, or select this feature's
non-Flash version.
