Ok so gen2 was wrong about when the sex hormones are produced but the rest of what he said was right. All animals begin their development essentially with a basic female form. The female form is the template. Yes you are genetically male or female from the moment of conception, but those genes that code for either body type are not expressed until later on. So you start off by developing a basic template (including nipples) and then when the sex hormones are produced later, the body starts to differentiate. And it gets more interesting: all tetrapods (all four limbed animals so thats amphibians, reptiles, birds, fish, mammals etc) share the same basic structure as us because we all evolved from the same ancestor. So if you look at a bird or frog embryo in early development, they will look almost identical. We have the same bone structure as all these animals, except the sizes, proportions and even positions vary according to, for example, whether they have evolved to fly, walk or swim!