Embryonic Development

If you look at a human embryo next to the embryo of a reptile, or that of a bird, you will see striking similarities between the groups. But why is this? If each goes on to take a completely different form, then why would each start from nearly the same point? These embryonic forms have remained the same since the branching of these species from a common ancestor some millions of years ago. I suppose evolutionarily, these embryonic forms provided everything needed to grow into completely separate body types, and any change wouldn’t have been beneficial enough to take place. This is just one of the many vestiges left from our evolutionary past.