No difference is seen in the embryonic stage of different animals. But the difference is seen in the transformed state. The embryos of fish, frog, chicken, cow, goat, monkey, human etc. are almost exactly the same. The embryos of monkeys, gorillas, humans etc. are almost identical.
According to the scientist Von Baer, the set of general characteristics arose before the emergence of special characteristics. From general characteristics eventually special characteristics emerge. In the embryonic stage each animal quickly sheds the structure of other animals. A baby animal looks like a fetus.
Animals whose embryos are similar derive from the same ancestor. Due to variation their structural differences have arisen. As a result, different species have been created.