Ernst Haeckel (1834-1919), a German biologist and naturalist, disagreed with von Baer and further expanded on the Meckel-Serres law with his biogenetic law. What does the biogenic law claim?

Simply it states that the progress of development of an organism is a recapitulation of the organism's evolutionary history, and thus its progeny.

Explanation:

Hence based on this the stages of developments of an embryo are similar to developments stages seen in adult forms. Therefore it is can be stated as recapitulation of phylogeny by ontogeny. Thus the developmental stages observed in an embryo, are summary oft he adult forms of its progeny. Therefore, studying this developmental stages in an embryo is in effects studying the diversification of life on earth, and its history.

In summary the developmental stages of the embryo is a good method to detect the similarities between taxa, and findings from these can provide evidences that their is a common ancestry for all species of organism on earth.