Comparative embryology is the study of how different types of organisms compare to each other during their fetal stages. Scientists have used comparative embryology to study and gather evidence of evolution. Karl Ernst von Baer, a Russian scientist, made observations on embryos of different species and came up with four principles of comparative embryology:

1. General features of an embryo appear before specialized features.

2. General features of an embryo develop before the most specific features appear.

3. Every embryo, instead of passing through the other forms, rather becomes separated from them.

4. Embryos of a higher animal only resemble the embryos, not the adults, of more primitive animals.
 

First and Second Principles

Von Baer’s first two principles explain the development of animal embryos and contradict the notion of preformation, where a fully formed organism exists and grows from an embryo. The first principle states that general characteristics appear earlier than specialized ones, and so a general, homologous structure would exist in the early embryos of groups of similar species. This would infer these organisms had a common ancestor, and different species “branched off” from it.

Example
At similar stages of development, the embryos of birds, reptiles, humans and pigs have tails