 Aristotle was a keen observer of many things, including embryos. Looking at chicks, for example, and drawing on his interpretations developed earlier for the physical sciences, he saw material, final, formal, and efficient causes at work in the generation of individual organisms. The early egg was not formed; it did not already contain a little chick (or whatever the species). Instead, an individual animal only gradually acquired a heart that began beating; likewise for the other parts that make it a chick. The material may be there from the beginning, but the formal cause only gradually plays out along with the efficient cause of embryonic development.