Biogenetic law states that ontogeny (development of embryo) is the recapitulation of phylogeny (ancestral sequence). It was given by E. Haeckel. For example, both chick and human embryos go through a stage where they have slits and arches in their necks that are identical to the gill slits and gill arches of fish. This observation supports the idea that chicks and humans share a common ancestor with fish. Related Theory Totipotent cells can give rise to both the placenta and the embryo. As the embryo grows, pluripotent cells develop into specialized, multipotent stem cells. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize. The inner cell mass, the source of embryonic stem cells. becomes pluripotent. (Ref. to Fig. 25.1) 