Comparative Review: The Phylotypic Stage of Vertebrates, Common versus Distinct Features, and Aspects of Evolution

Abstract

Carl Ernst von Baer (1791–1876) is considered the pioneer and father of comparative embryology, von Baer discovered the notochord, and the mammalian and human egg. Comparing many embryos, he observed that early vertebrate embryos exhibit common features: “More general features that are common to all the members of a group of animals are, in the embryo, developed earlier than the more special features which distinguish the various members of the group.” This statement is known as von Baer’s law. Thus, the features that characterize all vertebrates, such as the dorsal brain and the spinal cord, the notochord, the somites, and the aortic arches, are seen earlier in development than features distinguishing between the various classes (limbs in tetrapods, feathers in birds, and hair in mammals). Thus, at an early stage following gastrulation and neurulation, embryos offish, amphibians, reptiles, birds, and mammals all look alike. As development progresses, their developmental paths diverge and embryos become recognizable as members of their class, their order, their family, and finally their species (Fig. 4-1).