General laws of development. The biogenetic law. The law of embryonic similarity
K. Baer (1828) found that embryos of animals of one type are very similar in the early stages of development — the law of embryonic similarity. In later stages first the features of the large group (class) appear, then of the family, genus and species; that is, embryos pass from similarity to difference — embryonic divergence. For example, embryos of fish, lizards, chicken and humans at first all have a notochord and pharyngeal arches; only later do their differences show. The German scientists F. Müller and E. Haeckel put forward the biogenetic law: ontogenesis is a short and fast repetition of phylogeny (phylogeny is the historical development of a species). Modern science sees this not as an exact repetition but as traces of common descent: the embryo keeps features of the ancestor’s embryo, not of the adult ancestor. Embryonic similarity shows that vertebrates came from a common ancestor.
“Embryo series”. Compare pictures of fish, salamander, tortoise, chicken, rabbit and human embryos at 3 stages from the textbook or a reliable source. At which stage are they similar and at which different? Write in your notebook.