One of the most important concepts to the field of embryology is homology. The term homology is often used to imply similarity by descent. The statement that a bat's wing is homologous to a human's arm implies that these two structures originated from a common ancestral forelimb. Because we cannot identify the exact ancestor to test this statement, embryological evidence is often used as the criterion for identifying structures as homologous. A bat's wing and a human's arm are homologous because developmentally they arise from the same tissues, are controlled by the same developmental regulatory genes, and produce anatomically similar structures in the same developmental order.
The concept of testing homology through developmental information can become obfuscated. For example, the eye of an insect and the eye of a vertebrate perform the same function, but they are not generally accepted as homologous structures, because they undergo distinct developmental pathways and are anatomically very different. Paradoxically, the vertebrate eye and the insect eye are both controlled by homologous developmental regulatory genes, Pax-6. It is now accepted that homologous genes can act on nonhomologous structures, and in turn the development of homologous structures can be controlled by nonhomologous genes. This new hierarchical view of homology reconciles some discrepancies that biologists encounter when using embryological data as a means of identifying homology.
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