Abstract
Identical anomalies produced by such different causes as aneuploidy, gene mutation, teratogenic chemicals, and certain surgical procedures show that embryonic primordia respond as units in the production of anomalies of anatomical structure. Hence, they must also act as units during normal ontogeny. The presence of identical malformations in different mammalian species identifies developmental and anatomical homology by virtue of descent from a common ancestor. These dys- and orthomorphogenetically reactive units are the equivalents of the classical experimental embryologist's epimorphic fields, which are those units of the embryo in which the development of complex structures appropriate to the species is determined and controlled in a spatially coordinated, temporally synchronous, and epimorphically hierarchical manner that expresses both species-nonspecific (that is, phylogenetic) and species-specific genetically coded developmental information. Thus, there is some merit in taking a long view of development.
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