In view of the great number of teratogenic factors known and the vast array of congenital defects, disorders and syndromes, it would probably be a waste of time to search for unifying mechanisms and principles in abnormal development. Instead, therefore, I shall describe a selection of teratogens and their consequences, and try to arrange them in a certain hierarchy based on a simplified model of how they act.

The assumption underlying the model (Figs. 1 and 2) is that the result of a teratogenic insult is determined by its site of action and the stage of development of the target organ. This is supposed to hold for all congenital defects, whether due to genes or caused by exogenous agents. In genetic defects the scheme indicates the site and stage of development at which the mutant gene is expressed; in nongenetic defects the site and stage refer to exposure to an exogenous teratogen.

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