Time-specificity has been well established in numerous experiments in which a teratogenic agent has been shown to cause different malformations when applied at different times in development. These time-specific effects are related to definite stages or events in embryonic development which might be regarded as periods of special susceptibility. Excessive doses tend to obscure time-specificity by causing teratogenic effects at times other than during periods of special susceptibility.Recent experiments have indicated that, irrespective of time, many teratogenic agents seem to produce distinctive patterns of anomalies which differ qualitatively and quantitatively from those caused by other agents. The association of a particular group of malformations with a particular agent may be termed agent-specificity. Agent-specificity is not always readily apparent for reasons such as: a) elimination of certain types of abnormalities by fetal death of affected individuals, b) fluctuations in the pattern resulting from variations in dosage of the agent, c) variable reactions to the same agent in different species or even different strains, and d) varying interests and methods of study on the part of the investigator. When due allowance is made for such variables, distinctive patterns of malformations can often be associated with particular teratogenic agents.Although mechanisms of agent-specificity were not discussed in detail, it was suggested that each agent acts by interfering with a particular metabolic process in a specific way in the differentiating and growing embryo. Such action can be localized, generalized or selectively distributed, depending on the distribution within the embryo of the process concerned.
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