Final answer:
Critical period in development where structures are sensitive to external factors.
Explanation:
A critical period is a specific time during development when certain systems, structures, or abilities are most susceptible to external factors, such as teratogens.
During these critical periods, exposure to substances can lead to severe damage or abnormalities in the developing fetus, like the example of arms and ears forming around 6 weeks after conception.
Genetic makeup, dose-response relation, and the type of teratogen can all influence the impact during these critical periods.
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