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In an experiment, the right forepaw of a developing mouse embryo is examined. The tissue and cellular structure are microscopically analyzed with histology at several developmental time points.

The student identifies a gradient of the protein “sonic hedgehog” that originates and is highest on the posterior aspect of the limb bud and lowest on the anterior aspect. Perturbation of this gradient is found to disrupt the formation of the digits in the paw.

This observation describes the phenomenon referred to as




apical dominance.


apical polarization.


morphogen gradient.

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