The existence of a period of latent but stable polarity, i.e. a period in which the polarity axis has been irreversibly established but no morphological asymmetry can be detected, was studied in germinating populations of zygotes ofPelvetia fastigiata. We found that the time course of the loss of sensitivity to a single polarity-axis-determining light stimulus coincided with the time course of germination (rhizoid outgrowth), up to the germination of about one-third the population, showing that the cells remained responsive to the light stimulus until or almost until the appearance of visible asymmetry. In the later part of the germination period, some of the zygotes may have lost their light sensitivity somewhat before rhizoid outgrowth, suggesting that at least some of the ungerminated cells may at this time possess, for a brief period, a latent but stable polarity axis.
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