Abstract
1. When unfertilized eggs of Urechis caupo are exposed to ultraviolet light for different lengths of time, then fertilized, polar body extrusion and first cleavage are suppressed. In addition to the suppression of these stages radiation causes an increase in the reaction rate of the egg which bears a direct relation to the length of exposure. 2. Possible factors in the suppression of these stages in maturation and first cleavage are the alteration or probable injury of the egg cortex and accompanying changes in its chemical and physical properties, and viscosity changes in the egg endoplasm which inhibit internal phenomena.

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