Spermatozoa have been observed with the electron microscope at various stages of their approach to and penetration of the rabbit ovum. No significant change in fine structure is observed in uterine sperm, or in many of the sperm at the periphery of the granulosa investment around the ovum. By contrast, a majority of sperm lying between the granulosa cells or on the surface of the zona pellucida display various stages of the “acrosome reaction”; this involves fusion and vesicle formation between the plasma membrane and the outer membrane of the acrosome. Loss of these vesicular elements, and content of the acrosome cap, takes place before sperm begin to penetrate the substance of the zona. The constricted posterior “equatorial” segment of the acrosome cap does not take part in the acrosome reaction and remains with its content intact during penetration of the zona; neither does the content of the apical sub‐acrosomal region (perforatorium) or post‐acrosomal region appear to change in traversing the zona. The hypothetical zona lysin is thus presumed to be closely associated with the persistent inner membrane of the acrosome, which now becomes the limiting membrane around the anterior part of the sperm nucleus.

No “penetration filament” has been observed, but sperm within the zona pellucida of ageing eggs are often preceded by a straight or curved fissure in the substance of the zona. The possible nature of capacitation and its relation to the acrosome reaction and to the process of penetration, are discussed briefly.