Abstract
Scanning electron microscopy (SEM) of specimens freeze-dried after fixation in Parducz fixative and ultrathin sections of the sea urchin,Strongylocentrotus purpuratusshow that the egg is covered by many papillae about 0.25μ in diameter and 0.5μ long (Fig. 1a). When the vitelline layer lifts away from the surface of the egg at the time of fertilization it has many uniformly spaced protrusions that persist as prominent and consistent structural features of the fertilization membrane, which forms when material from ruptured cortical granules is added to the inner surface of the raised vitelline membrane. Dimensions of the protrusions, their spacing on the membrane, and their projection in a direction outward from the egg (Fig. 1b) suggests that they originate when the vitelline layer lifts away from the egg surface in the form of a somewhat distorted and expanded replica of the papillae-bearing surface of the unfertilized egg.
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