Fine structure of gametes of <i>Nereis limbata</i> (Annelida) before and after interaction

Abstract

Abstract The acrosome of the Nereis limbata spermatozoon is a vesicular body which is shaped in the form of a double‐walled hollow cone. As such it encloses an axial chamber housin the anterior part of a rod‐like structure which, posteriorly, fits into a deep invagination in the nucleus. When the acrosome undergoes reaction preparatory to and during penetration of the egg envelope, it opens anteriorly, discharging its contents, and the axial chamber, together with its contained rod, is expanded and portruded as the acrosome filament. The chorion of the egg is about 0.7 μ thick, and is traversed completely by microvilli which project at intervals. Numerous broad canals extend about three‐quarters through the chorion. It is by one of these canals that the acrosome filament passes in the initial stages of sperm penetration. The cortex of the egg has a wide zone of densely packed alveoli which contain moderately electron‐dense material, the jelly precursor. Cortical reaction to sperm entry involves fusion between adjacent alveoli and opening up of alveoli onto the cytoplasmic surface of the egg. Released jelly precursor, evidently becoming hydrated, flows into the perivitelline space and thence through the pores in the chorion that accommodate the microvilli (flowing around the microvilli which persist), so as to form a thick layer about the egg.