STUDIES ON THE ACROSOME. II. ACROSOME REACTION IN STARFISH SPERMATOZOA

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Abstract
1. When the spermatozoa of three starfish species, Asterina pectinifera, Asterias amurensis and Astropecten scoparius, are suspended in a dilute (0.1-1.0%) solution of egg albumin in sea water and mixed with homologous egg-water, three effects can be noted: a. many of the spermatozoa are agglutinated by their heads, forming permanent clusters; b. from the center of the acrosome of each agglutinated spermatozoan there has been extended a long (ca. 25 µ), very slender, straight filament which possesses considerable rigidity; c. in all the spermatozoa which have so reacted, there is a rearrangement of the principal parts, so that the middle piece is less tightly apposed to the head and the tail appears to be inserted laterally, between the head and middle piece. 2. A critical examination of the widely accepted explanation of starfish fertilization—that the effective spermatozoan is drawn through the jelly layer to the egg surface by a filament originating in an "attraction cone"—shows that this depends mainly upon an assumption made by Fol, which various considerations show to be of doubtful validity. In the light of the fact that starfish spermatozoa produce a similar filament from their acrosomes, on contact with dissolved jelly substance, it is proposed that this sperm acrosome reaction is the source of the filament which, extending through the jelly, stimulates the egg cortex. Following this stimulation, the egg cytoplasm draws in the filament with the attached spermatozoan and simultaneously forms a fertilization cone beneath the vitelline membrane, which separates as the fertilization membrane. This sequence of events is the same as that constituting the fertilization reaction in other echinoderms.