Sperm fertilization reactions of Limulus polyphemus were examined by scanning electron and/or light microscopy. The following were considered: sperm motility, attachment of sperm to egg, acrosome reaction, and penetration of the acrosomal filament. The spermatozoa after semination are non-motile and become active only in close proximity to a defined region surrounding the egg. Egg materials diffusing into this region induce sperm motility and stimulate large numbers of spermatozoa to move towards the egg surface. Each sperm initially attaches by the apical tip and undergoes the acrosome reaction which causes a more permanent secondary attachment by the adhesion of acrosomal contents to the egg surface. The acrosome reaction also initiates the penetration of the acrosomal filament through the egg envelope, an event occurring in 70–80% of the attached spermatozoa (about 10(6). Shortly after this penetration, a secondary reaction occurs which involves a spiralling of the flagellum and an incorporation into the sperm body of the flagellar fibrous components, which then become closely apposed to the sperm nucleus. These sperm fertilization reactions were performed or initiated with 0-34 M CaCl2 in whole eggs, egg sections, excised egg envelopes and/or the outer basement lamina of the egg envelope. The Limulus fertilization system is very valuable since sperm reactions can be examined biochemically, which may lead to a better understanding of the chemical mechanisms involved in sperm-egg interactions in all animal species.

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