1. Evidence is given to indicate that the formation of insemination filaments is the normal procedure of fecundation in the starfish egg. These filaments extend from the egg's surface to the spermatozoa lying on the outer borders of the jelly surrounding the egg. 2. The spermtozoön on the end of an insemination filament moves to the egg through the jelly by no apparent motive power of its own. This movement is accompanied by a progressive shortening and thickening of the filament. 3. The fertilization membrane begins to rise off the cone by the time the spermatozoön has migrated about halfway through the jelly. The elevation of the membrane spreads from this region. 4. The filament is a peculiar adaptation to the presence of the relatively dense jelly surrounding the egg and to the inability of the blunt-headed spermatozoa to reach the egg. In the absence of the jelly only an ovoid entrance cone develops to receive the spermatozoön. 5. Polyspermy can be prevented by the breaking loose of supernumerary insemination filaments from their attached spermatozoa. The discarded spermatozoa remain motionless in the jelly while the filaments are completely withdrawn. 6. There is a definite relation between the time that two or more spermatozoa become attached to insemination filaments and the success of one or all to enter the egg. This bears no relation to the distance of their places of attachment on the surface of the egg but to the time when the filaments begin to retract. 7. The original cortex is the only part of the starfish egg which responds to insemination. Endoplasmic exovates do not become inseminated.