1. The early events of sperm entry in Saccoglossus and Hydroides are described and examined in relation to present knowledge of the acrosome reaction and of egg membrane lysins. In Saccoglossus and several other species these events occur in two phases. First. The acrosome filament of the spermatozoön spans the egg membrane barriers, reaches the reactive egg protoplasm, and causes the egg to begin its fertilization reaction. Second. The filament and its connected sperm head move through the egg membrane barriers and enter the egg proper. The first phase is completed in a matter of seconds but the second phase usually requires several minutes. 2. The peripheral areas of the eggs of the two species differ as seen in sections. In Hydroides, but not in Saccoglossus, the vitelline membrane is bounded by a distinct outer border layer of small concentrically differentiated bodies and penetrated by microvilli from the egg. 3. The acrosome filament, seen in the living condition as a delicate thread in Hydroides and as an exceedingly tenuous thread in Saccoglossus, appears to be tubular in both species when seen in electron micrographs of thin sections. 4. The acrosomal region of Hydroides appears to consist of two components—a peripheral one, which may collapse during the acrosome reaction, and a central one related to the acrosome filament. 5. Deliberately induced polyspermic material was used to increase the probability of finding examples of sperm penetration in thin sections. 6. As seen in sections, areas of low electron density, interpreted as spaces or pits from which the material of the membrane is absent, surround the attached or penetrating spermatozoa. (a) In Hydroides the spaces vary greatly in many characteristics including shape, position in the membrane, and size with relation to the enclosed sperm head. In one specimen a portion of the membrane is missing from border to border; no spermatozoön is seen but immediately beneath the space is the apex of a fertilization cone. (b) In every case in which a determination could be made, the spermatozoön in the membrane has undergone its acrosome reaction. (c) In Saccoglossus some pits are found with which several spermatozoa are associated. Generally, where the spermatozoa are more numerous the pit is larger. (d) Pits similar to those seen in Saccoglossus sections are observed in living eggs. They remain in Membrane I after sperm entry. (e) From the above and other considerations it is suggested that the pits and spaces are formed by local action of a lysin or lysins emanating from the individual spermatozoön at the site of sperm entry. 7. It is considered that the suggested lysin would participate in sperm entry by eroding the membrane barrier in the vicinity of the sperm head, thus permitting the sperm head to pass through the membrane. Since the acrosome filament much earlier stimulates the egg's initial fertilization response, this lysin would facilitate the second phase of the early events of sperm entry.