1. The egg-extracts as well as the egg-secretions of Arbacia and Asterias, in addition to characteristic effects on spermatozoa, are permeability increasing agents, and initiate development. For the type of initiation here dealt with I have chosen the term auto-parthenogenesis. 2. Auto-parthenogenesis is blocked in Arbacia by washing the eggs in sea-water. 3. A group "Purple X" which may be split off from the sperm of Arbacia, and which also appears in boiled egg-secretion, blocks parthenogenesis in low concentrations and fertilization in high concentrations. 4. Prevention of parthenogenesis by washing can hardly be the result of an absence of fertilizin; prevention by Purple X may be the result of occupancy of the egg-receptors. 5. It is not unlikely that all cases of parthenogenesis are in reality instances of auto-parthenogenesis. 6. With certain minor modifications it appears possible to construct an hypothesis of fertilization in which the basal facts of the surface-alteration theory, as well as the facts emphasized by the fertilizin theory may, without violence to our knowledge, find a place. From the standpoint here adopted, there is no antagonism between the two theories which emphasize different groups of facts associated with the process of fertilization. These facts cannot be in conflict with one another.