1. Experimental study of pathological polyspermy is discussed as a method for ascertaining the dynamic characteristics of the fertilization reaction and especially of monospermy. 2. A general procedure is described for treatment of unfertilized eggs, standardization of sperm suspensions, and determination of percentages of polyspermic fertilizations. 3. Comparable overinsemination of the eggs of Arbacia punclulata, Asterias forbesii, Cumingia tellinoides and Cerebratulus lacteus shows sharply decreasing resistance, in the listed order, to polyspermy caused by excess sperm. 4. Eggs of Asterias forbesii become susceptible to polyspermic fertilization after standing in sea water less than two hours. Eggs of Arbacia show little diminution in their normal resistance to polyspermy until after sixteen hours in sea water. 5. Unfertilized Arbacia eggs were exposed to dilute solutions of chioral hydrate, cocaine hydrochloride, nicotine, morphine sulphate, strychnine sulphate and quinine sulphate. The effectiveness of these substances in inducing polyspermy is discussed and compared. The results for Arbacia punctulata are found to be qualitatively similar to those obtained by O. and R. Hertwig (1887) with Strongylocentrotus lividus. 6. Qualitative experiments testing the effect of fat solvents on unfertilized Arbacia eggs showed a fundamental similarity between the effects of anæsthetic concentrations of ethyl alcohol, ether, chloroform and ethyl urethane. These substances have a narrow range of high effectiveness near the point at which the cell cytolyzes. 7. Studies of the effect of the four common cations of sea water in causing polyspermic fertilization of Arbacia eggs showed that, when unfertilized eggs are washed in isotonic solutions of the chlorides and inseminated on return to sea water, the ions are effective in the order Na > K > Ca > Mg. On the other hand, insemination in isotonic chloride-sea water mixtures gave the result, Mg >> Na, K, Ca, with the maximum percentage for Mg = 71, for Na, K and Ca = ca. 5. 8. Exposure of unfertilized Arbacia eggs to acidified sea water over the pH range from 5.8 to 8.2 gave a maximum of polyspermy at pH 7.0. This result agrees with the work of Smith and Clowes (1924).