Abstract
Following irradiation of fertilized eggs of the sea urchin Strongylo-centrotus purpuratus with known dosages of monochromatic u.-v. no acceleration of cleavage was observed, but retardation was obvious when the dosage was large enough. 2804A was most effective in retarding, 2537, 2654 and 3025A next most effective, 3130A next, while 2660A even at the highest dosages produced barely perceptible effects. Lack of correlation of the wave-length of maximum extinction of light by the eggs with the wave-length of maximal efficiency might be taken as an indication that the destructive effect is mediated by some substance especially sensitive to 2804A.