Membrane formation took place in P. lividus 30 sec. after insemination. Perivisceral fluid inhibited membrane formation; eggs of P. lividus allowed to stand in normal sea water produced a sperm agglutinating substance. In E. microtuberulatus membrane formation took place in from 45-100 sec. When membrane formation was delayed about \ of the eggs did not form membranes. The membrane was completely off the egg 150 sec. after insemination. Vesicles are formed in the perivisceral space; presence of perivisceral fluid strongly inhibited fertilization. Uninseminated E. microtuberulatus eggs produced a strong sperm agglutinin. Agglutination ran parallel to fertilization capacity.