The eggs of Dendraster excentricus, Strongylocentrotus purpuratus, Patiria (As-terina) miniata, and Vrechis caupo were studied. Study of the vitellus of the unfertilized egg micrurgically indicates the presence of a surface layer (vitelline membrane) in the 4 forms. For Dendraster and Strongylocentrotus it is a soft, flexible, gelatinous substance. By contrast, eggs of Patiria and Urechis have a definite tough membrane. In every case the fertilization membrane (F.M.) is developed from the superficial layer of the unfertilized egg. In Dendraster and Strongylocentrotus this takes place by a radical alteration of the surface layer. In Patiria and Urechis the process is essentially a separation of the tough outer layer (egg membrane) and its transformation into the F.M. that the superficial layer may be considered a precursor to the F.M. is splendidly shown in eggs of the sand dollar, where its development is greatly modified. Changes in consistency and properties of the F.M. may be correlated with definite stages in its development. They offer further substantial proof that the vitelline membrane and the F.M. are not identical. The difference of the F.M. from any pre-existing structure on the unfertilized egg indicates that it is actually a structure resulting from activation of the egg. It is concluded that eggs fall into 2 general groups in which the F.M. is developed (1) in large part from a pliable superficial layer of substance, and (2) by the separation from the egg of a tough preexisting membrane.