I. Direct microscopic examination of the unfertilised eggs of Psammechinus miliaris and Teredo norvegica merely shows the existence of a thin, granule-free zone covering the surface. Whether this is continuous with the general cytoplasm or not cannot be made out with certainty by direct observation.
2. A cone of clear material can be drawn out from the surface of the unfertilised egg of both species by means of the microdissection needle. A definite membrane cannot be separated in this way.
3. Hypertonic solutions cause the egg of Psammechinus to shrink smoothly at first and later to become wrinkled. This is consistent with the view that the egg is surrounded by an elastic, solid layer which is normally in a state of tension.
4. Cytolysis of the egg of Psammechinus in tap water is not accompanied by bursting. The egg swells and is perfectly smooth and spherical when cytolysis is completed. This points to the existence of an elastic, solid surface layer.
5. Plasmolysis of the egg of Teredo is of the type here called "polyhedral."The irregular shape of the egg in the hypertonic solution is only temporary, as a clear membrane separates from the concave surfaces and the egg then becomes more or less spherical.
6. The protoplasm of the plasmolysed egg of Teredo behaves as a viscous fluid.
7. Cytolysis of the egg of Teredo in tap water is accompanied by bursting and dispersion of the entire cell contents. A crumpled membrane alone remains.
8. It is concluded that the unfertilised egg of both Teredo norvegica and Psammechinus miliaris is surrounded by an elastic vitelline membrane which is much a Vitelline Membrane of the Egg of P. miliaris and of T. norvegica 105 thicker in the former than in the latter. The vitelline membrane in both cases is tightly attached to the egg surface.
9 . In calcium-free sea water the fertilisation membrane is elevated normally in Psammechinus miliaris. It does not harden, however, and gradually sinks back on to the surface of the egg owing, apparently, to the loss by diffusion of the osmotically active substance in the perivitelline space. It can be elevated a second time by puncturing the surface of the egg and allowing some of the cell contents to penetrate into the perivitelline space.
10. It is suggested that one action of hypertonic solutions in inducing artificial parthenogenesis may be to cause a loosening of the attachment of the vitelline membrane to the egg surface.