An electron microscope study of fertilisation of the sea urchin egg Psammechinus miliaris

Abstract

1. 1. An electron microscope investigation has been carried out on the changes occurring at the surface of the sea urchin egg Ps. miliaris during the first 90 seconds following insemination. 2. 2. The surface of the unfertilised egg is bounded by a membrane about twice as thick as that bounding the surface of the fertilised egg. At fertilisation, this membrane splits into two layers—the vitelline membrane and the plasma membrane. This is followed by the opening up of the cortical granules, and the vitelline membrane, together with the contents of the cortical granules gives rise to the fertilisation membrane. These observations confirm, in general, the commonly accepted view of the origin of the fertilisation membrane. 3. 3. The membrane bounding the cortical granules is probably identical with the plasma membrane. The relationship of the granules to the surface of the unfertilised egg is such that the contents of the cortical granules may lie outside the permeability barrier. The implications of this are discussed, both with regard to permeability data and the opening up of the granules. 4. 4. There is no observable change in structure at the surface before the vitelline membrane separates from the plasma membrane. 5. 5. Treatment of unfertilised eggs with trypsin, which prevents formation of the fertilisation membrane reduces the thickness of the vitelline membrane. 6. 6. The fertilisation membrane is initially structureless and later becomes a two layered membrane.