Studies of the ultrastructure of sea urchin oogytes and the changes induced by insemination

Sea urchin oocytes were examined with the electron microscope before and after insemination. In the uninseminated oocyte a typical endoplasmic reticulum was found which after insemination underwent profound changes. Upon insemination the yolk granules were observed to disappear from the region contiguous to the nucleus. The formation of the reception cones and the changes seen in the cytoplasm are described. The events following the entrance of spermatozoa into the oocyte are discussed.