Fertilization is accompanied by changes in the structure of the egg cytoplasm (cf. Rothschild, 1958; Raven, 1961). At the level of fine structure such changes have mainly been studied in some marine invertebrates with small eggs that can easily be fertilized in vitro (Pasteels & de Harven, 1963; Schäfer, 1966). Vertebrate eggs are less favourable in this respect, but electron microscope studies have been made on eggs of mammals (Fléchon, 1966; Zamboni & Mastroianni, 1966; Zamboni, Mishell, Bell & Baca, 1966) and Xenopus (van Gansen, 1966). Changes generally observed soon after fertilization include the formation of polysomes or an increase in their number, a hyper-trophy of the Golgi complexes, and the appearance of granulated endoplasmic reticulum and annulate lamellae. Afzelius (1957) observed the dispersal of mito-chondria in fertilized sea-urchin eggs. Pasteels & de Harven (1963) reported that the structure and distribution of cytoplasmic organelles in eggs of the bivalve mollusc, Barnea candida, are not altered by fertilization. The structural changes produced by fertilization in the small, telolecithal eggs of lampreys were re-peatedly studied during the late nineteenth century (cf. Herfort, 1901) and later by Kille (1960). Vivid cortical activity and complicated cytoplasmic movements were observed at the animal pole. Electron-microscope studies of lamprey eggs have apparently been restricted to the yolk platelets (Karasaki, 1967). During a study of sperm penetration in lampreys, the eggs were found to be very favourable for electron microscopy, and some observations of early changes in the thin layer of pole plasm will be reported below. The structure of the egg envelopes, the emptying of cortical vacuoles, and sperm penetration are des-cribed in separate papers (Afzelius, Nicander & Sjödén, 1968; Nicander & Sjödén, 1968).

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