Pelagic eggs and demersal eggs of teleosts both have osmotic concentrations similar to that of the maternal body fluids, less than half that of sea water. Pelagic eggs are buoyant because they contain such large quantities of this dilute aqueous fluid. While the demersal eggs of teleosts usually have a water content of 60–70%, the buoyant pelagic eggs of marine teleosts such as whiting, Norway pout, saithe, cod, haddock, turbot, dab, plaice, witch, long rough dab, halibut and sole typically have a very high water content (ca. 92 %) and a lipid content of 10–17% of egg dry weight. About 90% of the buoyancy of such eggs in sea water is caused by their high aqueous content, only about 10% being caused by lipid. The buoyant eggs of grenadier and ling have large oil globules and higher lipid contents, 27 and 35 % of dry weight respectively. Nevertheless, most of their buoyancy is provided by their high aqueous contents (89 and 81 % water). The high water content of pelagic eggs is brought about by a massive influx of water into the oocytes during meiotic maturation (ripening) after vitellogenesis but before ovulation. In cod and plaice, ripening is accompanied by a four- to five-fold increase in both water content and free amino-acids, and by a large influx of both potassium and sodium. In cod, free amino-acids contribute much more than these inorganic ions to the water influx and to the total osmotic concentration of the mature egg, but in plaice the relative contribution of inorganic ions approaches that of the free amino-acids.

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Plasma vitellogenin synthesis in adult female Scyliorhinus canicula L. continued throughout the year but the rate of synthesis was significantly lower between March and August than during the rest of the year. The hepatosomatic index (liver weight as percentage of body weight) underwent annual variation in both sexes but this variation was considerably more pronounced in the female. Increase in hepatosomatic index in the female was accompanied by increase in the percentage of lipid in the liver and was largely caused by increase in the total lipid content of the liver. Comparison of livers of males and females showed that the percentage of lipid was not significantly different while the percentage of solids insoluble in water and organic solvents was greater in the female. Oviposition, as indicated by the percentage of females with eggs in the oviducts, continued throughout the year, reaching a minimum in September-October. Between July and October, parameters associated with vitellogenesis began recovery from their respective annual minima in the following order: plasma oestradiol level, hepatosomatic index, rate of synthesis of plasma vitellogenin, gonosomatic index, plasma vitellogenin level, rate of oviposition. I N T R O D U C T I O NOviparous vertebrates typically have breeding seasons in which vitellogenesis (the synthesis of yolk and its appearance in the developing ova), ovulation and oviposition are confined to well-defined, usually brief periods of the year. During this time a characteristic sequence of ovarian events may be observed in the great majority of lower vertebrate species. Initially the ovary is small and the oocytes are devoid of yolk; during vitellogenesis the ova grow more or less synchronously so that at any one time they are all similar in size, and when fully grown they are ovulated and laid together over a short period. Normally one such cycle of a few weeks or months is completed in a year; this appears to be generally true, with certain exceptions, at all latitudes.Breeding in females of the elasmobranch Scyliorhinus canicula L. (lesser spotted dogfish) is cyclical but the period of the cycle is unusually long. Examination of the single ovary throughout much of the year reveals growing eggs in a distribution of sizes from minute previtellogenic oocytes to mature yolk-filled ova of 1-5 cm diameter. Within the upper size range the distribution is discontinuous and oocytes are found in pairs in a rapidly decreasing hierarchy of sizes. After ovulation of a mature pair, one ovum passes down each of the paired oviducts (Metten, 1941); during this passage it is provided with a horny shell. Estimates of the frequency of oviposition in the wild vary from one pair every six days (Harris, 1952) to 10-15 P au " s P e r annum (Le Danois, 1913). Superimposed on this pattern of egg production the following cyclical constraints have * Present address: