With the exception of Petromyzon and Caspiomyzon, the various genera of holarctic lampreys each consists of several species, some of which are parasitic and others nonparasitic. In the latter, the socalled brook lampreys, sexual maturity is attained within a few months of metamorphosis, and as a result the adult feeding and migratory phase has been eliminated from the life cycle. The parasitic lampreys, after metamorphosis, move downstream to feed in the estuaries or open sea, although all species of Ichthyomyzon and a few land-locked forms of other genera remain throughout life in fresh water (Zanandrea, 1959b). In the genera Ichthyomyzon, Eudontomyzon, and Lampetra the parasitic and nonparasitic forms can be arranged in pairs of sibling species, which except for differences in size resulting from the presence or absence of an adult feeding stage, may be almost indistinguishable on purely morphological grounds (Hubbs, 1925; Hubbs and Trautman, 1937; Zanandrea, 1954). It is generally believed that the brook lampreys have evolved from related parasitic forms by a process of partial paedomorphosis (Leach, 1951; Young, 1960), and since the different species of brook lamprey have presumably arisen independently, it must be supposed that a tendency towards precocious sexual maturity has been widespread in those genera where such sibling species occur. In this connection it may perhaps be significant that in the genus Petromyzon, which consists of only one species, the parasitic P. marinus, differentiation of the gonad and growth of the oocytes occurs at a relatively late stage of larval life as compared with the brook lamprey L. planeri (Hardisty, 1960). As in certain families of fishes, notably the Salmonidae, where there are both holobiotic and amphiobiotic forms, the size of the various species is broadly proportional to the amplitude of their migrations (Fontaine, 1948). Thus, intermediate in size between the large anadromous sea lamprey and the small nonmigratory brook lamprey, there are forms such as the European river lamprey L. fluviatilis (whose migrations are probably confined to brackish estuaries or coastal waters) and the dwarf sea lamprey of the Great Lakes of North America. Associated with these differences in size, there are corresponding differences in fecundity (table 1), and as in reptiles and fishes (Rensch, 1959) increased body size has generally resulted in greater egg numbers rather than in larger eggs. While these egg numbers may be the maximum that are physiologically possible (Lack, 1954), they may also be limited by the extent of the spawning journey which the female must undertake (Svardson, 1949) and by the number of primary oocytes which were present in the gonad of the ammocoete larva. As neither the duration of the larval period nor the final size reached by the ammocoete differs very widely in the various species of lampreys (Leger, 1920, 1924; Zanandrea, 1957; Bahr, 1952; Hardisty, 1961b), the phenotypic effects of larval growth prior to metamorphosis can have little bearing on interspecific differences in oocyte numbers and it may be safely assumed that such differences have a genetic basis. There can equally be little doubt that, within ammocoetes of the same species, the wide range of variability in oocyte numbers (Hardisty, 1961a) is a reflection of variations in nutritional and other environmental factors. Similarly, in the parasitic lampreys, where the final size of the adult is determined during the period of postmetmorphic growth, individual fe-