Fish Hatchery Management - fisheries & aquaculture

174 FISH HATCHERY MANAGEMENTthe fish culturist in providing an optimum environment for egg development.Eg DeuelopmentDuring oogenesis, when an egg is being formed in the ovary, the egg's futureenergy sources are protein and fat in the yolk material. At this earlystage, the egg is soft and low in water content, and may be quite adhesive.The ovum, or germ cell, is enclosed in a soft shell secreted by the ovariantissue. This shell, or chorion, encloses a fluid-filled area called theperivitelline space. An opening (the micropyle) provides an entryway forthe sperm. Inside the perivitelline space is a vitelline membrane; the yolkis retained within this membrane (Figure 60). Trout eggs are adhesivewhen first spawned because of water passing through the porous shell. Thisprocess is called water-hardening, and when it is complete, the egg nolonger is sticky. The egg becomes turgid with water, and the shell isseparated from the yolk membrane by the perivitelline space filled withfluid. This allows the yolk and germinal disc to rotate freely inside the egg,with the disc always being in an upright position.The micropyle is open to permit entry of the sperm when the egg is firstspawned. As the egg water-hardens, the micropyle closes and there is noM ICROPYLEGERMINAL DISCVITELLINEM EM BRAN ESH ELLPERIVITELLINESPACEYO LKOIL DROPLETFIcURE 60.Diagrammatic section of a fertilized trout egg. (Sorrrce' Davis 1g53.)