ONTOGENY OF CYTOLOGICAL STRUCTURES AROUND THE YOLK-SAC DURING EMBRYOLOGIC AND EARLY LARVAL DEVELOPMENT OF SOME CICHLID FISHES

Ontogenesis of cell structures around the yolk sac was followed in thr ee species of cichlids, the bottomspawner, Tilapia zillii, and two mou thbrooders, the parental Sarotherodon galilaeus and the maternal Pseud otropheus johanni. A description is given of the initial stages of emb ryogenesis and developments observed in the nucleoli released towards fertilization from the egg germinal vesicle into the yolk, and their p articipation in enzymatic food metabolism. Attention was focused on th e development and involution of a special microvillar network that evo lves from the perivitelline syncytium towards the overgrowing yolk sac blood net. In T. zillii this commences 16-18 h after fertilization; i n S. galilaeus 30-32 h after fertilization; and in P. johanni, with la rger eggs and slower development, it commences 38-40 h after fertiliza tion. The extending microvilli establish close contact with the yolk s ac blood capillaries thus enabling food transport from the yolk to the developing embryo. Involution of the microvillar layer begins in free -swimming larvae, dispersing in bottomspawners at age 6-7 days, and in mouthbrooders at 14-16 days. (C) 1995 The Fisheries Society of the Br itish Isles