POSTEMBRYONIC DEVELOPMENT OF THE CEPHALIC REGION IN HETEROBRANCHUS-LONGIFILIS

At hatching, Heterobranchus longifilis does not display any primordia of the cephalic skeleton. The latter appears 12 h post-hatching and de velops in three stages up to day 16. The first stage (12 h to 2 days) involves almost exclusively the development of the chondrocranium. Dur ing the second period (days 3-8), dermal elements of the splanchnocran ium appear. The final stage is marked by resorption of the cartilages, progressively replaced by ossifications (days 10-16). At their appear ance the elements of the splanchnacranium are fused together, as are t he first neurocranial elements. Later, the splanchnocranium splits up. By the time the yolk sac is completely resorbed, the buccal and phary ngeal jaws are present, the suspensoria and hyoid bars are partially d eveloped, and the parasphenoid partially closes the hypophyseal fenest ra. These structures delimit a buccal cavity that is probably function al, i.e. capable of participating in the intake of exogenous food. Nex t to continue its development is principally the splanchnocranium, com pleting the walls of the buccal cavity. Cartilage resorption parallels the appearance of endochondral ossifications (except for the trabecul ar bars). Braincase closure begins to accelerate once the buccal syste m is complete. (C) 1997 The Fisheries Society of the British Isles.