Abstract
Embryonic development of Clarias gariepinus was studied from oocyte activation to the end of endogenous feeding (164 h post-fertilization, 164 h-PF). The ontogeny of the eyes, the ear, the heart, the digestive tract and the notochord were described histologically: (i) eyes were not pigmented at hatching (40 h-PF) and the anlage of the retina was observed at 96 h-PF. At 164 h-PF, the pigmented epithelium was distinguished as a single layer of cubical cells and the outer nuclear layer was composed of cones; (ii) the inner ear was oval-shaped and two otoliths and a sensory epithelium were differentiated at hatching. Three semicircular canals with rudimentary cristae developed until 96 h-PF, leading to a fully developed auditory vesicle at 120 h-PF; (iii) at 40 h-PF, a beating heart appeared as a thin-walled tube and at 72 h-PF no blood elements were recorded in the two-chambered heart. The four compartments (bulbus arteriosus, ventricle, atrium, sinus venosus) and mature blood elements were found only at 144 h-PF; (iv) the hatchlings possessed an undifferentiated digestive system, and differentiation of a buccal cavity, oesophagus and intestine was only recorded 96 h-PF. The connection between the oesophagus and the intestine and the anus opened at 144 h-PF; (v) the notochord in newly hatched embryos was composed of vacuolated cells and a peripheral layer of notochordal epithelium inside a fibrous sheath. Subsequently, vacuolated cells proliferated and notochordal epithelium changed from cubical to squamous cells. Eye differentiation was concomitant with first feeding, thus suggesting a key role of visual senses in feeding behaviour.