DEVELOPMENT OF THE INNER-EAR OF THE BROWN TROUT (SALMO-TRUTTA-FARIO) .1. GROSS MORPHOLOGY AND SENSORY CELL-PROLIFERATION

The gross development of the trout inner ear between embryonic and juv enile stages was studied by light microscopy. The otocyst has already formed in 3-4 mm embryos. The semicircular canals begin to separate fr om the utriculo-saccular cavity in 6 mm embryos, the anterior canal fi rst, then the posterior and the horizontal canal later. The formation of the saccular cavity begins in 7 mm embryos, whereas that of the lag ena occurs in 18 mm fry. The first macular primordia appear before the separation of cavities. The anterior and horizontal crests arise from the primordium of the utricular macula, and the posterior crest, macu la lagena, and macula neglecta arise from that of the saccular macula. The macula lagena and macula neglecta appear later. The sensory areas of the labyrinth and the number of receptor cells grow continuously b etween the embryonic and juvenile stages.