Organogenesis of the Opercular Apparatus in Salvelinus fontinalis (Mitchill)

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Abstract
The opercular fold in Salvelinus fontinalis (Mitchill) begins as a bulging, outward and backward, of the undifferentiated dorsally-packed mesenchymal tissue of the hyoid arch in embryos of 6.0–6.5 mm total length. From 6.5 to 9.0 mm, backward growth takes place although no gill arches are covered. The epithelium of the opercular anlage is 2- to 3-layered laterally and 1- to 2-layered with smaller nuclei medially. From 9.0 to 14.0 mm, i.e. hatching, the membranous lateral opercular and ventral branchiostegal portions extend posteriorly to cover the four gill arches. Ossification commences in the opercle, the jaws and the pectoral girdle. The epithelium which has scattered goblet cells is 3- to 4-layered antero-laterally, 1- to 2-layered postero-laterally and one cell in thickness medially. From 14.0 to 30.0 mm length ossification of the skeletal elements of the opercular apparatus takes place. The musculature of the operculum is complete at hatching.

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