Oviducal Contribution to Alteration of the Vitelline Coat in the Frog, Rana japonica. An Electron Microscopic Study

The vitelline coat (VC) surrounding coelomic eggs of the frog, Rana japonica, comprises bundles of filaments running both parallel and perpendicular to the egg surface. The coat gives little or no staining reaction with PA-CrA-Silver methenamine. In contrast, in the VC of uterine eggs the filament bundles are less conspicuous. and the interstices between the filament bundles stain strongly for carbohydrate. This alteration occurs during passage of the eggs down the first 1/20 th of the oviduct, the pars recta. The epithelium of the p. recta contains secretory cells, which contain electron-dense granules distinct from those in the jelly-secreting cells in more caudal portions of the oviduct. Treatment of coelomic eggs with an extract of p. recta followed by exposure to a sperm suspension resulted in marked swelling and softening of the VC. These results indicate that the contents of the granules secreted from the epithelial cells in the p. recta are deposited in the VC to increase its susceptibility to a fertilizing sperm.