Experiments have been performed which lead to the conclusion that dichromated gelatin holograms arise as a result of the formation of an air-gelatin interface, with the observed high diffraction efficiencies resulting from the air-gelatin refractive index differential of 0.52 found at the interface. It is postulated that rapid removal of water by isopropanol in the development process creates strains in the gelatin film which are relieved by splitting. The resultant cracks in the hologram manifest themselves in an obvious location, between the highly exposed planes of cross-linked gelatin. Photomicrographic evidence on Bragg-Lippmann and on plane gratings, liquid gate data, lack of thickness reciprocity for the gratings, and observations of grating thickness employing bandpass measurements comprise the bulk of the experimental work. From these data, a model is proposed that is consistent with all the experimental results for both Bragg-Lippmann and plane gratings.