Oogenesis was studied in adult Rana temporaria and Rana esculenta with the electron microscope. It takes an almost identical course in both animals. Three types of yolk bodies were found. These differ in their genesis as well as in their morphological appearance in the mature oocyte. In accordance with their morphogenesis they were named: 1. mitochondrial matrix yolk (MMY), 2. intracristal yolk
... [Show full abstract] (ICY) and 3. vesicular yolk (VY). MMY is formed within the mitochondrial matrix and has a centre to centre spacing of the crystalline lattice of approximately 165 Å. ICY is formed within the cristae and has a centre to centre spacing of the crystalline lattice of 85 Å. Vesicular yolk is formed through uptake of pinocytotic vesicles into multivesicular bodies and has a centre to centre spacing of the crystalline lattice of approximately 95 Å. The formation of the three types of crystalline inclusion bodies is discussed in view of a possible intraoocytic or extraoocytic origin of the incorporated material.