In many vertebrates and invertebrates the development of the embryo depends on the proper, often asymmetrical, localization of maternal determinants throughout the egg cytoplasm (ooplasm). One of the best known examples of such asymmetrical localization is the localization of maternal RNAs in Xenopus oocytes. Here, different RNAs localized to the vegetal cortex of the oocyte participate in germ cell determination and in axial patterning of the embryo. There are two different pathways of RNA localization during Xenopus oogenesis.