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. Such mRNAs as Xcat2, Xwnt11, Xdazl, Xpat, Xotx1 and Xlsirts utilize the Early (METRO, message transport organizer) pathway. All these RNAs are localized in the mitochondrial cloud and are transported within its fragments to the vegetal cortex of the oocyte. After fertilization the fragments of mitochondrial cloud containing localized RNAs become a part of the germ plasm. It is believed that the components of germ plasm are responsible for the specification of germ cell fate in developing embryo. Such mRNAs as Vg1 and VgT utilize the Late (Vg1-like) pathway. This pathway does not involve mitochondrial cloud and it is microtubule and endoplasmic reticulum dependent. The Late pathway localizing RNAs play a role in endoderm and mesoderm specification and axial patterning during the development of the Xenopus embryo.