Anat. Embryol. 151,201-218 {1977) Quantitative Anatomical Studies on the Postnatal Development of the Cerebellum of the Albino Rat* H. Heinsen Abteilung Anatomie der RWTH Aachen, Melatener Str. 211, D-5100 Aachen, Federal Republic of Germany Summary. The quantitative postnatal changes of the cerebella of 65 Wistar rats aged 2-120 days have been examined. The cerebellar volume increases in two phases: The first phase lasts from birth to the seventh postnatal week. The second phase begins ten weeks post parturn and lasts for a Ionger period than the first phase. The cerebellar surface increases continuously from birth to the end of the seventh week. The volume of the external granular layer is maximal when the organ grows rapidly. The external granular layer has nearly disappeared 24 days after birth; the volume of the interaal granular layer is maximal at this time. Later on, the volume and the width of the interaal granular layer decrease. Myelinization of the cerebellar fibers and growth of the molecular layer run parallel to this decrease. The second late, but protracted growth of the cerebellum, ten weeks after birth, is due to an increase of the molecular and medullary layer. These findings are in good accord with histological, histochemical, and ultra- structural observations of other authors. Key words: Cerebellum- Albino rat- Ontogeny- Quantitative anatomy. Introduction During recent years the histo- and cytogenesis of the cerebellum of the white mouse and rat has been studied extensively (Uzman, 1960; Miale and Sidman, 1961; Altman, 1966, 1969, 1972a, b, c, 1975, 1977; Altman and Das, 1966; Das, 1976; Fujita, 196 7). As demonstrated for all parts of the cerebellum and its layers, there is no synchronaus development. Larsell {1952) already differentiated phylogenetically younger and older parts in the cerebellum. In cantrast to the extensive investigations on cerebellar morpho- and histogenesis only few quantitative data are available on the postnatal cerebellar growth. Raaf and Kernohan (1944) examined the develop- ment of the cerebellum in human beings, Haddara and Nooreddin {1966) in the albino mouse, and Oster-Granite and Herndorn {1976) in the Syrian Hamster. The development of the cerebellum is a complex process, on the one band composed of • Supported by the Deutsche Forschungsgemeinschaft LA 184/3.