The cytoarchitecture of the cerebellum has been studied in chicken embryos from day 3-20 using serial sections stained with cresylviolet, haematoxylin-eosin and toluidine blue. Three periods have been distinguished in cerebellar development on a basis of cytoarchitectonic characteristics. Of these periods the middle one, which lasts from the 8th to the 15th day, is marked by two subsequent transient longitudinal cytoarchitectonic patterns in the cortical anlage. The first pattern, which exists between days 8 and 11, consists of 4 longitudinal Purkinje cell clusters (of the first order) at either side of the midline. The second pattern, which is most distinct and complete during embryonic days 12-14, is caused by specific localizations of otherwise few, early inwardly migrating granule cells from the external cerebellar matrix (so-called granule raphes), which pass through the layer of Purkinje cell clusters of the first order and thus subdivide these latter into smaller entities: Purkinje cell clusters of the second order. The number of these latter (6 or 7 and 11 or 12 in the anterior and posterior lobes, respectively) correspond to the number of parasagittal modules, which can be discerned on a basis of the organization of fiber connections of the adult cerebellar cortex. Thanks to this similarity various hypotheses can be formulated concerning the significance of the transient cytoarchitectonic patterns in the primitive cortex for the genesis of the modular organization of the cerebellum.