Cerebellar foliation is executed by anchoring the base of the fissures and by folia lengthening. (a, b) The Cb vermis primordium at E16.5 is a smooth structure (a), which, at E17.5 (b), is divided into four lobes by three fissures (asterisks). (c) At E18.5 the four principal fissures (asterisks) and five cardinal lobes (dotted outlines) are evident. The cardinal lobes are designated as anterobasal (red outline), anterodorsal (green outline), central (blue outline), posterior (yellow outline) and inferior (white outline). (d) The adult Cb has ten lobules that develop from the cardinal lobes, marked by colored outlines in (c, d) (I-X, principal fissures are marked by asterisks). (e) Superimposition of midline vermis tracings from E16.5 to P21 show that the folia grow by lengthening, while the bases of the fissures are largely fixed in position. Mb, midbrain; Cb, cerebellum; pc, preculminate; pr, primary; sec, secondary; pl, posterolateral fissure. Scale bars: (a-c) 300 μm; (d) 500 μm; (e) 600 μm.