Mouse cerebellar foliation is initiated by the formation of multi-cellular anchoring centers. (a) At E16.5, the first stage of anchoring center formation is an inward accumulation of gcps (blue) resulting from increased proliferation. The Pc layer (red) simultaneously folds inward, precisely where the gcps accumulate. (b) Next, at E17.5, gcps elongate and align along the fissure, as the outer cerebellar surface begins to fold inwards (c) By E18.5, the Bg fibers near the anchoring centers converge on the base of the fissure. (d, e) Cerebellar foliation continues by a self-sustaining process of folial outgrowth. (d) Bg fibers in the anchoring centers fan out from a small source to cover a large area of the IGL, thus forming a thin IGL at the base of the fissure. (e) Conversely, granule cells converge on a smaller central area and, therefore, accumulate at the crown of the folia to promote folial lengthening.