Cobblestone malformation; fetus aged 20 weeks. In the centre is a coronal section through the frontal part of the brain. The classic cobblestone cortex is seen over the lateral hemisphere walls (box a). There is a gradual transition in the nature of the malformation over the vertex and into the medial frontal lobes where the appearance is of PMG. The adjacent medial frontal surfaces are fused and share a basement membrane (boxes b and c) (H&E). a: Higher power of classic cobblestone malformation in the lateral wall of the frontal lobe. A line, representing the original pial surface, runs vertically through the mid region of the cerebral wall. To the left the lateral brain surface consists of masses of ectopic neurones mixed with vascular meninges. To the right in the deeper part of the brain wall clusters of neurones of the original cortex directly abut the pial surface (H&E). b: Medial frontal lobes. There is no falx. The cortical neuronal band is festooned. The leptomeninges are thickened and dip into fused adjacent folds of the neuronal band. c: The surface collagen layers are reduplicated and fused in the midline (reticulin).