Premature cortical folding. a: Fetus of 18 weeks with an area of abnormal cortical folding in what will become the perisylvian area (arrows). The surface of the brain elsewhere is smooth, as expected at this age (H&E). b and c: two examples of abnormally festooned neuronal bands in fetuses aged 29 and 26 weeks. In (b) (29 weeks) a single neuronal band festoons below thickened and very vascular leptomeninges (reticulin). In (c) (26 weeks) a single neuronal band festoons below a straight subpial, densely cellular, band. Less densely packed cells occupy the tissue between these two more cellular bands.