Cortical fusion; fetus 34 weeks. Depths of a sulcus at low power (a. H&E stain, b. Reticulin) shows the sulcus extending into the cortex as a seam. There are abnormally large blood vessels in the superficial cortex around the seam. c: At higher power the reticulin stain shows collagen fibres (arrows) which are not found in normal cortex and represent the remnants of trapped leptomeninges. There are abnormal large blood vessels and many plump, reactive astrocytes.