During human fetal development, the hippocampal sulcus first appears at approximately 10 weeks of gestational age. At this stage it exists as a broad shallow fissure along the surface of the dentate gyrus. Gradually, the fissure deepens and shifts toward the cornu ammonis. After about 18 weeks, the walls of the fissure fold into each other and begin to fuse. By 30 weeks, the hippocampal sulcus is normally obliterated except for its most medial part, leaving a shallow surface indentation.
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^Bastos-Leite AJ, van Waesberghe JH, Oen AL, van der Flier WM, Scheltens P, Barkhof F (2006). "Hippocampal sulcus width and cavities: comparison between patients with Alzheimer disease and nondemented elderly subjects". American journal of neuroradiology27 (10): 2141–5. PMID17110684.