For discussing the functions of different structures of all organisms.
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They are vestigial structures. In origin, during embrional development, the future CNS is represented by a tube, the neural tube, firstly delimited by a single cell layer. Successively the grey and white matter of brain and medulla grow from the thickening of that single cell layer all over the neural tube, until their dimensions are consistently bigger than the central lumen, which remains as central channel of the medulla. In cranial position, before the walls start developing to form emishperes, diencephalus and cerebellum, even the internal lumen of the tube enlarges in vesicles, which, after the complete differentiation of the cellular walls, are present in the mature encephalus as ventricles, cavities coming from embrional ancient structures. I hope I have been clear
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