The Overall Histological Pattern

The grey matter of the spinal cord consists of two lateral, wing-like expansions in cyclostomes, whereas in chondrichthyans it is differentiated into distinct dorsal and ventral horns, an arrangement similar to that found in all other vertebrates. The spinal grey matter in cartilaginous fishes contains cells of a wide size range, although in adults the larger elements are mainly confined to the ventral horns. Myelinated fibres are absent from the cyclostomes but the white matter of the chondrichthyan cord comprises numerous well-myelinated fibres.