Histological definition of four developmental stages for caudal primary scale and tooth development. Transversal sections of a tail from a 3 cm long embryo (A-E) and of lower jaws from 4 cm (F-H) and 5.5 cm (I-J) long embryos coloured with Nissl stain (cresyl violet - thionin), scale bar: 25 μm. K: diagram summarising the four common stages for odontode development in the dogfish. EM, early morphogenesis- thickening of the dental epithelium (d.e.), condensation of the mesenchyme (m, coloured grey); LM, late morphogenesis- bell shape bud formation; ED, early differentiation- a constriction appears at the bottom of the bud (arrowheads); LD, late differentiation- matrix secretion (arrow).