Fig. 5. Rearrangement of the terminal part of the L-PLL and structure of the tail. The neuromasts and their axons (green) are visualised in fluorescence and the image is superimposed on a Nomarski view of the same field. The axonal paths are indicated with a dotted line because the fluorescence is too weak to show when superimposed to the diascopic view. Given the thinness of the tail, the neuromasts on the other side of the fish are also seen. Each panel represents a different fish. (A) Terminal neuromasts along the ventral edge of the tail before the onset of the rearrangement. (B) Elongation of the melanocytes (white arrows). (C, D) Formation of a ventral mesenchyme and beginning of the migration of the penultimate neuromast (white arrowhead). (E) Appearance of the caudal fin rays. (F) Rearrangement nearly complete. (G) At a slightly later stage, lower-scale view to illustrate how the homocercal, symmetrical caudal fin of the zebrafish is entirely derived from the ventral fin fold of an intrinsically asymmetrical, heterocercal tail. Anterior is towards the left and dorsal is upwards.