Fig. 3. Ventral migration of neuromasts during postembryonic growth. The position of the horizontal myoseptum is indicated by the white arrows. (A) Size difference between a primary (left) and a secondary (right) neuromast shortly after the latter has begun to differentiate. (B) Neuromast migration varies along the anteroposterior axis. L1 has not moved, yet the secondary neuromast (NMII) that has formed on the next somitic boundary has migrated a long distance. (C) Onset of neuromast migration: L2 has begun to migrate (and to a lesser extent NMII) but L1 and L3 are still close to the myoseptum. The two axons innervating the neuromast L2 are indicated by an arrowhead. (D) At later stages younger secondary neuromasts (arrowheads) begin to migrate as soon as they differentiate. (E) Two neuromasts are occasionally observed at the same position, and migrate together. (F) The axons of sensory neurones follow the neuromasts in their migration. The tracks of the axons presumably reflect the migratory pathway of the neuromasts. In general the migration follows the intersomitic grooves (arrowheads), but may migrate straight ventralwards (asterisks). (G) As migration proceeds, the neuromasts tend to follow less and less the somitic boundaries (arrowheads). (H) The neuromasts of the dorsal line (D1-4) also migrate ventrally. In this case there is no indication that migration follows intersegmental limits. The neuromasts of the D' line have already formed. Their axons, and the dorsal course of the D axons, are obscured by a stream of melanocytes along the dorsal midline of the fish. Anterior is towards the left and dorsal is upwards.