sentences:  The brain of the early zebrafish presents an ideal environment in which to study the behavior of developing axon tracts.The earliest differentiating pioneer neurons create a scaffold, with which growth cones of follower axons interact with.Deletion of pioneer axons which create the scaffold have an effect on the growth cones of the neurons of the nucleus of the posterior commissure, in that they cannot follow the normal path of extending ventrally, then posteriorly.