Fig. 2. Chick commissural axons follow three distinct contralateral pathways. (A) Combined phase and fluorescent micrograph showing three distinct trajectories followed by DiI labeled commissural axons on the contralateral side of an E5.5 chick open-book spinal cord explant (dorsal is upwards and rostral is towards the right). (a) A small population of axons crosses the floor plate and extends directly into more dorsal regions of the white matter before initiating a rostral turn into the longitudinal axis. (b) The major class of axons exhibits an arcuate trajectory that consists of a rostral turn at the contralateral floor plate boundary, followed by growth alongside the floor plate for 
100 µm, diagonal growth away from the floor plate, and then a rostral turn into the longitudinal axis within the dorsal marginal zone. (c) A third, small population of axons turns orthogonally at the contralateral floor plate boundary and projects rostrally adjacent to this structure for distances greater than 100 µm. (B) Schematic representation of an open-book spinal cord preparation (marginal surface facing upwards) showing each contralateral pathway depicted in A. For each application of DiI into the dorsal spinal cord (see Materials and Methods for details), the relative proportion of axons within a cohort of DiI labeled axons (n=50) that followed each pathway was calculated. Scale bar: 100 µm in A. d, dorsal; fp, floor plate; rp, roof plate; v, ventral.
