  Axonal growth and guidance at the ventral floor plate is here followed
$\textit{in vivo}$ in real time at high resolution by light-sheet microscopy
along several hundred micrometers of the zebrafish spinal cord. The recordings
show the strikingly stereotyped spatio-temporal control that governs midline
crossing. Commissural axons are observed crossing the ventral floor plate
midline perpendicularly at about 20 microns/h, in a manner dependent on the
Robo3 receptor and with a growth rate minimum around the midline, confirming
previous observations. At guidance points, commissural axons are seen to
decrease their growth rate and growth cones increase in size. Commissural
filopodia appear to interact with the nascent neural network, and thereby
trigger immediate plastic and reversible sinusoidal-shaped bending movements of
neighboring commissural shafts. Ipsilateral axons extend concurrently, but
straight and without bends, at three to six times higher growth rates than
commissurals, indicating they project their path on a substrate-bound surface
rather than relying on diffusible guidance cues. Growing axons appeared to be
under stretch, an observation that is of relevance for tension-based models of
cortical morphogenesis. The \textit{in vivo} observations provide for a
discussion of the current distinction between substrate-bound and diffusible
guidance cues. The study applies the transparent zebrafish model that provides
an experimental model system to explore further the cellular, molecular and
physical mechanisms involved during axonal growth, guidance and midline
crossing through a combination of $\textit{in vitro}$ and $\textit{in vivo}$
approaches.
