shown recently that the same system is also involved in the setting
up of longitudinal fiber tracts at various distances from the midline
(Rajagopalan et al., 2000, Zlatic et al., 2003). Whether this function
is conserved in vertebrates has not yet been established. What-
ever the case, the universal implication of the robo/slit system in
everything that concerns the relation between neurons and midline
indicates that this system must have been firmly rooted in the
development of the urbilaterian nerve chord.
Likewise, the roles of the netrin/DCC system have been con-
vincingly demonstrated in protostomes (fly, nematode) as well as
in deuterostomes (chick, mouse, zebrafish). The molecular and
functional conservation of axon guidance mechanisms suggests
that a robust orthogonal organization already characterized the
urbilaterian nerve chord. Whether elementary functional circuits
were already genetically designed within this orthogonal net, and
could have been inherited by all derived animals, has not been
demonstrated but seems plausible enough.