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Fig. 2. Development of axonal processes in the leech nervous system. A monoclonal
antibody to acetylated 
-tubulin (ACT; Sigma) was used to stain axons in
embryos of different stages. Tracings of nerves from representative segments
were fused into a single image that depicts the major events during T.
rude axogenesis. The time line indicates the approximate clonal age of
axon fibers with respect to the corresponding birth of blast cells within the
neuroectodermal (N) lineage. The first axon processes were observed at 
50
hours (arrowhead), appearing on either side of the prospective midline. These
projected medially and met at the midline 1 hour later, upon which they
extended anteriorly to pioneer Faivre's nerve (arrow identified by F). Paired
longitudinal axons occupying positions of the future connective nerves (arrow
C) appeared at 55 hours clonal age. The pioneering process of the MA and
AA segmental nerves (arrows MA and AA, respectively) appeared almost
simultaneously between 55-58 hours clonal age; the MA axon fiber(s)
originated from a peripheral location corresponding to pz8/cf3
(Braun and Stent, 1989a), while
AA fibers appear to extend both medially and laterally from unidentified cells
at the ganglionic margin. Processes of the PP segmental nerve (arrow PP)
appeared at 60 hours clonal age. A transient, lateral longitudinal nerve
(arrows) was apparent at 65 hours clonal age, positioned slightly medial
to peripheral floret cf3 along the MA nerve. An axon marking the UP nerve
(arrow UP) appeared at 70 hours clonal age. At 120 hours clonal age,
the medial portion of the DP nerve was apparent (arrow DP), the segmental
nerves were compressed longitudinally forming a cluster of three nerve roots
and the ganglion was surrounded by glial sheaths. Anterior is upwards.
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