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16 Hr
18 Hr
20 Hr
24 Hr
30 Hr
36 Hr
FIGURE 5.1 The increasing complexity of fiber tracts in the developing vertebrate brain. Antibodies
against axons in the embryonic zebrafish brain at successive stages of development over the course of just
20 hours reveal that a variety of new axons are added at each stage. (After Wilson et al., 1990; Ross et al.,
1992)
B
Trochanter
Cell bodies
Femur
Control
A
Coxa
Fe1
Ti1
Growth
cones
Tibia
Guide post cells
To C N S
C
Ablated
Ablated Cx1 cell
Growth cone fails
to cross into Coxa
FIGURE 5.2 The growth of the Ti1 pioneers is aided by guidepost or stepping stone cells. A. A gras-
shopper embryo showing the developing legs. B. The Ti1 pioneers in black reach from one of the guidepost
neurons to the next, successively contacting Fe1, Tr1, and Cx1 on their way to the CNS. C. When the Cx1
cells are ablated, the Ti1 cells lose their way and do not cross into the coxal segment of the embryonic leg.
(After Bentley and Caudy, 1983)
 
 
 
 
 
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