The SNa has a characteristic bifurcated morphology (not
visible in this focal plane, which actually does not show
the SNa itself but rather some of the muscles it innervates;
the SNa can be seen out of focus under muscle 5). This
bifurcation develops early, and SNa can usually be scored in
late 16 embryos. The lateral (or posterior) branch of SNa
innervates muscles 5 and 8, and the anterior branch
innervates muscles 21-24. It is often difficult to see these
synapses, however. What can usually be scored easily for SNa
is the presence of the bifurcation and the length of the
branches (whether the posterior branch has extended all the
way to the anterior edge of the m cell, and whether the
anterior branch reaches muscle 24). SNa is in a very deep
focal plane relative to SNb; it is almost at the focal plane
of the epidermal cells, which lie underneath the
muscles.
To early 17 SNa focal plane
2
To early 17
brightfield
To diagram of
muscles.
To 2nd diagram (muscles
and motor axons).
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