Figure 5:

The developing Brachial plexus and forelimb musculature in Alligator mississipiensis. The influence and instructing interactions of surrounding tissues is relevant for the developmental pattern of forelimb musculature. As observed in other species (not shown) the nerves of the brachial plexus (SN VII to XI in Alligator) extend in between the Divisions of the forelimb developing musculature, being possibly even responsible or in part involved in the separation. In early stages, the proximal base of the Dorsal cord separates the deep Subscapular division from the rest of the Dorsal muscle mass, and ventrally, the Ventral intrinsic division is separated from the Pectoralis and Supracoracoideus divisions that remain together. In later stages the nerves also extend to lie in between the separated Deltoid, Latissimus and Dorsal intrinsic divisions dorsally and separating the Pecotralis and Supracoracoideus divisions. The correspondence between the passage of the nerves and the separation of the muscle masses is stronger in earlier stages, and in older embryos the nerves associate less intimately with them, indicating that probably in later stages the separation of individual muscles relies on different cues than in the earlier cleavage steps. Scale bars 500 μm.