In summary: Two sources of mesoderm limb: lateral plate forming cartilage and bone; somite derived cells forming muscle. The trunk level of the lateral plate mesoderm determines whether it become forelimb or hindlimb. The ectoderm is a signaling center regulating growth and    it participates in patterning this outgrowth. Because limb innervation arrives just as the muscle masses are forming, the segmental pattern of innervation will be altered as limb rotate.