Limb summary 1. Two sources of mesoderm limb: lateral plate forming cartilage and bone; somite derived cells forming muscle. 2. The trunk level of the lateral plate mesoderm determines whether it becomes forelimb or hind limb. 3. The ectoderm is a signaling center regulating growth and it participates in patterning this outgrowth. 4. Because innervation of the limb arrives just as the muscle masses are forming, the segmental pattern of this innervation will be altered as limb rotate.