MORPHOGENESIS OF MUSCULAR SYSTEM 721 



origin of the limb musculature and connective tissues. Similar affirmations 

 and denials are found in the literature, relative to origin of the intrinsic limb 

 muscles in higher vertebrates, including man. For example, Ingalls ('07) 

 described myotomic cell migrations into the developing human limb, whereas 

 W. H. Lewis ('10a) was not able to subscribe to this view. 



Although actual muscle tissue from the myotomes to the limb buds cannot 

 be traced in all cases, the fact remains that the nerve supply to a myotome 

 or to a particular group of muscle-forming cells appears to be a constant 

 feature. For example, the facial musculature, which is derived from the hyoid 

 arch mesenchyme of the embryo as set forth above, retains its innervation 

 by the facial or seventh cranial nerve, even though the muscle migrates far 

 forward from its original site of development. The innervation of the trapezius 

 muscle by the spinal accessory nerve is another example of this same fidelity 

 of the nerve supply to the original site of the origin of the muscle-forming 

 cells. Mall (1898, p. 348) describes this relationship between the nerves and 

 myotomes as follows: "As the segmental nerves appear, each is immediately 

 connected with its corresponding myotome, and all of the muscles arising 

 from a myotome are always innervated by branches of the nerve which orig- 

 inally belonged to it." (See fig. 33 lA.) 



The development of the musculature of the tetrapod limb involves two 

 main premuscle masses of tissue: 



( 1 ) An intrinsic mass of muscle-forming mesenchyme within the develop- 

 ing limb which condenses to form separate muscle-forming associations 

 of cells around the developing skeleton of the limb. Each of these 

 cellular associations then proceeds to differentiate into a particular 

 muscle or closely integrated group of muscles (figs. 328B; 329A and 

 B). That is, the intrinsic mass of muscle-forming tissue gives origin 

 to the intrinsic musculature of the limb. 



(2) An extrinsic mass of premuscle tissue which ultimately gives origin 

 to the musculature which attaches the limb and its girdle to the axial 

 skeleton. This premuscle tissue arises from two sources: 



(a) Premuscle tissue from the limb bud which migrates from the limb bud 

 proximally toward the axial skeleton. In the forelimb, the pectoral, 

 latissimus dorsi, and teres major muscles develop from this mass of 

 tissue, while in the hind-limb the caudo-femoralis, iliopsoas, piriformis, 

 and certain of the gluteal muscles appear to arise from muscle-forming 

 tissue which extends axially to unite the limb with the axial skeleton. 



(b) Premuscle tissue which arises outside the limb bud mesenchyme. The 

 muscles which arise from this tissue serve to attach the limb girdle to 

 the axial skeleton. From premuscle tissue of this type arise the Mm. 

 trapezius, sternocleidomastoideus, rhomboidei, levator scapulae, ser- 

 ratus anterior, and omohyoideus. 



