THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



305 



core shades off into the surrounding tissue of a lesser density, which is destined 

 to give rise to the muscles and which is known as the premuscle sheath. 



During these processes of differentiation in the limb bud proper, masses of 

 premuscle tissue have also become differentiated around the base of the limb 

 bud. These are the forerunners of certain extrinsic muscles of the upper ex- 

 tremity, such as the pectoralis, levator scapula, trapezius, latissimus dorsi, ser- 

 ratus, etc. (Fig. 273; compare with Fig. 274). 



Spinal ganglion 



Intervertebral disk 



8th cervical 

 myotome 



Border vein 



Fig. 271. — Transverse section through the 8th cervical segment of a human 

 embryo of 5 mm. Lewis. 



By the end of the fifth week the premuscle sheath in the limb bud proper be- 

 comes more or less differentiated into muscles or groups of muscles. The 

 differentiation is most complete at the proximal end. From this the transition 

 is gradual to the distal end where the premuscle sheath is intact. 



By the end of the sixth week most of the muscles of the upper extremity are 

 recognizable (Figs. 274 and 275). 



By the end of the seventh week practically all the muscles can be recognized 

 and are composed of muscle fibers. 



During the differentiation of the muscles, the limb bud and certain ex- 

 trinsic muscles migrate a considerable distance caudally. For example, the 



