THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



309 



and 278.) At the same time they increase in number by longitudinal splitting. 

 The cytoplasm among the fibrils becomes the sarcoplasm. 



After the granules which first appear unite to form the fibrils, the latter 



FIG. 275. Medial view of a reconstruction of the muscles of the upper extremity of a human 

 embryo of 16 mm. (about 6 weeks). Lewis. 



The muscle arising on the scapula (at the left of the figure) and passing toward the right is the 

 subscapular. The small muscle just below the subscapular is the teres major; below the 

 latter and hanging downward is the latissimus dorsi. Note the cut end of the pectoralis 

 minor just to the right of the narrow portion of the subscapular. Running from this cut end 

 toward the right is the biceps. The muscle at the lower edge of the figure in the arm region 

 is the triceps. In the forearm region, the muscle crossing the end of the biceps is the pro- 

 nator teres. Below the pronator teres, extending from the elbow to the thumb region is the 

 flexor carpi radialis. Below the latter and extending to a point opposite the thumb, is the 

 palmaris longus. Beneath the palmaris longus and dividing into branches which pass to the 

 2d, 3d, 4th, and 5th digits is the flexor sublimis digitorum. The muscle passing to the 

 thumb is the flexor longus pollicis. . The muscle at the lower border of the figure in the fore- 

 arm region is the flexor carpi ulnaris. 



FIG. 276. Myoblasts in different stages of development. Godlewski. 



The upper cell represents a myoblast with granular cytoplasm (from sheep embryo of 13 mm) ; the 

 middle, a myoblast with fibrils in process of formation (from guinea-pig embryo of 10 mm.); 

 the lower, a myoblast with still further differentiated, segmented fibrils (from a rabbit 

 embryo of 8.5 mm.). 



are apparently quite homogeneous. Later they become differentiated into two 

 distinct substances which alternate throughout their length and produce the 



