108 



MUSCULAR TISSUE 



the early histogenetic changes already described for cardiac muscle. In 

 the frog, irritability was shown by Hooker to follow closely upon differen- 

 tiation of the fibrillse, and the establishment of nervous connections. 

 The adult skeletal muscle fiber is a multinucleated structure. Is this 

 condition the result of fusion of distinct myoblasts, or of growth of a 

 single myoblast accompanied by nuclear proliferation ? Both interpreta- 

 tions have been advanced; many observational data tend to show that 



Q- 



A C 



FIG. 120. A PORTION 

 OF A STRIATED MUS- 

 CLE FIBER SEEN IN 

 LONGITUDINAL SEC- 

 TION. 



The alternate light 

 and dark cross stria- 

 tions are well shown. 

 h, light line, Hensen's 

 line, in the middle of 

 the dark disk Q. z, 

 dark line, Krause's 

 membrane or Dobie's 

 line, in the middle of 

 the light disk. Hema- 

 tein. X 1200. (After 

 Bohm and von David- 

 off.) 



FIG. 121. A SMALL 

 PORTION OF A MUS- 

 CLE FIBER OF A 

 CRAB SHOWING BE- 

 GINNING SEPARATION 

 INTO FIBRILS. 



Drawn from a pho- 

 tograph. X 600. (After 

 Schafer.) 



a skeletal muscle fiber represents a myoblast which has elongated and 

 multiplied its nuclei. In the trout embryo, however, considerable fusion 

 of myoblasts occurs. The mode of nuclear division appears to be at first 

 mitotic, and subsequently amitotic. In the tongue a small number of 

 branched fibers have been described. 



Striped muscle fibers differ in the relative amounts of myofibrillar and 

 sarcoplasmic content. When the myofibrillae are relatively preponder- 

 ant and the interstitial granules sparse, the fiber is known as 'light'; 



