354 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES HI 



cleavability, which betrays no mechanical inhomogeneity at the 

 boundaries of the segment, is an argument in favour of their uni- 

 formity. Further, very young fibrils are uniformly birefringent (later 

 the striation appears gradually differentiated from the middle towards 

 the extremities), while the cross striation may disappear in explanted 

 skeletal muscle cells through dedifferentiation (Schmidt, 1937a, pp. 

 215, 223). 



Whereas the fibrils are probably continuous, the sarcoplasm appeaxs 

 to be subdivided by transverse septa; for in the centre of the dark I 

 band there is always a narrow Z band, easily identified by staining, 

 which shines brightly between crossed nicols (Fig. 176). It is supposed 

 to be a cross membrane, continuous with the sarcolemma, the myo- 

 fibrils thrusting through it without hindrance. When the muscle 

 contracts, these regions do not thicken appreciably, so that the 

 sarcolemma is thrown into festoons. 



The segment of the myofibril from one Z band to the next is called 

 the sarcomere. Its length is about 2 //. In a growing muscle fibre, the 

 sarcomeres are added to the end of the fibre originating from one 

 single cell. The sarcomeres at the two ends are less differentiated 

 during growth than in the middle of the fibre (Haas, 1950). 



On both sides of the Z band slightly birefringent N bands occur, 

 often joining the Z band. Matoltsy and Gerendas (1947) suppose 

 the lack of optical anisotropy in the I band to be caused by the inter- 

 calation, between the myofibrils, of an optically negative substance,, 

 called N-substance, which compensates the positive double refraction 

 of the actomyosin (Gerendas and Matoltsy, 1947). The UV ab- 

 sorption of the N-substance is the same as that of nucleic acid, which 

 is an optically negative substance (see p. 220). Muscle fibres extracted 

 with 0.3 Af KCl, which dissolves myosin, lose their isotropic bands 

 (Snellman and Gelotte, 1950). 



The retardation of the Q bands in the fibre decreases considerably 

 during contraction, notwithstanding the appreciable increase in 

 thickness; the optical term for this is negative fluctuation. The fact 

 established by v. Muralt (1932) that negative fluctuation also occurs 

 with isomeric contraction — i.e., when the muscle is forcibly held 

 to its original length during contraction — is of great importance. 



Besides intrinsic birefringence, which is manifested as birefringence 

 of flow in myosin solution (v. Muralt and Edsall, 1930), the myo- 



