STRIPED MUSCLE OF WASP 57 
traction band might continue to adhere to a portion of a cer- 
tain telophragma in an otherwise relaxed fiber, and so initiate 
this fourth distinct type of intercalated disc. Through subse- 
quent modification by the penetration of tissue fluid via the 
telophragma, such a thickened portion of this membrane might 
be caused to persist as this type of disc. 
VIII. SUMMARY 
1. Contraction in striped muscle is associated with a genuine 
reversal of striations as regards a deeply staining substance of 
the dark dise of the sarcostyle. This reversal of striations 
results in the formation of contraction bands in the contracted 
fiber. 
2. A contraction band is composed essentially of the fused 
opposite halves of two adjacent dark discs. In fibers containing 
accessory dises, the contraction bands involve also two of these 
discs, and in fibers where sarcosomes occur; some of these gran- 
ules may also become included within the intersarcostylic spaces 
of the definitive contraction band and contribute to its deeper 
color and intenser staining reaction. The method of formation 
of the contraction band explains its bisection by the telophragma. 
3. The striping of the striated muscle fiber results from the 
segregation of darker and lighter (chromatic and achromatic) 
substances in alternating dark and light dises. These discs are 
bisected by a mesophragma and a telophragma, respectively. 
The deeper staining disc is not coextensive with, nor the result 
of the presence and stratified distribution of, specific anisotropic 
materials. The darker appearance and deeper staining reaction 
of the dark dise and of the contraction band may be due, in 
part at least, to the segregation (demonstrated by Menten) of 
chlorides, phosphates, and potassium salts in the dark disc in 
the relaxed sarcomeres and in the contraction band in the con- 
tracted sarcomeres. 
4. A beaded condition of the sarcostyle is not an index of a 
phase of contraction, nor is it the result of contraction. It is 
