40 H. E. JORDAN 
transiently a thick, less deeply staining (diluted) contraction 
disc bisected by the telophragma. ‘This dise (contraction band) 
then condenses about the telophragmata to form the deeply 
staining, relatively thin, contraction band of the fully contracted 
fiber (fig. 44). The ‘diluted’ condition of the contraction disc 
described above (figs. 42 and 43) represents the so-called ‘homo- 
geneous’ phase of contraction. It is only rarely conspicuous in 
certain fibers, and seems to depend upon the rapidity of the 
contraction process. When the process is rapid, this very 
transient phase will generally escape detection, or may possibly 
be absent, and the halves of Q seem to have moved en masse 
against the telophragmata. 
In the contracted sarcostyle of figure 44, an irregular cloudy 
line appears midway between successive contraction bands. 
This may represent a remnant of deeply staining material of the 
original Q-dise of the relaxed fiber, clinging to the mesophragma. 
It is noteworthy also that the telophragma is not discernible 
in this later contraction band (compare figs. 42 and 44). The 
masking of the bisecting telophragma may be due to its having 
become closely adherent to, and so hidden by, one of the halves 
of the deeply staining contraction band, or to the fact that it 
has been stretched in the shortened, widened, contracted sar- 
comere to a point where it is no longer conspicuous under the 
ordinary powers of the microscope. The paired constitution of 
the contraction bands is a fact of prime importance in con- 
nection with our interpretation of the several varieties of the 
simple type of intercalated disc as irreversible contraction bands. 
These illustrations supply the answer to another important 
question regarding the structure of the sarcostyle, namely, 
whether it is naked or enveloped by a ‘sarcolemma.’ Meigs" 
states that the sarcostyle of the fly’s wing muscle is homo- 
geneous in structure and is not surrounded by a membrane. 
The modifications produced by hypotonic solutions (figs. 29 to 
32) and by alcoholic fixation (figs. 34, 35, 41, and 43) demonstrate 
the fact that the sarcostyle has a physically different peripheral 
layer which performs the function of an osmotic membrane. 
