STRIPED MUSCLE OF WASP 39 
Figures 39 and 40 illustrate the same point in connection with 
a sarcostyles at an early stage of contraction. However, it does 
not seem to me that the above interpretation is quite adequate 
to explain all the variations in length of the dark dise in dif- 
ferent sarcostyles. It undoubtedly accounts for a considerable 
portion of this variation, but I feel constrained to conclude after 
careful and prolonged study of many relaxed sarcostyles, that 
the amount of deeply staining substance varies considerably in 
different fibers at the same functional stage in the same micro- 
scopic preparation. It seems to me fairly certain that the 
amount of dark and light substance varies within certain limits 
in different, apparently normal, sarcostyles at the same functional 
stages, and under the influence of apparently identical mechanical 
and osmotic conditions. 
Figures 40 and 41, at approximately the same functional 
stage (midphase of contraction), should be considered together. 
One has a straight contour, the other is slightly beaded. Figures 
42 and 43, at a later functional stage, illustrate the same point. 
The sarcostyle of figure 43 is at the same functional stage as the 
middle portion of figure 42; one sarcostyle is beaded, the other 
is not. These four figures (40 to 48) demonstrate again that 
beading is not an index of a distinct functional condition, that 
is, of degree of contraction; it is obviously a fixation artifact. 
This conclusion is rendered certain by the condition of the 
sarcostyle of figure 44. This sarcostyle is in full contraction and 
is characterized by conspicuous contraction bands, but it still 
maintains a straight lateral contour. 
To return to figure 42, the lower portion of the sarcostyle is at 
the same functional phase as the sarcostyle of figure 41. In the 
upper and middle levels two contraction bands are beginning 
to form. The upper one is at a later phase than the next 
lower, which shows thicker deep-staining terminal portions. It 
is clearly seen here that the formation of the contraction band 
results from a movement of the deeply staining substance of the 
dividing Q-dise toward the telophragma. But this material does 
not here move en masse, but in such manner as to produce 
