26 H. E. JORDAN 
the dark disc, and in consequence cause it to decrease in length 
and increase in width (diameter), producing thus contraction of 
the fibers by reason of the beading and shortening of the com- 
ponent sarcostyles, represented in diagram B. 
I have added diagram C to complete the series of changes in 
the sarcostyle during contraction, in accord with the data to be 
presented in the descriptive portion of this paper. In order to 
use diagrams A and B in this series we must disregard the exag- 
gerated length of the sarcomere in A. My interpretation of 
these diagrams is then as follows: Sarcostyle B is in the relaxed 
condition; sarcostyle A in an early stage of contraction, as indi- 
cated by the presence of a median disc, and sarcostyle C is in 
the fully contracted condition. The contraction band (C.B.) 
of sarcostyle C includes fused opposite halves of adjacent Q 
dises, bisected by the telophragma. 
The common assumption that the substance of the light dise 
is relatively more fluid than the substance of the dark disc is 
also directly contrary to what the microscopic data seem to indicate. 
The principal transfer of substances during contraction is not 
from the telophragma to the mesophragma, as is required in 
the above diagram according to Schaefer, but in the opposite’ 
direction. The distortion effects of mechanical and osmotic 
factors show their respective initial modifications first in the 
dark discs, as is to be expected if this portion is relatively more 
fluid than the light disc. A relaxed sarcostyle placed under 
tension responds first by a lengthening of the dark disc. Under 
the dehydrating effect of fixation with the higher grades of 
alcohol, the dark dises respond by a shortening (condensation) 
relatively far in excess of that effected in the light dise. More- 
over, in stained preparations it can readily be seen that a dark- 
staining substance of the Q dise actually moves toward the 
telophragma during contraction, as indicated by a clearing of 
the median portion of Q and the knobbed character of the 
terminals of the Q portions of the sarcostyles. 
The foregoing discussion should have made it clear that the 
prevailing ideas of the morphologic changes suffered by striped 
muscle during contraction, as based largely on the illustrations 
