STRIPED MUSCLE OF WASP 53 
midphases of contraction, frequently show relatively elongated 
sarcomeres. It must again be emphasized that a condition of 
stretching may be superimposed on sarcostyles at any functional 
stage. This fact must be. kept in mind in interpreting the 
structure and length of sarcomeres of any particular sarcostyle 
or fiber. The effect of stretching is first shown in an elongation 
of Q. If the sarcostyle is passing into contraction, that is to say, 
when it is in a condition where a median (H) disc is present, the 
stretching effect shows itself largely in a widening of the median 
disc, that is, causing a wide separation of the two portions of Q). 
This result has led to the general opinion that stretching causes 
a separation of Q along the mesophragma with the appearance 
of an H-dise. This is the basis also for the interpretation of a 
fiber with an H-dise bisecting the Q-disec as one in extension. 
However, the evidence is fairly complete that the relaxed con- 
dition of the fiber is characterized by an undivided Q-dise. 
Superposition of stretching upon a relaxed condition causes 
chiefly a lengthening of Q. The presence of an H-dise indicates 
not extension nor stretching primarily, but the intermediate con- 
traction phases. Stretching of a sarcostyle in this condition 
produces chiefly a lengthening of the H-disc. 
The unmodified fully contracted fiber has a sharp lateral 
contour (figs. 8, 33, and 44). Beading is not an index of con- 
traction, as claimed by Ranvier,?° Schaefer,?’ Meigs, McDou- 
gall,“ and others. Contraction is accordingly not simply a 
matter of the imbibition of fluid by the sarcostyle. It is a 
matter associated with the division of the deeply colored sub- 
stance of the dark dise and its subsequent movement against 
the telophragma, resulting in the production of contraction 
bands. . 
Meigs has made one of the most consistent attempts to 
support by morphologic data the imbibition hypothesis of muscle 
contraction essentially as proposed by McDougall. Meigs 
summarizes the results of his study of the muscle of the fly and 
of the frog by stating a series of objections against the opposing 
hypothesis, namely, that contraction results from fundamental 
intrasarcostylic reactions. Meigs says that ‘The latter hypoph- 
