STRIPED MUSCLE OF WASP 15 
condition of the fiber and so, by reason of an inherent elasticity, 
permits the mesophragma to contract and coarsen to the point 
of possible observation. Such observation is further aided 
by the special functional conditions obtaining in this portion of 
the fiber; the presence of the H dise at the midphase of con- 
traction allows the darker mesophragma to stand forth more 
conspicuously within the clear (H) portion of the dark (Q) dise. 
Figure 3, of a fixed and stained muscle fiber from the lobster’s 
antenna, interpreted as relaxed above and contracted below, 
according to Dahlgren and Kepner,? shows with exceptional 
clearness exactly the same conditions with respect of the undi- 
vided character of the dark disc of the sarcomere in repose, and 
the steps in the formation of the contraction band. However, 
this illustration presents one serious difficulty: while the sarco- 
mere of the relaxed portion measures in the figure 11 mm. in 
length, that of the portion interpreted as in contraction measures 
only 3 mm. less. But the length of a contracted sarcomere as 
compared with its relaxed associate is approximately two-thirds 
less. Since the character of the ‘striations’ of this fiber indicates 
contraction at the lower end, the nearly equal length of the 
sarcomeres at the two ends must be explained on the basis, 
supplied by the data relating to the sea-spider leg muscle, of a 
superimposed tension at the lower end. 
Figure 4, showing a wave of contraction passing over a living 
(unfixed and unstained) leg-muscle fiber of the beetle (Dytiscus 
marginalis), according to Schaefer,?®> demonstrates the same 
essential points regarding the formation of the contraction band, 
namely, an origin by fusion along the telophragma of the opposite 
halves of successive dark discs. It illustrates likewise a genuine 
reversal of striation, contrary to the opinion of Schaefer, who 
interprets the specimen as demonstrating that the apparent 
reversal is not real, but simply an optical illusion. Schaefer’s 
explanation of the basis of this ‘optical illusion,’ however, har- 
monizes better with our description of what occurs during 
contraction than with his description of the essential phenomenon 
of contraction in wasp’s wing muscle with which the Dytiscus 
leg-muscle is said to agree. He says, ‘‘The dark bands of the 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 1 
March, 1920 
