STRIPED MUSCLE OF WASP 59 
fiber has become greatly disturbed. Moreover, the optically 
inactive character of certain sarcostyles may be explained on 
the basis of an inability on the part of the sarcoplasmic particles 
to become similarly oriented in the dark discs immediately after 
return from contraction to relaxation. 
7. That anisotropy and the deep-staining character of the 
dark dise are two essentially distinct phenomena seems proved 
by the fact that the sarcostyle of the wasp’s wing muscle is only 
very feebly anisotropic, while the dark disc stains quite as 
intensively as that of the fibers, like those of insect leg muscle, 
where the anisotropy is relatively intense and most sharply 
segregated. 
8. Reversal of striation concerns only the deeply staining 
substance of the dark disc, not at all the phenomenon of aniso- 
tropy. The contraction band is a genuine new structure, not 
an optical illusion. It is essentially the same in a single sarcostyle 
as in a fiber-complex of sarcostyles, where increase of intersar- 
costylic fluid at the levels of the telophragmata might be con- 
ceived to contribute to an optical effect (as claimed by Schaefer) 
giving the impression of the formation of a new structure. The 
contraction band, in wing, leg, and cardiac muscle, consists 
essentially of the dark staining (chromatic) substances origi- 
nally segregated within the dark disc of the relaxed fiber. 
9. The simplest type of intercalated disc is identical with a 
contraction band. Both structures consist of horizontally aligned 
modified portions of sarcostyles (myofibrils), the whole bisected 
by a telophragma. In view of the fact that the contraction band 
is actually composed of two relatively independent portions 
(that is, two halves of different Q-discs), it is easy to conceive of 
an independent reversal of these constituents in relaxation. 
Failure of reversal of one half of a contraction band would result 
in the variety of simple disc bounded on only one side by a telo- 
phragma. The more complex types are readily derived from this 
simplest type through the subsequent operation of principally 
mechanical factors. The new evidence here given furnishes 
further support to our hypothesis that the intercalated discs of 
striated muscle (cardiac and skeletal) are essentially modified 
irreversible contraction bands. 
