58 H. E. JORDAN 
an artifact, due either to the dehydrating action of certain 
fixing reagents like alcohol upon the relatively more fluid dark 
dises or to the endosmotic action of hypotonic solutions. In the 
former case the constriction separating successive ‘beads’ is at 
the level of the dark dise (mesophragma), in the latter at the 
level of the telophragma. 
5. The condition of bisection of the dark disc by a median 
(H) disc is not an index of extension or of stretching, as claimed 
by Rollet and by Schaefer, but of intermediate phases of con- 
traction. Error in the interpretation of this condition has 
resulted through disregard of the fact that a contracting fiber 
may be quite as readily stretched as a relaxed or a contracted one. 
Stretching produces its primary effect in the region of the dark 
dise causing a lengthening of this disc. At the beginning of 
contraction stretching seems to produce the median disc, while 
in reality it simply brings into clearer view, through extension, 
an extremely thin median disc already present as an accom- 
paniament of the initial phase of contraction. These consider- 
ations explain the apparently paradoxical condition of occasional 
greater length of sarcomere in a contracting sarcostyle as com- 
pared with the sarcomere of a relaxed sarcostyle. Stretching 
of a fiber at the beginning of contraction produces the illusion 
of the production of an H-dise by tension. 
6. The segregation of anisotropic materials (conditions) in 
strata alternating with isotropic levels, and corresponding more 
or less sharply, under certain conditions, with the dark disc, 
would seem to find its explanation in the relatively more fluid 
consistency of these dark dises. Interpreting the condition of 
isotropy in terms of similar orientation of the sarcoplasmic 
particles to lines of stress (following Ranvier’s explanation), it 
becomes apparent why in a structure like that of a functioning 
sarcostyle—where the particles suffer continual rearrangement 
during contraction and relaxation—an anisotropic arrangement 
could be more readily assumed in the relatively more fluid semi- 
solid dark disc. Along the same line follows the explanation for 
the more feebly anisotropic character of the contracted fiber, in 
which the original orientation of the particles of the relaxed 
