184 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
In the perfectly unaltered state of the muscle no other 
transverse markings than these are discernible. But it is 
always possible to observe certain longitudinal markings ; 
and these are of three kinds. In the first place, the nuclei 
which, in the perfectly fresh muscle, are delicate trans- 
parent oval bodies, are lodged in spaces which taper off at 
each end into narrow longitudinal clefts (fig.52, A,B). Pro- 
longations of the protoplasmic sheath of the fibre extend 
inwards and fill these clefts. Secondly, there are simila1 
clefts interposed between these, but narrow and merely 
linear throughout. Sometimes these clefts contain fine 
granules. Thirdly, even in the perfectly fresh muscle, 
extremely faint parallel longitudinal strie 1-7,000th 
of an inch, or thereabouts, apart, traverse the several 
zones, so that longer or shorter segments of the 
successive septal lines are inclosed between them. A 
transverse section of the muscle appears divided into 
rounded or polygonal arez of the same diameter, sepa- 
rated from one another here and there by minute inter- 
stices. Moreover, on examination of perfectly fresh 
muscle with high magnifying powers, the septal lines are 
hardly ever straight for any distance, but are broken up 
into short lengths, which answer to one or more of the 
longitudinal divisions, and stand at slightly different 
heights. 
The only conclusion to be drawn from these appear- 
ances seems to me to be that the substance of the muscle 
is composed of distinct fibrils ; and that the longitudinal 
