18-1 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



In the perfectly unaltered stat'j 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 

 Avhich, 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 similar 

 clefts interposed between these, but narrow and merely 

 linear throughout. Sometimes these clefts contain fine 

 granules. Thirdlj", even in the perfectly fresh muscle, 

 extremely faint parallel longitudinal striae 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 pol3'gonal areas 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 



