476 WILLIAM A. HASWELL. 
with nuclei. There run along the fibre in nearly its whole 
length two clefts by means of which it is divided, save at the 
extremities, into two closely-approximated halves, each of 
which has an irregularly crescentic cross-section. The sub- 
stance of the fibre (fig. 6) (i.e. all except the granular core), 
is composed of striated muscle-substance. The dim bands are 
broad, four or five times the breadth of the bright bands. 
The latter exhibit in their centre a narrow band having the 
appearance of a double or triple line of dark granules; this 
dark line is particularly conspicuous in fresh specimens, and 
is more strongly marked than in the muscular fibres of any 
Crustacean or Insect I have examined ; it is also well seen in 
specimens preserved in alcohol and stained with hematoxylin 
(fig. 10), but varies in conspicuousness both in fresh and 
preserved specimens in different parts of the same fibre in 
accordance evidently with the state of contraction or relaxation 
of the part. In alcohol specimens teased and mounted in 
Farrant’s solution (fig. 9) some of the fibres are found to have 
the bright bands in the form of more or less prominent ridges 
without a trace of the dark line. In fresh specimens, again, 
fibres will be found in which the dim zones are thickened so as 
to confer an annulated appearance on the fibre, the bright 
zones forming annular constrictions in which the dark lines are 
very conspicuous (fig. 6). Hematoxylin stains the dim discs 
and the dark lines, leaving the substance of the bright zones 
unstained. The employment of polarised light brings out 
strikingly the ordinary contrast between the optical properties 
of the dim and of the bright bands. 
Longitudinal fibrillation is very distinct in all conditions of 
the fibres, and after treatment for a few days with weak 
chromic acid the constituent fibrille become very well defined 
and readily separable (fig. 11); each exhibits a very well- 
marked, fine, longitudinal striation. In such a preparation 
the continuity of the fibrille from one end of the fibre to the 
other is readily traceable, the transverse striations (i.e. the 
bright bands) being represented only by a simultaneous bend 
in the course of all the fibrils, together with an ill-defined 
