THE LEG-MUSCLES OF THE WATER-BEETLE. 
443 
The Querwand or Zwischenscheibe may probably be similarly explained ; for where it 
appears the rows of rod-heads of adjacent series are so drawn away from one another by 
the extension of the fibre that their bright borders are no longer blended, and a dim 
line (in all probability rendered darker by interference) comes into view between them. 
The argument in favour of the existence of a continuous membrane across the fibre, 
drawn from the fact that indentations of the sarcolemma pretty constantly occur at 
this place, loses its weight when we reflect upon the extreme delicacy of the sarcolemma, 
and the necessity which must exist for it to follow any changes of shape which may 
occur in the enclosed substance. 
Description of the Plate. 
PLATE XXXIII. 
Fig. 1. Portion of a muscular fibre of Dytiscus in the state of rest. 
a a a. Dim stripes. 
b b b. Bright stripes. 
c. Double dotted line in bright stripe formed by the heads of 
d. The muscle-rods. 
Fig. 2. Muscular fibre undergoing contraction. 
It It. Portions still at rest. 
P P. Contraction proceeding. 
C. Contracted portion. 
aa r Dim stripes. 
b b t b ir Bright stripes. 
c c t c H . Lines formed by rod-heads. 
Fig. 3. Transverse section of muscular fibre of Dytiscus. The irregular patch in the 
centre, containing a round nucleus, is the so-called central protoplasm. 
Fig. 4. Transverse section of muscular fibre of Water-beetle, made whilst frozen, 
showing the appearance of radiating lines described in the text. 
