186 E. A. Schäfer, 



nary muscles, it is conceded that even in the living condition the 

 tissue exhibits fine longitudinal lines which under favourable conditions 

 may be seen to run continuously from end to end of the fibre (Fig. 5 A). 

 These lines most commonly have enlargements upon them at regular 

 intervals corresponding with the position of the clear transverse stripe, 

 and the enlargements are usually double, i. e., are disposed in double 

 rows in the clear stripe. Sometimes there is a distinct interval be- 

 tween each two enlargements, occupying the middle of the clear 

 stripe, often this is absent and the double rows of enlargements touch 

 one another in the middle of the clear stripe or even blend into a 

 single row. The ordinary appearance of these muscles is represented 

 in Fig. 1. It will be seen that the continuity of the longitudinal 

 lines is here somewhat masked, near and between the enlargements, 

 by the peculiar effect which the enlargements produce upon the 

 light 1 ); but they can often even in the living fibre be seen to be 

 continuous (Fig. 5 A) and after treatment with acid the continuity is 

 very conspicuous (Figs. 6 and 24). 



If the muscular fibre be treated with dilute acid, either while 

 still fresh, or as Eollett recommends, after they have been a short 

 time in alcohol, the ordinary dim and bright cross-striation of the fibre 

 almost or entirely disappears, the muscular substance becomes clear, 

 and the longitudinal lines with their enlargements are rendered very 

 distinct (Figs. 6, 10, 24). A fine transverse line is now seen passing 

 exactly across the middle of what was previously the bright stripe, 

 and lying between the double row of dots. In fibres such as those 

 of Dytiscus, Vespa and Bombus which have the transverse section 

 shown in Figs. 2 and 16, the double rows of dots are replaced, when 

 the middle plane of the fibre, with its longitudinal series of central 

 nuclei, is exactly in focus, by double transverse lines of a thickness 

 corresponding to the size of the apparent dots (Fig. 5i>). This appea- 

 rance of the middle planes of these fibres is not correctly represented 

 by any of those who have described the structure of insect muscle, 

 being, so far as I can find, always delineated of the same appearance 



l ) The causes of this masking will be afterwards referred to. 



