324 JOHN BERRY HAYCRAFT, 
Moreover, the fibre should be slightly stretched, and as small as 
possible. 
It has also previously been mentioned that in many prepara- 
tions the fibres split up transversely in a most regular manner, 
and, unless the cover-glass be pressed upon, the little discs 
remain in position with narrow chinks between them. These 
chinks will be filled with the isotropous fluid used for mounting, 
which will lead to very anomalous appearances, and which may 
perhaps help to account for some of Briicke’s statements. These 
fallacies may be avoided by a study of the fresh fibres of insects’ 
muscle. Dytiscus and hydrophilus muscle has received a large 
share of the attention of histologists, but that from the wasp or 
blue-bottle fly is quite as good. A leg should be pulled from 
the trunk of a blue-bottle fly, and this again forcibly separated 
at the middle joint. A piece of muscle will project from one of 
the segments, which may be cut off and examined in a drop of 
fluid expressed from the thorax of the fly. The polariscopic effects 
may then be made clearly out in the still contracting fibres. I 
have tested all these points by a careful examination of insects’ 
fibres with thin plates of selenite and mica. This method is not 
so satisfactory, nor do the differences of colour seem to give 
such reliable evidence as may be obtained by the crossed Nicols 
alone. 
The fibre during contraction.—Living insects’ muscle may be 
examined and the changes observed when the waves of contrac- 
tion pass along the fibre, or, perhaps better still, they may be 
fixed with osmic acid. The muscles from the leg of an insect 
are rapidly separated out on a slide, and a drop of weak osmic 
acid added, which kills the fibres instantaneously, fixing them 
in the position that they happen to be in. On examination one 
generally finds fibres which in part of their course are contracted, 
and in other parts relaxed, when the differences in appearance 
may readily be studied. It may here be observed that the 
fibres bulge at the contracted part, so that if the surfaces be 
examined the focus of the microscope must be accommodated. 
The cross stripes are nearer one to another and correspond, as 
before, with the ridges and valleys seen at the margin, which are 
much more prominent and bolder in outline. 
In the contracted fibre the striping is practically the same as in 
the stretched condition. The contracted fibre exhibits just the 
same reversing of stripes on alteration of focus, and Dobie’s line 
and Hensen’s stripe can both be seen in the same positions as in 
the uncontracted muscle, provided the fibre is suitably placed for 
‘examination, and not sheared in its length. We must entirely 
deny the common statement, first introduced, we believe, by 
Engelmann, that in the contracted state the bright band becomes 
