Fibrils from the indirect flight muscle of Drosophila melanogaster which have been teased into a solution containing 0.1 M KCl, 2 mM EDTA, 4 mM MgCl2, and 2.5 mM ATP at pH 7.0 can be made to shorten to 10 per cent of their initial length by reducing the level of ATP at a pH of about 8 or by briefly treating the fibrils with trypsin before lowering the level of ATP. Fibrils shortened in either of these ways, when dehydrated and immersed in nitrobenzene, display a strong positively birefringent band at the level of the Z band. In the trypsin-treated fibrils the width of this Z band increases as the fibril shortens. The data obtained are in agreement with the view that the positively birefringent Z band results from the interdigitation of A filaments in adjacent sarcomeres. With shortening to about 35 per cent of the initial length, the cytological pattern suggests that the A filaments of alternate as well as of adjacent A regions interdigitate.

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