These cross-striated muscle fibers are derived from chickenembryo heart muscle. The cultured cells are able to periodically contract. The degree of shortening of these fibers during contraction is especially striking. The bright segments of the fiber, the I-bands, contain the actin. Through the middle of each of these runs a thin stripe, called the Z-line. The darker segments, or A-bands, are where the myosin filaments are localized [1].

During contraction, here shown in slow motion, the A- and I-bands slide into one another. This results in the disappearance of the I-bands and the widening of the Z-line, although the width of the A-band remains constant [2].