Abstract: The sarcomere length in the sartorius muscle of the toad and the frog was measured with the laser diffraction technique. Measuremnts were made both in vitro and in situ. results are as follows. 1. In vitro experiments: In order to compare the sarcomere length in the two species, the sartorius muscle was fixed at its length in the functional position. The sarcomre length of the fixed-length muscle(SLf)was 2.7±0.1μm (n=11)in the toad and 2.32±0.04μm(n=11)in the frog. when the sartorius muscle was stretched from slack to break, the relationship between the sarcomere length and the muscle length is not linearas expected but sigmoid in shape. 2. In situ experiments: The in vitro finding that the SLf in the frog is smaller than that in the toad was confirmed in the in situ experiments. when the laser beam was moved in the medial to lateral direction along a line bisecting the muscle, the SLf was found to increase in value in the toad, from a value of 2.59±0.07μm(n=9)at the medial margin, to a value of 2.64±0.09μm(n=9)at the midpoint and to a value of 2.71±0.08μm(n=10)at the lateral margin. The difference in SLf was not apparent in the frog.

Key words: Sarcomere length, in situ, sartorius muscle, toad, frog