Fig. 1. (A,B) Video images showing a cast of the vascular bed of the main vessels in the head of a Xenopus laevis tadpole using the digital motion-analysis technique. In the anaesthetized tadpole, the movements of the erythrocytes are digitally contrasted. The arrows point to the location where vessel diameter measurements were made by defining a threshold for the grayscale value and applying the ‘rectangle-fit’ algorithm. This automatically defined the best-fitting rectangle covering the blood vessel within this region of interest. The length of the short axis of this rectangle was taken as vessel diameter (see Materials and methods for details). (A) Before endothelin application; (B) after endothelin (ET-1, 10^–6 mol l^–1) application. ha, head artery. Image width, 1.6 mm.