Fig. 7. Circulation through segments with functional connections to both the dorsal aorta and the posterior cardinal vein. (A) The ventral half of a primary segment connected to both the dorsal aorta (red) and the posterior cardinal vein (blue). Blood flow through the vessels is noted (gray arrows). Numbers shown are fraction of 21 vessels examined that initially (at 2 dpf) had functioning dual connections that later resolved in favor of either an intersegmental artery (top, 11/21) or an intersegmental vein (bottom, 10/21) between 2 and 4 dpf (see text for details). (B-D) Representative examples of `dual connection' segments in which blood is flowing from dorsal aorta to posterior cardinal vein. Images shown (top) are lateral views of the trunks and tails of fli-egfp transgenic zebrafish embryos at approximately 2-2.2 dpf. (C,D) The vessels on opposite sides of the tail of at the same anterior-posterior position. Images were collected by multiphoton microscopy. Accompanying illustrative diagrams (bottom) show blood flow patterns through the dual-connected intersegments (flow direction is noted with red arrows). 3D reconstructions of these images are available at http://dir.nichd. nih.gov/lmg/uvo/ISV3_D.html Movies showing blood flow through these vessels are available at http://dev.biologists.org/supplemental/.