GFP expression in circulating blood cells in Tg(pax2a:GFP) embryos. Fluorescent microscopy images of a live, prim-11 stage (27 hpf) Tg(pax2a:GFP) embryo showing expression of GFP in circulating blood cells (arrows). The embryo was anaesthetised with tricaine and images were taken at consecutive time points (in order A, B, C, D) approximately one second apart. GFP-labelled blood cells can be seen moving posteriorly through the dorsal aorta and returning anteriorly through the posterior cardinal vein. The embryo is oriented with anterior to the left and dorsal up. Scale bar = 50 μm.