Occlusion of circulation in the PHS does not affect early thymus colonization. At 52 hpf, just before the onset of thymus colonization by CD41-GFP^low cells, circulation in the PHS along the left thymus was selectively suppressed by laser-ablating many erythrocytes at the upstream position indicated by a red cross, until the cell debris stopped the blood flow. At 70 hpf, the embryos in which circulation in this vessel was still blocked were sorted, and the content in GFP^low cells of the left thymus was compared with that of the right thymus and with the left thymus of control siblings. (A) Scheme of the venous (gray) and arterial (black) circulation in the region of interest at 60 hpf, left lateral view,¹⁸  with the sense of relevant venous circulation indicated by red arrows, and the nascent thymus in blue. (B-F) Live CD41-gfp 70 hpf embryos. (B) Control embryo, left side; arrow points at a leukocyte rolling in the PHS, with the fast flow of erythrocytes evidenced by a reddish shade. (C) Sibling (left side) in which the left PHS was occluded 18 hours earlier (52 hpf) at the position indicated by the red cross; arrow points at stagnant erythrocytes in the occluded PHS (see also Video S10); (D) Right side of the same embryo, with full circulation in the right PHS; (C′,D′) Corresponding CD41-GFP^low cells in the left (C′) and right (D′) thymi. PHS indicates primary head sinus; OV, optic vein; CCV, common cardinal vein; PceV, posterior cerebral vein; thy, thymus; and ba, branchial arches.