part of develops from ends during starts during duct of Cuvier anterior cardinal vein medial cardinal vein pronephric sinus xenopus_anatomy XAO:0000385 UBERON:0006174 A system of vessels that initially swell from where the anterior and posterior cardinal veins meet before draining into the heart via the duct of Cuvier at NF stage 33 and 34, and then continue to narrow and fold into a characteristic S-shape by NF stage 37 and 38. It continues to loop on itself, at first a loose, then a dense knot of many vessels by NF stage 45. pronephric kidney anatomical space NF stage 31 NF stage 55