Model of SIVP development. In cross section and lateral view at early stages (A–B), Vegf is secreted by the pronephric ducts (green) while Bmp is secreted by the intestine (brown). VegfA (green arrows), promotes sprouting of cells to form the SIVP from the posterior cardinal vein (pcv, blue). Bmp4 expressed in the gut (brown arrows), promotes the migration of angioblasts from the vein. (C) At later stages, honeycomb-structured vascular compartments form. (D) The mature plexus comprises a remodeled inner and outer basket. Abbreviations: da (red): dorsal aorta; pd (green): pronephric duct; pcv (blue): posterior cardinal vein; (brown): gut; (yellow): yolk.