Embryonically fused chimeras reveal that the spleen is the adult and long-term hematopoietic organ. Embryonic fusions were created by surgical juxtapositioning of the anterior and posterior halves from GFP⁺ embryos and either white or nucCherryRed⁺ embryos. Depending on exact positioning of the split, the liver and spleen can be derived from the same or differing halves yielding the same or differing colors of blood cell produced by each organ. (A) Half GFP⁺, half white wild-type axolotl with a GFP⁺ liver. (B) Half GFP⁺, half nucCherryRed⁺ larva. (C) Underside of the larva in B displaying how various tissues and organs are composed of genetically different cells. (D) Forelimb with GFP⁺ skin and muscle and Red⁺ bone. (E) GFP⁺ hematopoietic cells in the GFP⁻ tail of a 4-month-old fused chimera. (F) The GFP⁻ tail of a 9-month-old fused chimera that previously had GFP⁺ blood. (G-H) The liver and spleen of a GFP⁺ cephalic half and white tail fused chimera with no visible GFP⁺ blood in circulation. The liver is GFP⁺ but the spleen is GFP^- and has no GFP⁺ blood cells. (I-J) GFP⁺ and Red⁺ larva with a Red⁺ spleen and a GFP⁺ liver visible through the skin. (K) nucCherryRed⁺ spleen-derived blood cells visualized in the GFP⁺ Red⁻ skin of a green/red fusion animal at 7 months of age. (L) GFP⁺ liver-derived blood cells visualized in the GFP⁻ Red⁺ skin of the same fusion animal at the same time point as in K. In these fusions, the liver-derived contribution slowly diminishes over time. See Figure 7 and supplemental Figures 5 and 6. The FACS plots shown in Figure 7 and supplemental Figure 6 for the GFP/white fusion are of the peripheral blood in the same animal but taken several months apart. The FACS plot in supplemental Figure 6 was taken earlier and displays a greater percentage of GFP⁺ cells than the plot in Figure 7, again supporting the results observed in the pictures.