The Localization of Precursor Cells for Larval and Adult Hemopoietic Cells of Xenopus Laevis in two Regions of Embryos

For determination of the localization of lymphoid and erythroid precursor cells in embryos of Xenopus laevis, diploid‐triploid chimeras were produced either by joining embryos antero‐posteriorly or by orthotopic grafting of various tissues into Nieuwkoop‐Faber st. 22–23 tailbud embryos. The sources of the hemopoietic cells were determined in the chimeric animals at various stages by microspectrophotometry of Feulgen‐stained cells. Analyses of chimeras produced by joining embryos antero‐posteriorly at different levels showed that the precursor cells that contribute to the hemopoietic cells are localized in the posterior half to three quarters. Orthotopic grafting of ventral or dorsal tissues revealed that the precursor cells that contribute to hemopoietic cells in early larvae are mostly localized in the ventral blood island (VBI) mesoderm, whereas those for late larvae and adults are localized both in the dorso‐lateral plate (DLP) mesoderm comprising the prospective mesonephros and in the VBI mesoderm. Reciprocal heterotopic grafting of VBI‐ and DLP mesoderms showed that the two compartments differ in their capacities to differentiate into hemopoietic cells. It is proposed that the VBI‐derived cells migrating towards the primary lymphoid organs constitute the transient hemopoietic population of early larvae, and the importance of the mesonephric region for definitive hemopoiesis is pointed out.