transition. In adults, haematopoiesis occurs in the red bone marrow, in the core of most bones. The red bone marrow is derived from the layer of the embryo called the mesoderm. Recent study marks the first global discovery of hematopoietic stem cell (HSC) niches within invertebrate skeletons--overturning the long-held belief that skeletal hematopoiesis is unique to vertebrates.