Although the primary object of this Clinics issue is to delineate various aspects of stem cell biology in humans that may underlie clinical problems, most specialists will readily admit that the mouse is a relevant model, experimental studies from which have brought out critical understandings and suggested tactical approaches. I hope to make clear that other models, designed in other classes of vertebrates such as birds and amphibians, have yielded important clues toward unraveling the early ontogeny of the hematopoietic system, in particular the emergence of stem cells.