 While the hemangioblast theory appears to be generally supported, most of the studies done have been "in vitro", indicating a need for "in vivo" studies to elucidate its existence.Definitive hematopoiesis then occurs later in the aorta-gonad-mesonephros (AGM), a region of embryonic mesoderm that develops into the ventral wall of the dorsal aorta, at E10.5 in mice and 4wpc (4 weeks post-conception) in humans.New HSCs either enter the aortic circulation or remain within the endothelium.