In vertebrates, definitive hematopoietic stem cells (HSCs) first emerge in the ventral wall of the aorta in the Aorta-Gonad-Mesonephros (AGM) region of the embryo, where they differentiate from a specialized type of endothelium termed Hemogenic Endothelium (HE). While the transition from HE to hematopoietic tissue has received much experimental attention, much less is known regarding generation of HE itself. The current study investigates the emergence of the HE in the chick embryo aorta. Using the HE marker Runx1 as well as a new chicken-reactive antibody to the endothelial marker VE-Cadherin, we document the relationship between the emerging HE and surrounding tissues, particularly the coelomic epithelium (CE) and CE-derived sub-aortic mesenchyme. In addition, the fate of the CE cells was traced by electroporation of a GFP-expressing plasmid into the CE, followed by analysis using immunofluorescence and in situ hybridization. We make the novel observation that CE-derived mesenchyme transiently invades through the ventral wall of the aorta during the period of establishment of HE and just prior to the emergence of hematopoietic cell clusters in the ventral aortic wall. These observations emphasize a hitherto unappreciated dynamism in the aortic wall during the period of HE generation, and open the door to future studies regarding the role of invasive CE-derived cells during aortic hematopoiesis.