E9.5 EKLF^−/−KLF2^−/− yolk sac blood islands have abnormal morphology. Panel A is a representative section from a wild type, panels B and C are from 2 different EKLF/KLF2 double knockout, and panel D is from a KLF2^−/− embryonic yolk sac. The abbreviations used are columnar epithelium (Ep), erythroid cells (Ery), endothelial cells (En), mesothelial cells (Me), endothelial/mesothelial layer (En + Me L), and ingrowth of epithelial cells (IGEp). Photographs were taken at 400 × magnification. See “Analysis of EKLF^−/−KLF2^−/− embryos” for more image information.