a, Mid-sagittal view of a human (CS6b/7) and mouse (E 7.0) embryo at the onset of gastrulation highlighting the morphological differences as well as corresponding tissue types. In the human, the endoderm adjacent to the epiblast is the Hypoblast but extends to line the extraembryonic secondary yolk sac. In the mouse, the endoderm surrounding the egg cylinder is termed visceral endoderm but extends to line the trophoblast and is termed parietal endoderm. b, Schematic diagrams highlighting differences in the size of human and mouse embryos at comparable stages of development. The dotted lines represent mesoderm migration, which due to both size and morphology is further in the human than in the mouse.