Amnion Formation In Mouse Embryos, Illustrated By Transverse Sections. Transverse sections of embryos between E6.5 and E7.5 illustrate the different stages of amnion development. Each section plane is represented by a corresponding line in the picture of a matching whole-mount embryo. Artificial colors as in Figure 2. No attempt has been made to distinguish the head process and any definitive endoderm from the embryonic mesoderm and visceral endoderm, respectively. (A) Early streak stage: extraembryonic mesoderm has inserted between extraembryonic ectoderm and visceral endoderm at the posterior side of the embryo. (B) Midstreak stage: extracellular spaces are present within the extraembryonic mesoderm. (C) Late streak/no bud stage: lacunae in extraembryonic mesoderm coalesce to form the exocoelomic cavity (EC). (D) Between late streak/no bud and late streak/early bud stage: the EC has enlarged. (E) Late streak/early bud stage: the EC extends laterally around the egg cylinder, converging on the anterior midline at the embryonic-extraembryonic junction. (F) Between late streak/early bud and neural plate stage: the proamniotic cavity (PAC) is constricted by the embryonic ectoderm and extraembryonic ectoderm of the amniochorionic fold, preceding the closure and separation of amnion and chorion. The allantoic bud (Al-bud) is prominent. (G) Neural plate stage: the amnion (Am) and chorion (Ch) are completely segregated dividing the proamniotic cavity into the amniotic cavity (AC), exocoelomic cavity and ectoplacental cavity (EPC). Scale bar: 200 μm.