Development of the mouse conceptus from the mid-primitive streak to early somite stages. All sections are longitudinal with the exception of (E) and (F ), which are frontal. (A) Mid-primitive streak stage conceptus (E7.25) with the proamniotic cavity (pc) lined by ectoderm cells. Mesoderm cells are beginning to form a cavity (arrow) that will expand to form the exocoelom (ex). (B) Late-primitive streak stage conceptus with an expanded exocoelomic cavity (arrow) in extraembryonic mesoderm. (C) Neural plate stage conceptus (E7.5). Extraembryonic mesoderm cells forming the lateral wall of the exocoelom are proliferating to form a mesodermal cell mass (arrow). (D) Higher magnification of a mesodermal cell mass, consisting of flattened mesothelial cells (arrowhead) lining the exocoelom and proliferating undifferentiated mesoderm cells (closed arrow). The mesodermal cell mass is apposed to the single cell layer of visceral endoderm (open arrow) with its brush border facing the yolk sac cavity. (E) Early somite stage conceptus with multiple yolk sac blood islands (2 blood islands are indicated with arrowheads). Trophoblast giant cells (arrow) are in contact with the maternal decidual tissue (d). (F ) Blood islands at the early somite stage consisting of differentiating erythroid cells (open arrow) and endothelial cells (closed arrow). ac, amniotic cavity; d, deciduum; ec, ectoplacental cavity; epc, ectoplacental cone; ex, exocoelomic cavity; pc, proamniotic cavity; ep, embryo proper. Each scale bar represents 100 μm and the magnification of (F ) is the same as (D).