Gastrulation stage of mouse embryo.
A. Cells of the primitive endoderm colonize the blastocoelic surface of the mural trophectoderm and become the parietal endoderm, which, together with the trophoblast that derives from the mural trophectoderm, forms the parietal yolk sac.
B. As the epiblast and extaembryonic ectoderm become elongated into a cylinder, the visceral endoderm covers their outer surface.
C. The trophoblast and parietal endoderm together with the thick basement membrane between them surround the entire embro, and is later removed (indicated by a dotted line) in panels F-H.
D. Gastrulation stage in which the primary germ layers are formed (ectoderm, mesoderm and definitive endoderm), from which all the fetal tissues will develop.