Three-dimensional anatomy of the 8-day mouse concepts: a study by scanning electron microscopy.

The three-dimensional anatomy of the 8-day mouse concepts was studied by scanning electron microscopy aided by microdissection within the microscope specimen chamber. Attention was given to the relationship of the extra-embryonic membranes and their subtended compartments and particular emphasis was placed on the 'inverted' condition of the embryo at this stage of development. The main points brought forth in this study are: (1) the five basic brain segments are discernible on the basis of surface contour; (2) the cervical fold, extending from the ventrum of the metencephalon to the somatopleure, forms a partition which separates the branchial region from the rest of the amniotic space; (3) the procephalic membrane bifurcates in a vertical plane to form the splanchnopeure and somatopleure lateral to the forebrain, and it bifurcates horizontally to form the dorsal and ventral coverings of the pericardial coelom; (4) the anttrum of the pericardial-peritoneal canal opens into the lateral coelom posterior to the cervical fold; (5) the midgut of the embryo is delineated laterally by longitudinal grooves connecting the foregut and hindgut antra; (6) embryonic ectoderm in the neural-fold region is formed by a single layer of pseudostratified columnar cells; (7) the allantois is hollow near its base and the inner surface is formed by a discontinuous layer of squamous cells; (8) blood islands in the extra-embryonic mesoderm form a ring of bulges around the middle of the exocoel. Other structures such as the ectoplacental cavity, the ectoplacental cone and the parietal capsule are also described.