Fig. 7. Cellular proliferation after pulse BrdU labelling in vitro. (A) Stage 1 with uniform labelling of the embryonic disc and background labelling in the bordering extra-embryonic tissues. A' and A'' anterior and posterior border, respectively, of embryo in A to show unlabelled (horizontal arrowheads) and labelled (vertical arrowheads) nuclei. (B,C) Early and late stage 2, respectively, with PGE (anterior border marked with broken line) less densely labelled than remainder of embryonic disc. Vertical bar indicates position of section shown in F. (D,E) Early and late stage 3, respectively, with intense labelling of AMC (compare with C) and primitive streak. Vertical bar indicates position of section shown in G. (F,G) Posterior sagittal sections from embryo in C,D, respectively, confirm the absence of mesoderm cells at stage 2 (C,F) and vigorous mesoderm ingression at early stage 3 embryo (D,G). At stage 2 (F), labelled nuclei are almost exclusively found in the epiblast (e) and at stage 3 (G), the hypoblast (h) is still unlabelled, while many nuclei are labelled in epiblast (e), in ingressing (i) and definite (m) mesoderm. Double asterisks mark posterior embryonic border at the junction with the extra-embryonic trophoblast. Scale bars: 250 µm in A-E; 60 µm in A' and A''; 30 µm in F,G.