Fig. 6. Cellular migration in the posterior half of 6.2 d.p.c. embryonic discs as determined by DiI labelling and culturing for 8 (A-C), 12 (D-H) and 16 hours (I-M). Dark-field photographs taken immediately after the injection (A,D) and after culture (B,E,I). DiI deposits are numbered in the order of injection in each embryo and appear with a shadow image under asymmetrical dark field illumination (A,B,I) and ring-like under circular illumination (D,E). Direction and distance of dye spread (white bars in B,E,I), as determined in fluorescence micrographs of embryonic discs mounted on microscope slides (C,F,G,J-L). (C) Dye spread in posteromedial direction (white line) following injection of bolus #1 (position indicated by white ring) placed mediolateral and anterior to the PGE of embryo shown in A,B. The DiI bolus was lost during mounting after removing the zona pellucida but left a mark of reduced cellular density seen in DAPI labelling (see inset). Translocated DiI appears as minute spots in the area of the PGE; these lie in one plane except for some spots that produce typically larger out-of-focus images (arrowhead). (F,G) Dye spread in posteromedial (F) and anteromedial direction (G) after injection anteromedially (#1) or posteromedially (#2) in the PGE in the embryo shown in D,E. (H) DAPI-labelled frozen section (position marked with broken line in F) showing epiblast (arrowheads pointing downwards) and not hypoblast cells (arrowheads pointing upwards) being labelled with DiI. (J,K) Dye spread after injections near the lateral margin anterior to the PGE (#1), posteromedially in the PGE (#2) and in the anterior centre of the PGE (#3) in the embryo shown in I. Injection #3 was inadvertently positioned between epiblast and hypoblast and gave rise – in addition to the typical spot-like labelling (compare C,F,G) – to complete cells being labelled (e.g. arrowhead). (M) Nomarski image of semi-thin section (position marked with broken line in K) of the specimen in K after photoconversion. DAB precipitates are found in the strongly labelled (hypoblast) cells only (arrowheads as in H; m, mesoderm). (L) Posterior and some anterior dye spread after injection similar to #3 in I and after development to late stage 3 (dark-field view of embryo not shown). Scale bars: 300 µm in A,B,D,E,I; 55 µm in C (inset 80 µm); 75 µm in F,G; 20 µm in H; 70 µm in J-L; 35 µm in M.