Limits of region A and B in early primitive streak embryos.

*Developmental stage indicated as Hamburger Hamilton (HH) stages [30].∧Developmental stage indicated as Primitive Streak (PS) stages [31].Table indicates the length of the primitive streak during development from stage HH3 (PS2) to HH3+ (PS4) and the limits of region A and region B along the anterior-middle primitive streak. An eye piece reticule with 100 subdivisions (0 to100) was used to delineate region A and region B. The length of the primitive streak is shown in the number of subdivisions and in microns (µm). The limits of region A and region B for each developmental stage are shown as subdivisions. The primitive streak of all embryos used in the study was measured to determine developmental stage and a total of 348 embryos were labelled and analysed to determine the limits of each region. Labelling experiments were performed by either injecting vital fluorescent dyes at one to three subdivisions within each region or by grafting GFP fluorescent primitive streak tissue from most of region B of a donor embryo into the equivalent region B of a host wild type embryo.