Fig. 4. Embryos with one type of X-gal distribution after activation of Cre recombinase in a blastomere at the four-cell stage. (A-G) The results of embryo 4-7. In this embryo, X-gal-positive cells were distributed randomly following labeling at the four-cell stage. (A) Whole mount, (B-F) sections showing that the stained cells were in all cell layers. The numbers of positive cells were less than in those embryos labeled at the two-cell stage. (C) In the extra-embryonic ectoderm of this embryo, clusters of relatively large numbers of X-gal-positive cells were observed. (G) Quantitative analysis. X-gal-positive cells accounted for between 5% and 31% of the total cells depending on the tissue. (H-O) The results of another example, embryo 4-6, which showed a similar pattern of distribution of labeled cells. (L,N) Sections at different positions of the same embryo. (I,J,K) High magnifications of L. Histograms show that the contribution of labeled cells is between 5% and 41%, however localization to any specific tissue or position in the body was not observed. Scale bars: 300 µm (A), 150 µm (B), 30 µm (C).