Fig. 2. Relationship between lineages of the pre-implantation blastocyst and post-implantation egg cylinder with summary showing asymmetric distribution of visceral endoderm cells at E5.5 and E6.5. (A-C) Core of the ICM in the blastocyst (A, blue) contributes to the epiblast of the egg cylinder at E5.5 (B) and E6.5 (C). Primitive endoderm in the blastocyst (A, yellow) contributes to parietal endoderm (not shown) and visceral endoderm of the egg cylinder (B,C). Polar trophectoderm (A, green) develops into extra-embryonic ectoderm of the egg cylinder (B,C). Colour has been added to DIC images of embryos to indicate lineage relationships between cells. (D) Schematic representation (box-whisker plots) of findings of Weber et al. (Weber et al., 1999) tracing the fate of ICM cells near the polar body (as indicated by the green star) (N/PB, blue) and away from it (A/PB, red) during development from blastocyst to egg cylinder stages. Bar indicates median; the lower and upper limit of the boxes and their whiskers illustrate 25%, 75% and entire range of distributions, respectively. N/PB descendants in visceral endoderm tend to be distributed more distally (embryonic), and A/PB descendants are distributed more proximally (extra-embryonic). This reciprocal fate of visceral endoderm descendants is already apparent at E5.5 and accentuates with development to E6.5.