Figure 3.

Illustration of the concept of an embryonic-extraembryonic continuum onto which all tissues of the developing embryo can be mapped. Only selected tissues are shown as examples. Tissues are initially composed of cells with both embryonic and extraembryonic potential, and they gradually move towards either extreme, depending on the proportion of their constituent cells with particular fates. Examples of exclusively embryonic or extraembryonic tissues are not as common as previously thought. (TE, trophectoderm; ICM, inner cell mass; ParE, parietal endoderm; VE, visceral endoderm.)