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Abstract:
A recurring question in developmental biology has been whether localized determinants play any role in mammalian preimplantation development1. This is a controversial issue that brings back the idea of prepatterning2-4 and is explored further by Plusa et al.5, who claim it is the first cleavage of the mouse zygote that predicts the blastocyst axis, rather than the animal pole or sperm entry point, as previously suggested3. However, other evidence indicates that the blasotcyst axis is not predetermined and there is no prepatterning in the mouse egg6-12. Here we investigate the origin of these different views and conclude that they arise from differences in the data themselves and in their interpretation.