Weaknesses in the case against prepatterning in the mouse.

Serious weaknesses are exposed regarding recent claims to have demonstrated that there is no prepatterning of axes in the mouse. That the orientation of the axis of polarity of the blastocyst is dictated by the shape of the zona pellucida is contradicted by systematic observations on living conceptuses. Moreover, this 'mechanical constraint' hypothesis fails to account for key findings on which the case for prepatterning is based.