In the mammalian embryo, the first axis to appear is at the time of the fifth cell division when the inner cell mass (ICM) becomes visible. The localization of the ICM on one side of a cavity formed within the cluster of dividing cells marks the embryonic-abembryonic (E-Ab) axis. This name derives from the fact the most of the embryo will develop from the ICM, whereas other tissues (the placenta, etc.) will develop from the other cells. There has been a long-standing controversy as to what determines the mammalian E-Ab axis; is the information inherently in the zygote, or is it determined after several cell divisions? In an elegant series of studies whereby dividing cells were labeled using new molecular genetic tools and then carefully followed during development, Yoko Kurotaki, Kohei Hatta, Kazuki Nakao, Yo-ichi Nabeshima, and Toshihiko Fujimori have provided an answer in a mouse model.