Cells near the opposite end, the 'vegetal' pole, become other parts, or grow into entirely separate structures to nurture the embryo. The growing embryo is organized around an axis between these two poles. Shift the cells of an embryonic frog around, and the future frog will be scrambled, too. Mammals with placentas, like mice and humans, have embryos that make no particular organizational commitment early on. This was thought to mean that the cluster of cells of the early embryo had no particular plan either.
Mammals do have the same structure that marks the animal pole in frogs.
But this 'polar body' seems to exert little control over the destiny of the mammalian cells. The cells can be moved, removed or even swapped with others and the resulting mouse is just as healthy as its untouched brother. Magdalena Zernicka-Goetz at the University of Cambridge, UK, and her colleagues used fluorescent proteins to track the development of cells in mouse embryos from only a few cells to when a hollow ball of 32 cells implants itself in the uterus. To test whether the cells retained an organizing pattern throughout development, the team made combined embryos made up entirely of animal or of vegetal pole cells. But the resulting embryos still grew into normal healthy mice.