An embryo floats in a bath of warm liquid. Transparent, folded into itself, like all embryos. There’s the large head with a round eye in it, the red heart and the spinal cord that extends, vertebra by vertebra, to the curled-up tail. Anyone might think it’s a regular embryo (human or mouse – at this stage of development both species sport a tail). But no. This embryo is not developing within a mother’s womb, but between the glass sides of a revolving vial, at a temperature of 37 degrees Celsius (98.6 Fahrenheit), set by a faithful thermostat.