The human genome can create cells with a remarkable range of capabilities and shapes. Looked at under the microscope, the enterocytes which line the gut bring to mind the ghosts from a Pac-Man video game. Neurons look like medieval morningstars, but with long hair. But none is more special than the big, round egg. It is not just the largest of the cells. It is also, once fertilised, the cell from which all other sorts of cell are descended. The size of the egg allows it to carry the nutrients and chemical building blocks needed for the first days of…