Sea urchin embryo. Immunofluorescence micrograph of a sea urchin embryo at the 8-16 cell stage. The orange structures are the cell nuclei. The bound- aries between different cells show up as green lines. Following fertilization an embryo undergoes multiple rounds cell division, dividing each time into double the number of cells (1, 2, 4, 8 ...). Eventually a hollow ball of hundreds of cells is produced, which then starts to differentiate. This picture was made by exposing the embryo to fluor- escent antibodies that bind to certain proteins in the cell. The embryo was then viewed with a laser- scanning light microscope, which makes the anti- bodies fluoresce. Magnification unknown.