Recent advances in microscopy and in fluorescence labeling of proteins allow the observation of complex biological processes in living organisms. This movie shows the early stages of embryonic development of a fruit fly egg, which is half a millimeter in size. In the first part of the movie, the observation of fluorescently labeled histones (proteins that help packaging the chromosomes into a compact nucleoprotein structure in the cell) reveals the dynamics of the mitotic divisions of the nuclei that occur in the egg during the first two hours of development. Following the synchronous nuclear divisions, cellular membranes form and cell movements initiate a process called gastrulation, during which cells invaginate in the embryo to form tissue layers and organs following a genetically determined developmental program. Early steps of gastrulation are shown in the second part of the movie through fluorescence labeling of membrane proteins. As suggested in the movie by the biophysicist writing down differential reaction-diffusion equations on a blackboard, the ability to observe these kinds of processes at the molecular level and in real time has opened new venues in modern biology, by applying tools from mathematics and physics to understand complex biological processes in a quantitative manner.