|A major benefit of using the zebrafish as a model is its rapid early development. For example, the time lapse movie to the right depicts the first 18 hours of development. By this time, the basic vertebrate body plan is recognizable and several organ systems are beginning to form. Just ten hours later, the embryo will have a fully functional circulatory system (bottom panel). An additional benefit is that zebrafish embryos are fertilized and develop externally allowing easy access to embryos for the purpose of time lapse analysis and other manipulations, such as microinjection with nucleic acids.