To study the mechanisms controlling embryonic development, experimental analyses must be performed on animal models. There are different species that are currently used as models in developmental biology. In invertebrates, drosophila remains a very useful model thanks to its rapid development and to the extensive study of genes involved in the control of its development. C. elegans has been extensively used because of its simplicity due to the total restricted number of adult cells in this species. In vertebrates, amphibians remain a very useful model with the extensive analysis of Xenopus laevis. The zebrafish is now a widely used model because of its genetic analysis. Birds are still used for the accessibility of their embryo allowing experimental procedures to be performed in ovo. The mouse is now very widely used because it is possible to obtain transgenic mice to study both loss or gain of gene function.