Model organisms each have some particular experimental advantages which have enabled them to become popular among researchers. In one sense they are "models" for the whole animal kingdom, and in another sense they are "models" for human development, which is difficult to study directly for both ethical and practical reasons. Model organisms have been most useful for elucidating the broad nature of developmental mechanisms. The more detail is sought, the more they differ from each other and from humans. Plants:  Vertebrates:  Invertebrates:   Also popular for some purposes have been sea urchins and ascidians. For studies of regeneration urodele amphibians such as the axolotl "Ambystoma mexicanum" are used, and also planarian worms such as "Schmidtea mediterranea". Organoids have also been demonstrated as an efficient model for development. Plant development has focused on the thale cress "Arabidopsis thaliana" as a model organism.