Xenopus laevis
The rat ("Rattus norvegicus") is particularly useful as a toxicology model, and as a neurological model and source of primary cell cultures, owing to the larger size of organs and suborganellar structures relative to the mouse, while eggs and embryos from "Xenopus tropicalis" and "Xenopus laevis" (African clawed frog) are used in developmental biology, cell biology, toxicology, and neuroscience. Likewise, the zebrafish ("Danio rerio") has a nearly transparent body during early development, which provides unique visual access to the animal's internal anatomy during this time period. Zebrafish are used to study development, toxicology and toxicopathology, specific gene function and roles of signaling pathways.