Simple vertebrates such as the zebrafish (Danio rerio) and the clawed frog (Xenopus laevis) are also commonly used in scientific research. These animals are cheap, small, easy to keep, have a very short developmental period, and can produce large amounts of offspring (148). Like C. elegans, early stage embryos have a transparent egg, and the mature zebrafish or immature Xenopus tadpole are also relatively transparent, allowing internal structures to be imaged very easily. Because the stages of development are thoroughly understood and can be visualized easily, it is possible to expose embryos to ethanol during very distinct and short periods of development, which can be very important for determining critical periods of ethanol exposure (149).
