Light micrograph (SEM) of a zebrafish (Danio rerio) embryo. This fish is used to study embryonic development. Zebrafish eggs are transparent. This allows observation of the differentiation of the cells into different body organs. Zebrafish also develop rapidly (2-4 days) and the eggs develop outside the mother's body. As vertebrates, these fish are more appropriate models for some genetics research than fruit flies and nematode worms. They are cheaper than mice, but not as well understood. Zebrafish are freshwater fish that are found in India.