Zebrafish young. Coloured scanning electron micrograph (SEM) of newly-hatched zebrafish (Danio rerio) young or fry. These fish are used to study embryonic development. The eyes (pink) can be seen in the heads. 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 nemotode worms. They are cheaper than mice, but not as well understood. Zebrafish are freshwater fish that are found in India. Magnification: x30 when printed 10cm wide.