Zebrafish can make a great research model. They share genetic features with many other organisms, and they undergo a rapid developmental process. For the first few days of their life, their bodies can remain transparent, so it's relatively straightforward to use fluorescent tools to highlight the molecular processes that are happening in the embryo.
In this video from The Company of Biologists, and produced by Benjamin Stevenson and colleagues, you can watch the action in a developing zebrafish embryo. The researchers are using a technique called lineage tracing; the early cells of the embryo were fluorescently labeled so they could be imaged with microscopy. As the labeled cells rapidly divide over and over again and the zebrafish grows, the fluorescent markers stay in the cells where they were first injected, indicating which new cells were created from those original cells - tracing their lineage.
Source: Development