It is very labor intensive to lineage an embryo‘s development. If you skip one day, you lose track of some cell divisions (Sulston, 1983). Now, with the help of advanced technologies and software development, researchers are able to record the development of an embryo as a 4-dimensional (3 dimensions in space + time = 4 dimensions) movie. They then are able to observe and annotate the movie using programs such as VisBio.
This is a 4D movie that was composed by 3 focal planes – focal plane 2, 4 and 6 (for demonstration purpose we only picked 3 focal planes to use). Hit play, then try to go up, down, forward and backward to see how the movie changed with the changing depth and time. For instance, you might see nuclei in focal plane 6 but not in focal plane 2 and 4.