After the initial phase of rapid cell divisions, vertebrate embryos must go through a variety of cell movements to form the primary axis of the body and arrange cells to give rise to different tissues.  Here an embryo of the Discus fish (Symphysodon discus) is seen undergoing these essential cell movements in a process called epiboly, during which cells migrate from the top of the embryo to cover the yolk cell. By the end of this movie many of the major organ systems have formed, including the nervous system and muscles, as demonstrated by the embryo wiggling. If you look closely at the top of the embryo near the end of the movie you can also see the formation of the head and the beginning of the heartbeat. This movie was captured by bright field microscopy over the course of 22 hours.

More information about epiboly movements can be found here. 

Contributed by Zachary Collins, a third-year graduate student in the Biological and Biomedical sciences Program at Harvard Medical School.