This is a short video clip of myotome formation in a zebrafish embryo — it’s the subject of an upcoming column in Seed, so I’m putting a short visual aid here.
If you’re totally mystified still, let me orient you. Here’s a whole zebrafish embryo. There’s a large yolk filling most of the center of the image, and the embryo itself arcs along the dorsal side, stretching from where I’ve labeled the eye to the tailbud.
Along the length of the trunk, you might be able to see some periodic seams, a few of which I’ve marked with black arrows. These are the muscle segments or somites. They start forming in the trunk, and progressively pinch off new segments/somites sequentially from the undifferentiated cellular mass farther towards the tail until the final total of 30-34 are formed.
The blue box is the region where somites are still forming. What I did in the movie was to zoom into just that area and watch the process as it occurred, and then played it back at a higher speed.
At the start of the movie, the area in the blue box looks like this:
That blue arrow marks the position of the last formed somite boundary. Everything to the left is nicely blocked off into a series of somites (which are more clearly seen in this closeup). Everything to the right is relatively undifferentiated mesoderm.
40 minutes later, another somite boundary is formed (green arrow).
And 40 minutes later still, another boundary forms (red arrow).
Note that this movie shows a process that’s going on a little more slowly than you’ll find in the standard staging series for the zebrafish. The staging series is defined for 28.5°C, which is the temperature the animals prefer, and at that temperature they make a new somite every 20-30 minutes. This movie was made at room temperature, a little cooler than the standard, and development was concomitantly slowed.