Embryologists historically used the appearance of somites within the mesoderm layer as an accurate "clock" of early embryonic development. Classic chicken staging was by the number of somites present at a particular time. This led to the question of how this "clock" was actually leading to the pattern we observe being generated in the unsegemented presomitic mesoderm (PSM). Today we know some of the signaling processes (Notch and Wnt signaling) involved in establishing pattern in the mesoderm.
This current page has a number of research movies and teaching animations that illustrate mesoderm development.
Masamizu Y, Ohtsuka T, Takashima Y, Nagahara H, Takenaka Y, Yoshikawa K, Okamura H, Kageyama R. Real-time imaging of the somite segmentation clock: revelation of unstable oscillators in the individual presomitic mesoderm cells. Proc Natl Acad Sci U S A. 2006 Jan 31;103(5):1313-8. Copyright (2006) National Academy of Sciences, U.S.A (More? Mesoderm Movies - Hes1 | abstract excerpt)
The movie shows real-time images of the presomitic mesoderm (PSM) expression of Hes1 (helix-loop-helix transcription factor) involved in Notch signaling for the somite segmentaion clock in a embryo (day 10.5). Images were taken over a period of 15 h, with oscillations propagated from the caudal end to S0 in the PSM. Movie © National Academy of Sciences (2006)