The cells of individual somites in 2-day-old chick embryos were marked by injecting a fluorescent dye into the somitocoele. This procedure permanently marked the cells and allowed their subsequent development and distribution to be followed. The cells were found to remain in close association with each other within limited boundaries and did not mix to any great extent with similar cells from adjacent somites. Fluorescent cells from single somites were found in the intervertebral disc, connective tissue surrounding two adjacent neural arches, all the tissues between the neural arches, the dermatome, and the associated myotome. No fluorescent cells were found in the notochord or in any nervous tissue apart from accompanying connective tissue. Surprisingly, the vertebral bodies and neural arches did not contain any fluorescent cells apart from those in the connective tissue surrounding them, but this absence of fluorescent cells was thought to be due to the dilution of the fluorescence following cell proliferation. These results provide further experimental support for the theory of resegmentation in vertebral formation, and also provide evidence of a compartmental method of development along the rostrocaudal axis in vertebrates, similar to that already discovered in insects. On the basis of cell lineage criteria, the sclerotome might be considered as a developmental compartment.