The effects of somite removal on vertebral formation in the chick.

We have examined the effects on vertebral development of various combinations of somite removal in two day old chick embryos as shown by vertebral formation after a further seven days of incubation. Each combination produced one of a variety of results ranging from completely normal vertebral formation, through fusion of various vertebral elements, to the absence of complete vertebral halves and the formation of hemivertebrae. Assessment of our operating ability showed that we were removing at least 90% of the somitic material and therefore these results suggest that there is a regulating mechanism available to the embryo, at least with regards to vertebral development. When two consecutive somites were removed, vertebrae frequently developed that were lacking certain elements. This suggests that the somitic cells are already determined with regards to formation of specific vertebral elements. Experiments involving the removal of a bilateral pair of somites (a repetitive unit) also provided evidence of a counting mechanism which ensures that the correct number of total vertebrae are present.