Chick somite determination: the role of factors in young somites and the segmental plate.

One class of hypotheses proposed to explain the anterior to posterior sequence of chick somite formation envisions the movement of morphogenetic substances or processes along the segmental plate prior to somite formation. An effort was made to demonstrate any such phenomena in the region of somite formation by placing grafts that contained forming somites into the segmental plates of cultured host embryos. No somite-inducing properties on the part of the grafts were detected. Host somite formation proceeded in a normal manner. In additional experiments, posterior portions portions of segmental plates were isolated from the more anterior parts of the segmental plate which might contain the hypothetical morphogenetic factors. It was found that anterior portions of the segmental plate representing as many as eight prospective somites could be removed without impairing the ability of the remaining posterior portion to form somites in vitro. It is concluded that other hypotheses of the type studied here are not likely to be useful and that the present data further strengthen the argument that the cells of the chick segmental plate present at any time between the 8- and 20-somite stages of development are committed to form somites. In addition, the explant experiments described here indicate that a somite pattern representing about 11 prospective somites, becomes established in the segmental plate.