An experimental analysis of somite segmentation in the chick embryo.

In a previous paper it was suggested that collagen fibrils play an important role in the process of somite segmentation. This paper was designed mainly to test that concept. In one series of experiments, embryos were treated with either alpha, alpha'-dipyridyl or L-azetidine-2-carboxylic acid, which are analogues that interfere with the formation of normal collagen. The reagents led to a reduction in the numbers of somites that formed, as well as to the production of other anomalies such as overall diminution in size and retardation. The older the embryo at the time of treatment, the further posteriorly were the major anomalies located. It is concluded that these results lend some support to the concept. In a second series of experiments an incision was made along one side of the neural tube and notochord to separate it from the segmental plate on one side. The result was that many more somites formed on the unoperated (control) side of the embryo than on the operated side. It is concluded that these results also lend support to the concept; but that they are of interest also in relation to the mechanisms involved in the control of somite numbers. In a third group of experiments, attempts were made to obtain somites in the absence of endoderm. Although this was not possible using surgery, it was achieved by treating the young embryos with U.V. irradiation. It was concluded that the presence of endoderm is not essential for the segmentation of mesoderm.