La formation des somites chez l’embryon d’oiseau: étude expérimentale

ABSTRACT Somitogenesis in the avian embryo: an experimental study The determinism of somitogenesis has been studied. A complete segmental plate can form somites even when isolated from cord, neural tube and somites. Somites are always formed if the segmental plate remains in continuity with the somites which have just formed. When the segmental plate is separated from the axis and deprived of its apical end, it undergoes no differentiation or differentiates incompletely. In the latter case the quality of differentiation decreases in a cephalo-caudal direction. When the segmental plate, without its apical end, is left in contact with neural tissue or chorda, it forms somites normally. The growth of the segmental plate is reduced whenever it has been isolated from neural tube and chorda altogether. Behind its apical end the segmental plate can regulate excesses as well as deficiencies. It appears, then, that the intersomitic fissures are determined at the apical ends of the segmental plates, but not behind them. This determination is progressively established backwards. It can be transmitted by the last differentiated as well as the differentiating somites. However, the neural tube, and possibly the cord, play the leading role in this process, for the intersomitic clefts are fixed at their proper place in connexion with them.