An Experimental Study of the Development of the Notochordal Sheath

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Abstract
Pieces of presumptive notochord of amphibian embryos, carefully cleaned of other mesodermal cells, fail to form notochordal sheath when cultivated in isolation, but a sheath is formed around them when they are transplanted to any other site in the embryo in which they come in contact with mesoblastic cells. The sheath is absent from those regions of transplanted notochords which are in contact with non-mesodermal tissues such as endoderm. No sheath is formed around pieces of human hair placed in the position from which the notochord has been excised. If the presumptive notochord material is excised from late gastrulae of Trituras, a new notochord may be formed by regulation from the more lateral mesoderm. The regulated notochords are always provided with a sheath, and conversely the sheath is absent whenever the notochord is missing. It is concluded that the sheath is laid down by extra-chordal mesoblastic cells, which are induced to carry out this secretion by contact with the notochord. It is pointed out that this contradicts the conventional view held by most comparative anatomists, who have attributed the secretion of the sheath to the external layer of notochord cells (the ‘notochordal epithelium’).

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