Abnormalities of notochordal cells and of mesenchymal cells in embryos of Danforth's short-tail (Sd) and C57BL mice were examined by means of electron microscopy and cytochemical staining at 11.0 and 11.5 days of gestation. In abnormal (Sd/+; Sd/Sd) embryos, the notochordal cells were markedly deficient in bundles of filaments and lacked surface protrusions, and the notochordal basal lamina was continuous; in contrast, notochordal cells of normal (+/+) littermates and of C57BL embryos contained numerous bundles of filaments and showed fingerlike surface protrusions and discontinuous basal laminae. The pathologic notochordal cells also lacked the accumulations of glycogen revealed in the normals by means of thiocarbohydrazide cytochemical staining at the electron microscopic level. The mesenchymal cells of abnormals also were deficient in filaments but did stain for glycogen, though less prominently than did normal mesenchymal cells.