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The pathogenesis of vertebral defects in mouse embryos is reconstructed after a single intraperitoneal injection of 6-aminonicotinamide, fifteen milligrams per kilogram, on Day 9.5 of pregnancy. This period, at about mid-pregnancy in the mouse, was found to be the critical stage for production of lower vertebral malformations in the embryo.Various kinds of defects—fusions, hemivertebras, hypoplasias, spina bifidas—were observed from lower thoracic through the sacral regions. Most vertebral malformations were associated with rib deformities. Cellular damage of the ventral region of the neural tube rather than sclerotomes was the most striking finding within a day or two of treatment.Vertebral malformations were preceded by failure of condensations of sclerotomal cells which later become intervertebral elements, and failure of such vertebral segmentation seemed to be due to the affected neural tube.