Open malformations of the central nervous system may involve the brain or spinal cord, or both. Preliminary experiments in which a window was cut in the shell overlying early chick embryos (with removal of 2 ml of albumen) produced a range of neural and non-neural malformations. Exposure of Stage 5--10 embryos at 26 hours of incubation produced open brain and cord defects. Embryos were recovered at 11--12 days for gross examination. Open cord defects in 12 day experimental embryos could be divided morphologically into 2 types. One group showed an everted symmetrical plaque of neural tissue. In the other group the cord defect was more irregular, partly covered by skin, and often combined with rump and trunk defects. Skeletal staining showed that vertebral lesions increased in severity in a cranio-caudal sequence. Spina bifida occulta was found in the cervical and upper thoracic regions; spina bifida manifesta, associated with open cord defects, occurred from the lower thoracic to the sacral regions; vertebral deletions were almost confined to the caudal region. Spina bifida manifesta at the site of open cord defects also showed 2 distinct patterns. Regular cord defects were associated with regular spinal defects, showing loss of spinous processes, reduction of laminae and eversion of the pedicles. Irregular cord defects were associated with more irregular spinal defects showing vertebral deletions or fusions, rumplessness, and pelvic reduction. Neither group, however, showed local kyphosis or scoliosis. Early neurogenesis in the avian and human embryos is very similar with development of the spinal cord from neural plate and tail bud materials which fuse in an overlap zone. These experimental defects in the chick embryo, separable into regular and irregular types thus provide a useful model for investigation of the embryogenesis of spina bifida.