The crosses, T/+ or T/t^w2 ×btm/btm, give rise to 50% incidence of the tailless mice, development of which was investigated. No difference was seen in external appearance of the embryos at 9 days of gestation. However, some embryos showed fusion of the notochord and the neural tube at the posterior part of the body on the histological examination. The prospective tailless individuals were distinguishable from the normal littermates by the constriction of the root of the tail at 10 days of gestation. Thereafter, they showed several abnormalities such as the poor growth of the posterior part of the body, thinning of the tail and a blood blister at the tail tip or in the lumbosacral region. The abnormal embryos of 11-12 days showed severer abnormalities in the medio-dorsal area, i.e., the notochord was branched or degenerated at several places and the neural tube was distorted, duplicated or fused with the mesenchyme. All the tailless newborn young had blood blisters or red scars on the dorsal skin at the middle of the lumbosacral region. Histologically, the spinal cord posterior to the lumbosacral level was revealed to be severely distorted or duplicated and completely devoid of the bony vertebrae, and the dorsal blood blister was found to be the meningomyelocele derived from the abnormal development of the spinal cord. Skeletal abnormalities of the tailless young were as follows. The sacral and caudal vertebrae were absent. The cervical vertebrae were mostly normal, but the thoracic and lumbar vertebrae showed several abnormalities such as fusion of the ribs, lack of the vertebral body and vertebral arch.