Abstract
Diplomyelia (duplication of the neural tube or spinal cord) was studied histologically in nine cases of T/+ mouse embryos at 10–15 days of gestation. Grüneberg investigated T/+ fetuses and interpreted the extra neural tube to be notochord, but a reexamination of this material demonstrated that the interpretation is incorrect. Diplomyelia is produced in the mutations Fused, Kinky, vestigial tail, homozygous Brachyury, and t‐haplotypet9 and in the new mutation, NM 529, described here.