In vertebrates, the brain and spine develop from the neural tube of embryos. Spina bifida is a birth defect that results from incomplete closure of the embryonic neural tube. Although the cause of spina bifida is unknown, women with low levels of folic acid have a higher chance of giving birth to a baby with spina bifida. Spina bifida results from a defect in the development of the ________.
1) blastocyst
2) ectoderm
3) mesoderm
4) endoderm