Among the most dramatic malformations of the body and central nervous system (CNS) are those that involve abnormal lateral separation of the neural tube along its long axis. Often referred to as “monsters” in the old literature, these include the spectra of conjoined (or conjoint) twinning and holoprosencephaly. In the former, the body axis splits inappropriately, conceivably anywhere along the long axis, leading to duplication of body parts. In the latter, the prosencephalon fails to separate normally (Figure 38.1). Close embryologic connections between the brain and face dictate that many of these disorders have abnormalities of the face (Figure 38.2).