Exencephaly, Pseudocephaly, and Anencephaly
In exencephaly, the brain is formed without the brain casing - the cranium. It is generally missing the forebrain (the prefrontal lobe), but is otherwise mostly formed. Despite this, the spinal cord is rarely formed with the brain, as there is a disconnect between the telencephalon and the hindbrain during very early development, and only the midbrain is formed.
In pseudoencephaly (a term rarely used in medicine nowadays), the midbrain and spinal cord are formed, but the forebrain is not.
In anencephaly, the most common of the three conditions, the brain and spinal cord are only tiny specs of what they should be. Curiously, despite them being a literal extension of the brain, the eyeballs sometimes fully form, though with no receptors to process the information they receive, even if the fetus were to survive, they would not see anything.
Human Monstrosities, Part IV. Barton Cooke Hirst and George A. Piersol, 1893.