St. Hilaire defines a monstrosity as a serious deviation from the specific type, complex, apparent on external view and congenital. His classification of anomalies and monstrosities has generally been followed by teratologists since his time. A large family of monsters is characterized by a defective closure of the cranial vault or the vertebral laminæ. There are many varieties according as the defect is in the cranial bones, the spine, both cranium and spine, or parts of each or both. In a large number the brain is entirely wanting or rudimentary. Such are the anencephali or derencephali, the commonest of all, the so-called frog or owl fetuses. In another class the brain is present, although the mal-development of the bones has caused all or part of it to lie outside the cranial cavity. Sometimes the posterior defect is confined entirely or mostly to the vertebral laminæ and we have spina bifida,
LEWIS HF. ON INIENCEPHALUS. JAMA. 1897;XXVIII(9):388–390. doi:10.1001/jama.1897.02440090006002b
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