Forebrain in human anencephaly.

Gross and microscopic study of twelve human anencephalics revealed existence of the forebrain with variable growth in all specimens, meaning thereby closure of the neural tube at its cephalic extremity. This is supported by the findings of choroid plexus, pineal tissue and rudimentary pallium within the anterosuperior margin of a median dorsal opening situated behind the forebrain mass. The opening bears an intimate relation to the undue dorsal convexity of the sphenooccipital junction, and leads cranially to a cavity representing the third and lateral ventricles. This opening is situated in the posterior part of the roof of diencephalon, and seems to be the primary site of disturbance causing anencephaly. Weakness of the diencephalic roof is indicated by its epithelial nature and its ballooning in cyclopia and cebocephaly. A corresponding potential weakness in the overlying tissues is evidenced by the sagittal fontanelle of early fetal life and by congenital scalp and skull defects of the parietooccipital region.