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However, several early embryos in the I iterature show wellpreserved
brain tlssue protruding through the cranial defect, formîng
an exencephaly rather thên the anencephaly of later stages (Huntet, 1|g34-?¡51 .
1 ,6 .2 t4en î njc!:e I e , Enrephq I gmed rìgoce I e
ln another group of lesions cranium bifidum is accompanied by
hernîation of cranial contents,without direct exposure of the brain tissue.
A cranial meningocele involves. herniation of meninges through the skull
defect, and may be compared to a meningocele of the spine.
Protrusion of brain tlssue as wel I as meninges through a cranium
blfidum constîtutes an encepha I omen i ngoce I e. This is perhaps nÌo re comparable
to â myelocystocele than to a mye I omen i ngoce I e în the spine,
as the herniated brain is invariably enclosed by meninges and sometimes
covered by normal skin (Bal lantyne, l!04).
1.6.3 Cran ium Bifidum Occultum
Finally, examples of cranium bifîdum occultum,without -hern¡ation of
the underlying brain or meninges,are occasional ly seen (Caffe)r, 1972)..
1.7 DYSRAPHIC STATES
The dîversity of neural malformations reflects the complexity of
neural development. However, ê fundamental áistinction may be made between
open and closed defects of the central nervous system. .Spina biîida and
cranium bifidum constitote the open defects (or dysraphic conditîons),
and are of considerable clinical împortance.
Open defects have been described in both human and experimental
embryos at stages of development as early as the perîod of normal neural
closure. Any experimental study of the pathogenesis of spina bifida and