Failure to separate into hemispheres.

Alobar: complete failure, no falx, single mono-ventricle, fused thalami.

Semi lobar: Partial separation of the posterior occipital and temporal lobes. Frontal brain is fused, thalami partially fused.

Ace of spades configuration of the ventricles.

Lobar: fusion of only anterior inferior frontal lobes so no faux in that location. Otherwise the brain appears to be quite normal except for lack of septum pellucidum.

Septal optic dysplasia: most mild form in which there is no septum pellucidum and the optic nerves are very atrophic. Schizencephaly may be present in fifty percent of these cases. Corpus callosum agenesis may also be seen with this entity.

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