A CASE OF CYCLOPIA 233 



The major portion of the posterior two-thirds of the cerebral 

 rim on both sides has been found to represent a but shghtly 

 modified hippocampal formation (fig. 20). The line of attach- 

 ment of the thin roof (Ep.R.) corresponds to the edge of the lim- 

 bus medullaris or fimbria (Fm.). A well marked fascia dentata 

 {F.d.) is present, bounded laterally by the fissura hippocampi 

 (F.h.), and mesially by the fimbrio-dentate sulcus (S.F.d.). 

 Above the fimbria, the thin roof is invaginated by vascular tissue 

 to form a band of choroid plexus {Pl.Ch.L.). This 'choriogenous 

 zone' extends forward only as far as the extreme anterior tip of 

 the fascia dentata {F.d., fig. 19). It is to be noted that if the 

 normal lateral plexus could be straightened out it would repre- 

 sent an area such as is found here and bearing precisely similar 

 relations to the fimbria. 



Throughout the major portion of the anterior third of the cere- 

 bral rim, the fascia dentata can not be made out. Here the 

 limbus corticalis has not become rolled upon itself {Lh.C, fig. 18). 

 The limbus medullaris {Lh.M.) becomes on this account more 

 extensive. No choroid invaginations are present over this area. 



The identity of the limbus corticalis between the anterior ex- 

 tremities of the two definitely identified hippocampal areas is as 

 yet obscure. It is hoped that further study in comparison with 

 the other cases in my hands may clear up this point. 



As has already been mentioned, no medullated fibers are pres- 

 ent in this area of the cortex. In this connection it is to be noted 

 that there was a complete absence of medullated tissue through- 

 out the hippocampal area in the normal term foetus used as 

 control. 



Ventricular anomalies 



The cavity of the fourth ventricle {Vent. IV) is essentially 

 normal in its relations. It is lined throughout by a well devel- 

 oped layer of ependyma. 



Turning now to the aqueductus {Aq.C, figs. 34 to 42), it is 

 to be noted that numerous irregular ependymal diverticula arise 

 from its ventral and lateral walls in the region just caudal to 

 the posterior commissure. One of these evaginations passes 



