342 



THE MORPHOGENESIS OE THE CENTRAL NERVOUS SYSTEM 



hemisphere). The median lamina between the hemispheres lags behind in its 

 development and thus there is formed the great longitudinal fissure between the 

 hemispheres. The lamina is continuous caudally with the roof plate of the dien- 

 cephalon; cranially it becomes the lamina terminalis, the cranial boundary of the 

 third ventricle (Figs. 332 and 342). 



Chorioid Plexus of the Lateral Ventricles.— It will be remembered that the 

 chorioid plexus of the third ventricle develops in the folds of the roof plate of the 

 diencephalon. Similarly the thin, median wall of the pallium at its junction 

 with the wall of the diencephalon is folded into the lateral ventricle. A vascular 



Fissura prima 

 Chorioid plexus of lal. ventricle 



Pallium 



Pineal body 

 Superior colliculus 



Corpus striatum 

 Hippocampus 



Roof plate 



Mesencephalon 



Fig. 345. — ^The fore-brain and mid-brain o£ an embryo 13.6 mm. long seen from the dorsal surface. 

 The pallium of the telencephalon is cut away, exposing the lateral ventricle (His). 



plexus, continuous with that of the third ventricle, grows into this fold, and pro- 

 jects into the lateral ventricle of either side (Figs. 345 and 347) . The fold of the 

 pallial wall forms the chorioidal fissure and the vascular plexus is the chorioid 

 plexus of the lateral ventricle. This is a paired structure and with the plexus 

 of the third ventricle forms a T-shaped figure, the stem of the T overlying the 

 third ventricle, its curved arms projecting into the lateral ventricles just caudal 

 to the interventricular foramen. Later, as the pallium extends, the chorioid 

 plexus of the lateral ventricles and the chorioidal fissures are extensively elongated 

 into the temporal lobe and inferior horn of the lateral ventricle (Fig. 348). 



