340 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



cephalon, cranially it becomes the lamina terminates, the cranial boundary of the 

 third ventricle. 



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

 the folds of the roof plate of the diencephalon develops the chorioid plexus of the 

 third ventricle. Similarly the thin median wall of the pallium at its junction 

 with the wall of the diencephalon is folded into the lateral ventricle. Into this 

 fold grows a vascular plexus continuous with that of the third ventricle and pro- 

 jects into the lateral ventricle of either side (Figs. 327 and 329). The fold of the 

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



Fissura prima 

 Chorioid plexus of lat. ventricle 



Pallium 



Pineal body 

 Sup. colliculus 



Corpus striatum 

 Hippocampus 



Roof plate 



Mesencephalon 



Fig. 327. — The fore-brain and mid-brain of 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 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. 330). 



The interventricular foramen (of Monro) is at first a wide opening (Fig. 

 325) but is later narrowed to a slit, not by constriction but because its boundaries 

 grow more slowly than the rest of the telencephalon (Fig. 329). 



