342 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



the diencephalon ; cranially it becomes the lamina terminalis, the cranial 

 boundary of the third ventricle (Figs. 332 and 342). 



The Chorioid Plexus of the Lateral Ventricles. It will be remembered 

 (p. 338) 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 plexus, continuous with that^of 

 the third ventricle, grows into this fold, and projects into the lateral 



Fissura prima 

 Chorioid plexus of lat. ventricle 



Pallium r 



Pineal body 

 Superi-or colliculus 



Corpus striatum 

 Hippocampus 



Roof plate 



Mesencephalon 



FIG. 345. The fore-brain and mid-brain of an embryo of 13.6 mm., seenfrom the dorsal surface. 

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



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). 



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

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

 boundaries grow more slowly than the rest of the telencephalon (Fig. 347) . 



