336 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



the basal plates ventral and somewhat cranial to the nucleus of the oculomotor 

 nerve. The origin of the cells forming the red nucleus is not definitely known. 

 The alar plates form the paired superior and inferior colliculi which together 

 constitute the corpora quadrigemina (Fig. 331). The plates thicken and neuro- 

 blasts migrate to their surfaces, forming stratified ganglionic layers comparable 

 to the cortical layers of the cerebellum and the cerebellar nuclei. With the 

 development of the superior and inferior colliculi the cavity of the mesencephalic 

 region decreases in size and becomes the cerebral aqueduct. 



The mantle layer of the basal plate region is enclosed ventrally and laterally 

 by the fiber tracts which develop in the marginal zone. Ventro-laterally appear 

 the median and lateral lemnisci and ventrally develop later the descending tracts 

 from the cerebral cortex, which together constitute the peduncles of the cerebrum. 



Roof plate (with chorioid plexus) 



Alar plate orThalamus 



Sulcus limitans or 

 S. hypothalamics 



Basal plate or 

 Hypothalamus 



~ Mammillary recess 

 Fig. 322. — Transverse section through the diencephalon of a five weeks' human embryo (His). 



The Diencephalon. — In the wall of the diencephalon we may recognize 

 laterally the alar and basal plates, dorsally the roof plate and ventrally the floor 

 plate (Fig. 322). The roof plate expands, folds as seen in the figure, and into the 

 folds extend blood capillaries. The roof plate thus forms the ependymal lining 

 of the tela chorioidea of the third ventricle. The vessels and ingrowing mesenchy- 

 mal tissue form the chorioid plexus. Cranially, the tela chorioidea roofs over the 

 median portion of the telencephalon and is folded laterally into the hemispheres 

 as the chorioid plexus of the lateral ventricles. Laterally, the roof plate is attached 

 to the alar plates and at their point of union are developed the ganglia habenulce. 

 The pineal body or epiphysis is developed caudally as an evagination of the roof 

 plate. Tt appears at the fifth week (Fig. 327) and is well developed, by the third 

 month (Fig. 324). Into the thickened wall of the anlage is incorporated a certain 

 amount of mesenchymal tissue and thus the pineal body proper is formed. The 



