338 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



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. 



— Alar plate 



fKr -Marginal layer 



Nucleus 

 N. III. 



Root fibers N.I II. 



Fig. 339. — ^Transverse sections through the mesencephalon of a 10.2 mm. embryo (His). A, 

 Through the isthmus and origin of the trochlear nerve; B, thit>ugh the nucleus of origin of the oculomotor 

 nerve; D. IV, decussation of oculomotor nerve; MJ., mantle layer. 



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. 340) . 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 



Roof plate {with chorioid plexus) ■ 



Alar plate or Thalamus 



-^Sulcus limitans or S. hypothaiamicus 

 Basal plate or Hypothalamus 



~ Mammillary recess 

 Fig. 340. — Transverse section through the diencephalon of a 13.8 mm embryo (His). 



X 29. 



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. 



