338 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



caudal (Fig. 339 B). In addition to these nuclei of origin, the nucleus 

 ruber (red nucleus) is developed in 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 (Figs. 337 -B and 349). The plates thicken and neuroblasts 

 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 mesen- 

 cephalic 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. Ven- 

 tro-laterally, appear the median and lateral lemnisci; ventrally, the de- 

 scending tracts from the cerebral cortex, which together constitute the 

 peduncles of the cerebrum, develop later. 



Roof plate (with chorioid plexus) . 



Alar plate or Thalamus 



'ulcus limitans 

 Basal plate or Hypothalamus 



Mammillary recess 

 FIG. 340. Transverse section through the diencephalon of a 13.8 embryo (His). X 29. 



The Diencephalon. In the wall of the diencephalon we may recog- 

 nize laterally the alar and basal plates, dorsally the roof plate, and ven- 

 trally 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 of the tela chorioidea of the third ventricle. 

 The vessels and ingrowing mesenchymal tissue form the chorioid plexus. 

 Cranially, the tela chorioidea roofs over the median portion of the telen- 

 cephalon 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 habenula. 



The epiphysis, or pineal body, is developed caudally as an evagination 

 of the roof plate. It appears at the fifth week (Fig. 335) and is well 

 developed by the third month (Fig. 342). Into the thickened wall of the 



