330 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



cephalon (Fig. 332). A dorsal depression separates the latter from the mesenceph- 

 alon. The lateral wall of the diencephalon is thickened to form the thalamus, 

 the caudal and lateral portion of which constitutes the metaihalamus. From the 

 metathalamus are derived the geniculate bodies. In the median dorsal wall, near 

 the caudal boundary hne of the diencephalon, an outpocketing begins to appear 

 in embryos of five weeks (Fig. 332). This is the epithalamus, which later gives rise 

 to the pineal body, or epiphysis. 



The thalamus is marked off from the more ventral portion of the diencephalic 

 wall, termed the hypothalamus, by the obhquely directed sulcus hypothalamicus 

 (Fig. 341). Cranial to the optic chiasma is the optic recess, regarded as belonging 

 to the telencephalon (Fig. 332). Caudal to it is the pouch-Kke infundibulum, an 

 extension from which during the fourth week forms the posterior lobe of the 

 hypophysis. Caudal to the infundibulum the floor of the diencephalon forms the 

 tuber cinereum and the mammillary recess; the walls of the latter thicken later and 

 give rise to the mammillary bodies. An obKque transverse section through the 

 telencephalon and hypothalamic portion of the diencephalon (Fig. 343) shows the 

 relation of the optic recess to the optic stalk, the infundibulum, and Rathke's 

 pocket, and the extension of the third ventricle, the proper cavity of the dien- 

 cephalon, into the telencephalon between the corpora striata. 



The mesencephalon in 13.6 mm. embryos (Fig. 332) is distinctly marked off 

 from the metencephalon by the constriction which is termed the isthmus. Dorso- 

 lateral thickenings form the corpora quadrigemina. VentraUy, the mesencephahc 

 wall is thickened to form the tegmentum and crura cerebri. In the tegmentum are 

 located the nuclei of origin for the oculomotor and trochlear nerves. The former, 

 as we have seen, takes its superficial origin ventrally, while the trochlear nerve 

 fibers bend dorsad, cross at the isthmus, and emerge on the opposite side. As 

 the waUs of the mesencephalon thicken, its cavity later is narrowed to a canal, 

 the cerebral aqueduct (of Sylvius). 



The walls of the metencephalon are thickened dorsaUy and laterally to form 

 the anlage of the cerebellum. Its thickened ventral wall becomes the pons 

 (Varolii). Its cavity constitutes the cranial portion of the fourth ventricle. 



The caudal border of the pons is taken as the ventral boundary line between 

 the metencephalon and myelencephalon. The myelencephalon forms the medulla 

 oblongata. Its dorsal wall is a thin, non-nervous ependymal layer, which later 

 becomes the posterior medullary velum. From its tliickened ventro-lateral 

 walls the last eight cerebral nerves take their origin. Its cavity forms the greater 

 part of the fourth ventricle which opens caudally into the central canal of the 



