330 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



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 obliquely directed 

 sulcus limitans (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-like infundibulnm, 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 oblique 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 pouch, and the extension of the third ventricle, the proper 

 cavity of the diencephalon-, 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. 

 Ventrally, the mesencephalic 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 walls of 

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

 cerebral aqueduct (of Sylvius) . 



The walls of the metencephalon are thickened dorsally 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 thickened 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 spinal cord, cranially into the 

 cerebral aqueduct. The increase in the flexures of the brain and the relative 

 growth of its different regions may be seen by comparing the brains of 

 embryos of four, five, and seven weeks (Fig. 333). 



