328 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



ventricles and these communicate through the interventricular foramina (Monroi) 

 with the cavity of the diencephalon, the third ventricle. The cavities of the ol- 

 factory lobes communicate during fetal life with the lateral ventricles and were 

 formerly called the first ventricles. 



The crossing of a portion of the optic nerve fibers in the floor of the brain 

 forms the optic chiasma and this, with the transverse ridge produced by it inter- 

 nally, is taken as the ventral boundary line between the telencephalon and dien- 

 cephalon (Fig. 314). 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 metathalamus. From the 

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

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

 in embryos of five weeks (Fig. 314). 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 hypothalamicus. 

 Cranial to the optic chiasma is the optic recess, regarded as belonging to the 

 telencephalon. Caudal to it is the pouch-like 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 oblique transverse section through the telencephalon 

 and hypothalamic portion of the diencephalon (Fig. 325), 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 diencephalon, into the telen- 

 cephalon between the corpora striata. 



The mesencephalon in 13.6 mm. embryos (Fig. 314) is distinctly marked off 

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

 laterally thickenings form the corpora quadrigemina. Ventrally, the mesenceph- 

 alic wall is thickened to form the tegmentum amd crura cerebri. In the tegmen- 

 tum 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 



