THE NERVOUS SYSTEM. 503 



next the brain, and then extending through the pouch and finally nearly oblit- 

 erating its cavity (third month). The shape of the organ (the hypophysis) 

 formed by the union of these two parts is subsequently changed by its relations 

 to surrounding parts. Its posterior lobe is derived from the infundibular por- 

 tion, its anterior lobe from the pouch. 



An expansion of the floor of the brain caudal to the infundibulum has been 

 mentioned as the mammillary region. Subsequently there is formed from its 

 cephalic part another evagination, the tuber cinereum. The mammillary region 

 forms the mammillary bodies. The region caudal to the mammillary region 

 later receives many blood vessels, thereby becoming the posterior perforated 

 space. 



At the end of the fourth week the roof plate of the diencephalon is smooth. 

 At about this time the greater part of the roof expands, forming a median 

 longitudinal ridge (Fig. 426). This ridge, which remains epithelial throughout 

 life, is broader at its anterior end where it passes between the beginning pallial 

 hemispheres. As the roof plate expands further, the anterior part is next 

 thrown into longitudinal folds. The ridge forms the epithelial lining of the 

 tela chorioidea of the third ventricle (diatela). By further growth and vas- 

 cularization of its mesodermal covering at the beginning of the third month, 

 there is formed the chorioid plexus of the third ventricle (diaplexus). Lateral 

 extensions of the tela form the chorioid plexuses of the lateral ventricles (see 

 p. 5 17) . In the fifth week a protrusion appears at the caudal end of the median 

 ridge which is the beginning of the epiphysis. Soon after this, the furrow which 

 forms its caudal boundary extends forward along the upper part of the sides of 

 the walls, marking off a fold which is the lateral continuation of the median 

 protrusion. From the median protrusion is later formed the pineal body, 

 while from the lateral folds are formed the pineal stalk, and in front the 

 habenula, with its contained nucleus (ganglion) habenulce, and the stria 

 medullaris. Still further caudally, the anterior part of the mid-brain forms 

 a horseshoe-shaped fold the arms of which extend forward over the dien- 

 cephalon, ventral to the pineal folds. The median part of this fold forms the 

 anterior corpora quadrigemina. From its lateral extensions are formed the 

 anterior brachia of the anterior corpora quadrigemina, the pulmnar and the 

 lateral and medial geniculate bodies, all of which (pulvinar ?) later receive optic 

 fibers. The transverse furrow which forms the boundary between the rudi- 

 ments of the pineal body and of the anterior corpora quadrigemina marks the 

 location of the future posterior commissure (Figs. 426, 427 and 428). 



The part of the roof anterior to the pineal fold, as already stated, forms the 

 tela chorioidea of the third ventricle. Certain folds appear in it, however, 

 which are more clearly indicated in later stages of embryonic development 

 than in the adult and which probably represent structures already mentioned 



