THE BRAIN 



339 



The pineal body, or epiphysis, 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 anlage is incorporated 

 a certain amount of mesenchymal tissue and thus the pineal body proper is 

 formed. 



The alar plate is greatly thickened and becomes the anlage of the thalamus 

 and metathalamus. The latter, really a part of the thalamus, gives rise to the 

 lateral and median geniculate bodies. 



The sulcus hypothalamicus (Fig. 341) forms the boundary line between the 

 thalamus (alar' plate) and the hypothalamus (basal plate plus the floor plate). 



Suh us Inpollulamuus 

 \ 

 Pallium \ 



Hypothalamus 



Manimillary recess 

 Coypus striatum j I njinidibulum 



Optic ridge 



Fig. 341. — Median sagittal section of the fore- and mid-brain regions of a brain from a 10.2 mm. embryo 



(after His). 



' This sulcus thus corresponds to the sulcus limitans of the spinal cord and brain 

 stem. The basal plate is comparatively unimportant in the diencephalic region, 

 as no nuclei of origin for motor nerves are developed here. In the floor plate 

 the ridge formed by the optic chiasma constitutes the pars optica hypothalamica. 

 The Hypophysis. — The infundilulum develops as a recess caudal to the 

 pars optica hypothalamica (Figs. 342 and 343) . At its extremity is the sac-Hke 

 anlage of the posterior lobe of the hypophysis or pituitary body. During the fourth 

 week the infundibular anlage comes into contact with Rathke's pouch, the epi- 

 thelial anlage of the anterior lobe of the hypophysis (Fig. 343). The epithehal 



