THE BRAIN 



337 



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. 323) forms the boundary line between the 

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

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

 brain stem. The basal plate is comparatively unimportant in the dien- 

 cephalic region as no nuclei of origin for motor nerves are developed here. 



Sulcus hypothalamicus 



Hypothalamus 



Pallium 



. 



Mammillary recess 

 Corpus striatum \ Infundibulum 



Optic ridge 



FIG. 323. Median sagittal section of the fore- and mid-brain regions of a brain from a 10.2 mm. embryo 



(after His). 



In the floor plate the ridge formed by the optic chiasma constitutes the pars 

 optica hypothalamica. 



The Hypophysis. The infundibulum develops as a recess caudal to the 

 pars optica hypothalamica (Figs. 324 and 325). At its extremity is the sac-like 

 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. 325). The epithelial 

 anlage is at first flattened and soon is detached from its epithelial stalk. Later, 

 it grows laterally and caudally about the anlage of the posterior lobe and during 

 the second month its wall is differentiated into convoluted tubules which obliterate 

 its cavity. The tubules become closed glandular follicles surrounded by a rich 



