THE BRAIN 



339 



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 limitans (Fig. 341) forms the boundary line between the 

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

 plate). The basal plate is comparatively unimportant in the diencephalic 



Sulcus limitans 



Hypothalamus 



Pallium 



Mammillary recess 



Corpus striatum \ Infundibulum 



Optic ridge 



FIG. 341. Median sagittal section of the fore- and mid-brain from a 10.2 mm. embryo (after 



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 hypophysis, or pituitary body, has a double 

 origin. Its glandular portions develop from the ectodermal Rathke's 

 pouch, which appears at about 3 mm. just in front of the pharyngeal 

 membrane (Fig. 86). This pouch early comes in contact with a sac-like 

 extension of the infundibulum, the anlage of the neural hypophyseal 

 lobe (Figs. 122, 341 and 343). Rathke's pouch, at first flat, grows laterally 

 and caudally about the neural lobe, and loses its stalked connection with 

 the oral epithelium at the end of the second month (Fig. 146). The 

 original cavity of the pouch becomes the residual lumen of the adult 

 gland. In embryos of about 20 mm., its walls differentiate into the 



