THE BRAIN. 125 



the time of completion of the tube (Fig. 60) it projects inwards 

 as a small, solid, tongue-like process beneath the brain, between 

 this and the dorsal wall of the pharynx. 



The projection continues its growth inwards, and expands at 

 its end into a somewhat flattened mass of cells, which lies imme- 

 diately beneath the infundibulum, close to the anterior end of the 

 notochord, and which becomes the pituitary body itself (Fig. 64, 

 PT); the rest of the process forms a slender stalk, which connects 

 the pituitaiy body with the surface epiblast. About the time of 

 opening of the mouth, the pituitary body becomes hollow and 

 separates from the stalk, which atrophies and soon disappears 

 completely. The pituitary body (Fig. 65, PT), which is now all 

 that remains of the original ingrowth, acquires close relations 

 with the hinder end of the infundibulum, which it retains through- 

 out life (Fig. 63, PB). It becomes partially divided into anterior 

 and posterior portions, of which the latter forms a complicated 

 mass of convoluted tubes. 



The optic vesicles. From the sides of the fore-brain, about 

 the time that closure of the neural tube is effected, a pair of 

 hollow lateral outgrowths, the optic vesicles, arise. Each optic 

 vesicle soon becomes constricted at its base, so as to form a 

 bulb, opening by a tubular stalk into the fore-brain. From the 

 bulbs, the eyes are developed in a manner that will be described 

 later on ; while the optic stalks form paths along which the fibres 

 of the optic nerves pass from the eyes to the brain. 



About the time of opening of the mouth (Fig. 64), a trans- 

 verse groove runs across the floor of the fore-brain, in front of the 

 infundibulum, IN, and between this and the vesicle of the hemi- 

 spheres, BC. This groove is bounded in front and behind by 

 transverse ridges, and is produced outwards at its two ends 

 into the tubular optic stalks. At a slightly later stage, about the 

 time of appearance of the hind limbs (Fig. 65), the stalks be- 

 come solid along their whole length ; the further changes in 

 connection with them will be described in the section dealing 

 with the development of the eye. 



The cerebral hemispheres. The hemispheres, although the 

 largest part of the adult brain (Figs. 62, 63), are the last to 

 appear. About the time of hatching of the tadpole, the anterior 



