38 Vertebrate Embryology 



At a very early period, a thickening of the 

 nervous layer of the ectoblast is formed just 

 below the anterior end of the neural canal. This 

 collection of cells grows inward as a tongue 

 of ectoblast tissue between the anterior end of 

 the brain and the digestive tract (Figs. 14, PT, 

 17, and 1 8, P). The inner end of this tongue 

 of cells becomes broader and hollow, and even- 

 tually separates from its stalk to form the 

 pituitary body, which lies just under the in- 

 fundibulum (Fig. 18, /). From the roof of 

 the fore-brain, at the point where the neural 

 tube finally closed, a small, hollow diverticu- 

 lum is pushed out and becomes enlarged at 

 the end (Figs. 14 and 17, /W). This is the 

 pineal body, and when the skull is formed it 

 cuts off the enlarged knob from its hollow 

 stalk, the knob, which has become solid, re- 

 maining outside of the skull, and the stalk 

 retaining its connection with the fore-brain. 



The cerebral hemispheres, which form the 

 larger part of the fore-brain, do not form until 

 a comparatively late period. They begin as a 

 large, median diverticulum from the front of 

 the fore-brain (Figs. 17, CV, and 18), which 

 diverticulum is at first unpaired, but later be- 

 comes divided into the two hemispheres. 



