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, 7W). 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. 



