104 THE BIOLOGY OF THE FROG chap, v 



The forebrain soon becomes separated into two parts, 

 the thalamencephalon behind, and the cerebral hemispheres, 

 which grow out from the latter in front. The floor and 

 walls of the former become thickened to form the optic tha- 

 lamic the roof remains thin and membranous, and the cavity 

 becomes the third ventricle. From the roof of the thala- 

 mencephalon there arises a median hollow outgrowth, the 

 pineal gland, which extends dorsally, reaching the surface 

 ectoderm, where it becomes expanded into a small knob. 

 The knob becomes constricted off when the bones of the 

 skull develop and forms the brow spot, previously described. 

 The floor of the thalamencephalon gives rise to a hollow 

 outgrowth, the infundibulum, which extends downward. It 

 comes into close contact with another structure, the pituitary 

 body, which is developed from the ectoderm of the dorsal 

 wall of the stomodeum. The sides of the thalamencephalon 

 give rise to a pair of lateral diverticula, the optic vesicles, 

 which grow out until they come in close contact with the 

 surface ectoderm. The distal end of the vesicles widens 

 out to form the retina of the eyes, the stalk giving rise to 

 the optic nerve. 



The anterior wall of the forebrain produces a pair of 

 pouches, the cerebral hemispheres, which finally become 

 the largest part of the brain. Their cavities, the lateral ven- 

 tricles, communicate with the third ventricle by an opening, 

 the foramen of Monro. 



The nerves arise as paired outgrowths both from the brain 

 and cord, pushing their way between the cells of the other 

 organs, dividing and ramifying, as they push outward 

 toward the various parts they supply. The spinal nerves 

 begin as two independent outgrowths, representing the 

 dorsal and ventral roots ; these soon unite into a single 

 nerve. 



