THE ERA IX. 



119 



third ventricle ; from it the pineal body and the iafundibulum 

 are developed as median diverticula, dorsal and ventral respec- 

 tively ; while the optic vesicles and cerebral hemispheres arise 

 as paired lateral and anterior outgrowths. 



The mid-brain undergoes comparatively little change ; from 

 its roof the optic lobes of the adult are formed. 



The hind-brain becomes the medulla oblongata of the adult : 

 from the roof of its anterior part the cerebellum is formed. 



Before considering the development of the several parts of 

 the brain in detail it will be well to notice the general propor- 



C.P, 



FIG. H3. 



FrG. 62. The brain of the adult Frog : dorsal surface. 

 FIG. 63. The brain of the adult Frog: ventral surface. 



x4. 

 x4. 



C, cerebellum. CH, cerebral hemisphere. CP, choroid plexus of third ventricle. 

 F, fourth ventricle. IN, infuudibulum. M, medulla oblongata. O, olfactory lobe. 

 OC, optic chiasina. OL, optic lobe. P, stalk of pineal body. PB, pituitary body. 

 T, thalarnencephalon. I, olfactory nerve. II, optic nerve. Ill, third or motor oculi 

 nerve. IV, fourth nerve. V, fifth or trigeminal nerve. VI, sixth nerve. VII and 

 VIII, combined root of facial and auditory nerves. IX and X, combined root of 

 glossopharyngeal and pneumogastric nerves. 



tions and relations of the brain during the successive stages of 

 its formation. These will be readily understood from comparison 

 of Figs. 60, 61, 64, 65, and 89. 



At the time of the first formation of the brain-tube, before 

 the hatching of the tadpole (Figs. 60, 61), cranial flexure is very 

 strongly marked, and the fore-brain, BF, projects far in front of 

 all other organs of the body. Later on (Figs. 64, 65), both 

 these relations are changed ; the brain appears to become 



