8o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



in front and the cerebellum behind (Fig. 52). This 

 corresponds to the condition of the average mammal, 



Fig. 49. — Fish stage. 



and may be called the mammal stage. (6) Finally, it 

 grows forward, wholly covering the olfactory lobe, and 



Fig. 50. — Reptile stage. 



backward, wholly covering the cerebellum, and we have 

 the human stage (Fig. 53). 



Fig. 51.— Bird stage. 



Meanwhile several other changes are in progress : (1) 

 In proportion as the head is raised nearer and nearer to 

 a vertical position, the base of the brain, which was at 



