SCYLLIUM CANICULA 



257 



thickened to form the crura cerebri, and consequently reduce its 

 cavity to a fairly small passage, the iter. 



While these processes have been taking place the hind-brain has 

 also been differentiating into its two subdivisions, the metence- 

 phalon and the myelencephalon. The roof of the former gives rise 

 to a large median dorsal outgrowth, the cerebellum, while that of 

 the latter remains thin and epidermal. The sides and floor of the 

 whole hind-brain thicken considerably. At the hinder end the 

 myelencephalon thickenings pass over into those of the spinal cord. 



The nasal organ and the auditory organ are not derived from 

 the brain like the optic vesicle, but from ingrowths of the ectoderm. 



The various parts of the brain in the embryo and the adult 

 and the structures characteristic of each of them may be tabulated 

 in the following manner : 



THE DIVISIONS OF THE BRAIN IN SCYLLIUM CANICULA, AND THE STRUCTURES 

 ASSOCIATED WITH EACH. 



