368 GENERAL CONCLUSIONS. 



ment) giving rise to a pair of segmental nerves is, roughly speaking, 

 homologous with the whole mid-brain. 



The type of differentiation of each of the primitively simple 

 vesicles forming the fore-, the mid- and the hind-brains is very 

 uniform throughout the Vertebrate series, but it is highly instructive 

 to notice the great variations in the relative importance of the parts of 

 the brain in the different types. This is especially striking in the case 

 of the fore-brain, where the cerebral hemispheres, which on embryo- 

 logical grounds we may conclude to have been hardly differentiated 

 as distinct parts of the fore-brain in the most primitive types now 

 extinct, gradually become more and more prominent, till in the highest 

 Mammalia they constitute a more important section of the brain than 

 the whole of the remaining parts put together. 



The little that is known with reference to the significance of the 

 more or less corresponding outgrowths of the floor and roof of the 

 thalamencephalon, constituting the infundibulum and pineal gland, 

 has already been mentioned in connection with the development of 

 these parts. 



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