NERVOUS SYSTEM OF THE VERTEBRATA. 447 



the nerves of the fore-brain have a different character to those of 

 the mid- and hind-brain. 



This primitive division of the central nervous system is lost 

 in all the true Vertebrata, and in its place there is a secondary 

 division corresponding with the secondary vertebrate head 

 into a brain and spinal cord. The brain, as it is established in 

 these forms, is again divided into a fore-brain, a mid-brain and a 

 hind-brain. The fore-brain is, as we have already seen, the 

 original ganglion of the praeoral lobe. The mid-brain appears 

 to be the lobe, or ganglion, of the third pair of nerves (first pair 

 of segmental nerves), while the hind-brain is a more complex 

 structure, each section of which (perhaps indicated by the con- 

 strictions which often appear at an early stage of development) 

 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 instruc- 

 tive 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 hemi- 

 spheres, which on embryological 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 infundibulunv and 

 pineal gland, has already been mentioned in connection with the 

 development of these parts. 



(332) C. J. Cams. Vcrsnch einer Darstellnng d. Nervensy stems, etc. Leipzig, 



1814. 



(333) J. L. Clark. " Researches on the development of the spinal cord in Man, 

 Mammalia and Birds." Phil. Trans., 1862. . 



