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of sensory centres. The homology is most readily tr^iceible 

 in the ventr-il region. The neurones of the ILL and IV ner«/e3 

 are true somatic nootor neurones, corresponding entirely to 

 those of the ventral cornua of the cori. 



The dorsal region of the midbrain has become more and 

 more specialized as an optic termination. At a phylogenetically 

 early period, the optic fibres grew backward from their orig- 

 inal relatioTS to establish terminations here. Probably the 

 most primitive connection is the one .vith the giant neurones 

 constituting the apparatus of the roof-nucleus and Heissner's 

 fibre. Through this means, the optic neurones were chained 

 directly to the somatic motor neurones innervating the body 

 musculature. Later, the midbrain roof became thickened by the 

 wandering oat'vari of neurones from the central gray matter, 

 and by the development of new optic terminations associated 

 with them. Thus has arisen the tectum mesencephali, an end- 

 station which has remained important in the vertebrate series 

 as a visual centre until secondary connections were establish- 

 ed with the pallium. Hence the tectum is of great magnitude 

 in the lower vertebrates, where the pallium is weak, but be- 

 comes dwarfed in the mammalian brain in which there is an over- 

 shadowing development of pallial connections. 



5. Summarif of the Miibrain. 

 The tectum mesencephali of Mustelus receives practically 



