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all of the optic fibres, apparently only collateral branches 

 being given to the interbrain. Three structural zones of the 

 tectum are to be recognized: the superficial, the middle, and 

 the deeper zones, respectively. The superficial layer has 

 chiefly fibres, »ith a favv minate, tangentially elongated neu- 

 rones. The middle layer is composed of a densely crovided tan- 

 gle of neurones of the Golgi II type, the axones of which spread 

 laterally. The deeper layer has neurones which send long den- 

 drites iTto the outer levels, while their axones penetrate tne 

 stratum medullare profundum. Optic terminations occur in all 

 of these layers. The structure of the deeper layer places the 

 selachian more nearly in the direct phylogenetic line than the 

 teleost. 



Tne stratum medullare profundum receives optic fibres, 

 axones from the tectum, fibres of the olfactory mechanism from 

 relays in the thalamus, as well as fibres from posterior re- 

 gions. 



The central gray matter has become differentiated at cer- 

 tain points to form the roof-nucleus, and the nuclei of the 

 III and IV ner/es, respectively. 



The roof-nucleus is a collection of very large neurones 

 lying dorsal to the aqueduct of Sylvius, the axones of which 

 form, ultimately, the fibre of Reissner. Terminations of fi- 

 bres from the stratum medullare profundum occur near the neu- 

 rones of the roof-nucleus. The roof-nucleus and Reissner's 

 fibre constitute a direct path for motor reflexes between certain 



