412 THE TORUS LONGITUDINALIS OF THE TELEOST BRAIN: 



VIII. CONCLUSION. 



The torus longitudinalis is, then, an archaic portion of the mesencephalic roof 

 formed in the teleosts from the mesal and primitive portion of the tectum opticum, 

 constricted off, and, as it were, left behind in the enormous development of the tectum 

 of this aberrant group. In its early development in the ganoids it is the result of 

 purely mechanical causes, the rapid growth in the adult of the "nucleus magnocel- 

 lularis" producing a downward bending of the mesencephalic roof on either side of 

 the median plane. Mechanical causes are still plainly operative in the Siluridse, 

 but in other teleosts the torus appears, at an early stage of ontogenetic development, 

 as the result of phylogenetic causes. Though the torus longitudinalis is a structure 

 which first attains an independent and definite form in the teleosts and in that 

 group only, its essential elements are perhaps the most archaic of the mesencephalic 

 roof. 



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