404 THE TORUS LONGITUDINALIS OF THE TELEOST BRAIN: 



the median sulcus of the torus is early formed. The growth of the torus pushes 

 apart the halves of the tectum (Fig. 5), and eventually the torus comes to project 

 below the level of the tectum (Fig. 7). 



In the Salmonidse this longitudinal thickening develops into an almost cylindrical 

 rod-like structure embedded between the halves of the tectum (Fig. 2). The median 

 fissure of the torus develops in this secondarily, and never becomes accentuated (Figs. 

 2, 3). Later the torus becomes dorsoventrally elongated, but in the Salmonidse it 

 never projects below the tectum (Fig. 3). 



In every case the first differentiation of the torus is at the anterior end of the 

 mesencephalon and progresses caudad with advancing development. A series of 

 transverse sections will therefore show many different stages of development (Figs. 

 6, 7). In nearly all teleosts the torus is early to develop and usually has attained 

 its typical form at the time of hatching, but continues to increase in size up to the 

 adult stage. 



III. MORPHOLOGY. 



In the Silurida? the brain is of relatively simple morphological structure, recalling 

 the generalized ganoid type. The optic lobes are not more highly developed than 

 in Amia. The median portion of the roof of the mesencephalon is thin, almost mem- 

 branous and without nerve elements. The mesal portion of the tectum opticum 

 of each side becomes differentiated from the rest of the tectum and separated from 

 it by a shallow, longitudinal groove (Fig. 8). The torus lobes so formed never have 

 a thickness greater than the tectum, with which they are intimately associated. 

 Anteriorly the lobes of the torus are almost in contact, being separated only by 

 the narrow fissure. Caudad of the commissura posterior the fissure widens and the 

 lobes diverge, until in the middle portion of the optic lobes they are widely separated 

 (Fig. 8), and connected only by the thin membranous roof of the mesencephalon. 

 This is here made up of ependymal cells and contains no other nerve elements than 

 the fibres of the dorsal decussation of the tectum which pass through it from side to 

 side. Posteriorly the lobes diminish in size and disappear. 



In the Cyprinidse (Notropis) and Poeciliidse (Fundulus) the torus longitudinalis 

 has a form typical of it in a large number of teleosts. It is a conspicuous and well- 

 defined structure, and attains its greatest development anteriorly, diminishing in size 

 posteriorly (Fig. 23). It hangs freely suspended in the median plane, projecting into 

 the mesencephalon below the level of the tectum. The torus lobes are in close prox- 

 imity throughout their length, the median fissure between them being much reduced. 



