SAEGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 197 



selachians, but resembles more closely the condition in ganoids. The 

 median zone of the mesencephalon, however, is probably more complex 

 in some teleosts than in any other group. This is due to the develop- 

 ment of the dorsal decussation of the tectum opticum and the crowding 

 into the median plane of structures derived from the lateral zones. 



2. The Torus longitudixalis. In the adult teleost braiu the roof 

 of the mesencephalon is complicated by the presence of the torus longi- 

 tudinalis. In the adult this is typically a medianly grooved ridge 

 extending downward from the thin median portion of the mesencephalic 

 roof (Fig. H ; Plate 8, Fig. 58). It usually extends from the posterior 

 commissure through the length of the optic lobes, but is best developed 

 at the anterior end. The relative size of the torus, and consequently 

 its relations to the surrounding structures (tectum, posterior commis- 

 sure, valvula cerebelli, etc.), vary greatly in the different groups of 

 teleosts (see Sargent, :03''). 



Rabl-Eiickhard ("87) first pointed out that each half of the torus is 

 developed from the tectum of the corresponding side. The torus first 

 appeal's outogenetically, as a longitudinal thickening of the tectum 

 on each side of the median plane, and becomes separated from the tec- 

 tum by a longitudinal fissure. At first these longitudinal lobes are 

 considerably separated, being connected by only tlie thin median roof 

 of the mesencephalon. As differentiation advances and the lobes in- 

 crease in size, this fissure is narrowed and in higher teleosts becomes 

 obliterated. In the Siluridae the typical larval condition persists, the 

 lobes remaining separated except at the extreme anterior end (Plate 9, 

 Fig. 65). 



The first differentiation of the torus lobes is at the anterior end of the 

 tectum, and progresses caudad with advancing development. In nearly 

 all teleosts the torus is early developed, and has attained its typical 

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

 adult stage. 



Phvlogenetically the torus longitudinalis, as we have seen, has its 

 beginning in the ganoids. With the increasing complexity of the adult 

 ganoid brain, and the development of the dorsal decussation of the 

 tectum, the cells of the optic reflex apparatus are crowded downward 

 so as to form two longitudinal ridges, one on either side of the median 

 plane, extending backward from the posterior commissure. 



The Siluridae naturally supply the necessary connection in the 

 phylogeny of the torus between the primitive condition in ganoids, and 

 the highly differentiated state in the higher teleosts. In Amblyopsis 



