198 bulletin: museum of comparative zoology. 



(Fig. G) the torus, as a result of arrested development, has ganoid 

 characteristics. 



The torus longitudinalis, then, is merely the niesal and primitive 

 portion of the tectum constricted oft", and as it were left behind in the 

 enormous development of the tectum in this aberrant group. But 

 though the torus is a structure which first attains an independent and 

 definite form in the teleosts, and exists in that group only, its essential 

 elements are perhaps the most archaic of the mesencephalic roof. 



With this enormous development of the tectum opticum in teleosts, 

 important morphological changes have been brought about. The optic 

 lobes, carrying with them the torus longitudinalis, have grown cephalad, 

 so that they project above and overlie the diencephalon (Fig. /; 

 Plate 7, Fig. 51). The posterior commissure, developing ontogenetically 

 before this enormous anterior development has taken place, and being 

 fixed in position, is not carried forward with the tectum. In the adult 

 brain the posterior commissure, therefore, seems to have migrated cau- 

 dad, and in median sagittal sections is seen projecting far backward 

 from the anterior part of the tectum (Figs. 49, 51). A somewhat similar 

 condition occurs in other vertebrates, but is not carried to nearly the 

 same extent. 



The torus longitudinalis curves around the margin of the recess above 

 the posterior commissure, and at its anterior and ventral margin be- 

 comes fused with the posterior commissure (Plate 7, Fig. 50 ; Plate 8, 

 Figs. 55-57). In the more highly differentiated and active teleosts the 

 valvula cerebelli attains a great development and is crowded forward, 

 largely filling the mesocoele (Fig. /). In these forms, as a result, 

 the torus is crowded forward, so that it is shortened and comes to 

 lie largely dorsal to the commissure (Plate 10, Fig. 69) ; or it may be 

 flattened along the roof, and in close contact with the valvula. 



In the teleosts a greater or less portion of the torus lies dorsal to, 

 and a considerable distance cephalad of, the posterior commissure. It 

 thus comes about that the developing axons of these torus cells, in 

 making their way into the ventricles, take the shortest path and enter 

 the ventricle anterior to the posterior commissure. A somewhat similar 

 condition was noted in Polypterus, where several of the optic reflex 

 cells were found anterior to the posterior commissure. This phyloge- 

 netic change is explained by the ontogeny. The cells of the optic 

 reflex appai'atus are early developed, before hatching, and while the 

 torus longitudinalis is as yet incompletely difi"erentiated from the tec- 

 tum. They send their axons into the ventricle from the median plane. 



