42 bulletin: museum of comparative zoology. 



in the dorsal root, wliich spreads on the roof of the tectum — did not 

 degenerate. In a very old one-eyed fish both the geniculate ganglion 

 and the torus longitudinalis were, he found, much atrophied and the 

 fillet was reduced. The spindle-cell layer contained fewer cells than 

 were found in fishes more recently operated on. 



Turning next to the finer anatomy of the tectum a diagrammatic rep- 

 resentation of a parasagittal section is shown in Figure 22 (Plate 5). 

 This exhibits the types of cells found in the tectum by the aid of the 

 silver method. 



In layer 1 few cells were impregnated. Of these the more common 

 type (Fig. 22, a) vvas oval and bipolar, its two processes running parallel 

 to the fibres of layer 2. In some instances, however, the cell had a 

 third and even a fourth process. Similar cells have been described by 

 Fusari, except that the cell bodies described by him were spherical. 

 Neumayer ('95) has shown elongated bipolar cells with })rocesses parallel 

 to layer 2, and also rounded cells whose neurites fibrillated in the layer 

 of optic fibres. Mirto ('96) indicated cells in corresponding positions, 

 but with triangular bodies. I, also, have found a few peai'-shaped cells 

 (Fig. 22, j8) in this layer. These lay near the surface and sent off their 

 processes from their deeper, smaller ends. Some of these processes 

 passed through the optic layer (2) into layer 3, while others turned at 

 right angles and ran in layer 1 parallel to the surface. 



Layer 2 is composed of the medullated fibres which enter the tectum 

 as the optic ti'act. At the beginning of the tectal region the fibres of 

 the tract, after having passed beneath the geniculate body, bend toward 

 the surface of the brain to form this second layer. Some of the cells 

 of the nidulus corticalis {nid. ctx.) lie in this layer, since the nidulus ex- 

 tends from ventricle to surface. The bulk of the dorsal bundle of fibres 

 from the tractus passes too near the sagittal plane to touch the nidulus 

 corticalis, and the ventral division does not reach as far dorsally as the 

 nidulus. So tliere is little disturbance in the course of the fibres of the 

 tractus in passing these very large cells. The diminution in the thick- 

 ness of the optic-fibre layer in passing from before backwards, which is 

 due to the fibres continually spreading out over more of the surface of 

 the optic lobe, and to the termination of many of them in anterior 

 regions, is shown in Figure 25 (Plate 5). 



Here and there other cells, besides those of the nidulus corticalis, 

 which lie at the anterior end of the tectum, are seen in the optic layer; 

 these have fibres, some of which extend inward, others outward. The 

 cell-body of one of these (Fig. 22, y) was pear-shaped, the smaller end 



