200 bulletin: museum of comparative zoology. 



portion which is later partly constricted off to form the longitudinal 

 lateral lobe of the torus longitudinalis. During tliese early stages the 

 sulcus between the halves of the tectum is of extraordinary width, i. e. 

 the roof of the mesencephalon is very thin for a considerable distance on 

 either side of the median plane (Plate 9, Fig. 0)5). 



The axons enter the ventricle from the mesal and anterior portions 

 of the torus lobes immediately above the posterior commissure (Fig. 

 47). The cells of the torus show something of the arrangement, in rows 

 radial to the mesocoele, characteristic of most teleosts. In the anterior 

 part of the torus the cells are larger than posteriorly, perhaps because 

 they are further advanced in development. They have in general a 

 bipolar appearance, but as some of them have been seen to have three 

 chief processes (Fig. 47), it is probable that all have. The axon, coming 

 from the tapering end of the cell, is usually directed ventrad, and passes 

 in this direction into the ventricle, as before described. In larvae 3 cm. 

 long the structures of the mesencephalic roof have attained adult con- 

 ditions. 



At the posterior end of the body the spinal cord bends dorsad, and is 

 continued into the heterocercal tail. The walls of the cord are here 

 thin ; in larvae of 2 to 3 cm. they are made up for the most part of a 

 single layer of cells (Plate 8, Fig. 61). In that portion of the central 

 canal beyond the flexure there is a large number of characteristic pos- 

 terior canal-cells. Roughly estimated they ma}'^ number from 20 to 30 ; 

 they are usually of the elongated spindle-shape, and frequently are seen 

 to be continued into a neurite at each end (Fig. 61, a). The neurite, or 

 axon, which runs forward is the larger and is always present. These 

 axons run parallel for a considerable distance before uniting (Fig. 61, ax.). 



The cells give off a number of dendritic processes, which pass for the 

 most part to the ventral side of the canal and enter the cord (Plate 6, 

 Fig. 43 ; Plate 8, Fig. 61, y8). 



Cyprinidae (37). In young Notropis cornutus (a shiner) 2 cm, long 

 (Plate 7, Fig. 48, fas. Beis. See also Sargent, :03 h, Fig. 23), 

 the optic reflex apparatus is fully established. In the adult of 6 cm. 

 Reissner's fibre is relatively large (diameter 1.5 micra), and well defined 

 in its course through the canal and brain ventricles. The fibre emerges 

 almost as a whole from just in front of the posterior commissure, but is 

 joined by some fine fibrils coming from the pocket just behind and above 

 the commissure. 



There are no conspicuous cells in the anterior portion of the roof of 

 the mesencephalon, and the constituent axons of Reissner's fibre have 



