ITS ONTOGENY, MORPHOLOGY, PHYLOGENY, AND FUNCTION. 



415 



EXPLANATION OF THE PLATE. 



ABBREVIATIONS. 



Pars intercalatus (Schaltstiick) . 



Mesencephalic recess, dorsal and anterior 

 to the posterior commissure. 



Tectum opticum. 



Torus longitudinalis. 



Torus semicircularis. 



Tractus toro-fibrae Reissneris anterior. 



Tractus toro-fibrse Reissneris posterior. 



Tractus opticus. 



Tractus toro-cerebellaris. 



Tractus toro-tectalis. 



Valvula cerebelli. 



Mesal edge of tectum opticum (con- 

 sidered by Auerbach the " Anlage " of 

 the torus). 

 Mesencephalic groove above the torus. 



PLATE XXIX. 



In all figures the dorsal edge of the section is uppermost. All figures are from transverse sections, except 

 Figures 20 to 24 inclusive which are from parasagittal sections. In Figures 20 to 23 inclusive the anterior end 

 is at the left. In Figure 24 the anterior end is at the right. All the figures were outlined with the camera lucida. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Fig. 5. 



Fig. 6. 



Fig. 7. 



Fig. 8. 



Fig. 9. 

 Fig. 10. 

 Figs. 11 



Figs. 15-: 



Salvelinus fontinalis, at time of hatching. Section through the anterior portion of the mesencephalon 



caudad of the posterior commissure, showing the median longitudinal fold from which the torus develops. 



The median sulcus of the torus is indicated by the absence of cells in that region. X87. 

 Salvelinus fontinalis, one day older than that shown in Figure 1. The section was taken a little anterior 



to that for the preceding figure. X87. 

 Salvelinus fontinalis, 4 cm. long. Section through the same region as that shown in Figure 2. X40. 

 Acanthocottus seneus, newly hatched. Section through the anterior part of the mesencephalon showing 



the torus longitudinalis in an early stage of development. 

 Acanthocottus seneus, a little older than that seen in Figure 4. Section through the same region as in the 



preceding figure. X87. 

 Tautogolabrus adspersus, young specimen 6 cm. long. The torus presents the adult condition. X 15. 

 Tautogolabrus adspersus. Section from the same specimen as that from which Figure 6 was taken, but 



from a position considerably posterior to that of Figure 6. The torus is seen in an early stage of 



development. X 15. 

 Amiurus nebulosus, young specimen. Section through the anterior portion of the mesencephalon, caudad 



of the posterior commissure. X30. 

 Amblyopsis spelseus, adult. Section through the same region as in the preceding figure. X30. 

 Menidia notata, young specimen. X20. 

 -14. Pseudopleuronectes americanus, adult. Four sections from the median portion of the mesencephalon. 



Figure 11 represents the most posterior of the four sections, and the others follow in sequence. Xl5. 

 19. Cynoscion regalis, adult. Five sections of the mesencephalon; that shown in Figure 15 is the most 



anterior, and the others follow in sequence. XlO. 



