Johnston, TJie Brain of Petroinyzon. 7 



In the mean time the acustica of the two sides arch over 

 the ventricle toward one another and fuse to form a bridge of 

 tissue scarcely as thick as the acustica themselves, the cerebel- 

 lum (Fig. 12). In the cerebellum are recognizable the charac- 

 teristic granular and molecular layers, although they are poorly 

 differentiated. 



Above the cerebellum there appears the bilobed tectum 

 opticum (Figs. 12, 13), which presents an appearance in trans- 

 verse section very closely resembling that of the tectum of 

 Acipenser in the form of the cavity, the arrangement of the 

 white and grey matter, and in the dorsal decussation. A little 

 farther forward the dorsal decussation disappears and the median 

 part of the roof of the mid-brain becomes a choroid plexus 

 (Fig. 14). Conspicuous in the base of the mid-brain are the 

 III nerve with its nucleus and decussation, and the decussation 

 of the bundles of Meynert (Fig. 24). In the cephalic part of 

 the mid-brain the tectum has receded far to the sides, leaving 

 the whole dorsal wall to be made up of the choroid plexus 

 (Fig. 14), and the transverse section of the nervous tissue has 

 nearly a simple U-form. In the lateral wall appear the bundles 

 of Meynert, the right one being several times larger than the 

 left. Farther forward the dorsal edges of the lateral walls be- 

 come connected by the large posterior commissure (Fig. 15). 



Beneath the base of the mid-bVain appears in section the 

 most caudal portion of the 'tween-brain, the corpus mammiilare. 

 It has a thick nervous caudal wall and a thin ventral wall which 

 constitutes part of the saccus vasculosus. The cavity of the 

 corpus mammiilare enlarges and opens into the 'tween-brain 

 ventricle in about the same transverse sections in which the 

 posterior commissure comes to be replaced by the ganglia hab- 

 enulae and the superior commissure. The right ganglion 

 habenulae is very much larger than the left (Fig. 16), the dif- 

 ference in size being much greater than in Acipenser. Immedi- 

 ately in front of the posterior commissure and behind the supe- 

 rior commissure a small irregular diverticulum of the third ven- 

 tricle gives rise to the very attenuated stalk of the epiphysis 

 (Fig. 2). This extends forward a little to the left of the middle 



