340 loi'KNAI. Ol' C()M1'AKAT1\K NkI' 1U)L<)(;Y . 



is remarkably verified in transverse section. A caudad 

 diverticle of the cul-de-sac from the exterior extends to the 

 level of the valve where the fourth nerve enters without the 

 curvature described in the moon-eye, but which is here un- 

 necessary by reason of the divarication of the optic lobes. 

 At this level the cerebellum is obviously folded upon a 

 longitudinal fissure for almost its whole height, though the 

 fissure has been practically obliterated by the union of the 

 walls. A little caudad a rudiment of the caudal diverticle of 

 the fourth ventricle can be detected, and the arrangement of 

 the gray and white matter shows that the duplication has 

 been affected in the usual manner, yet the cavity is absent, 

 and the whole organ is so closely adherent to the combined 

 lateral lobes that only a comparison with simpler types 

 explains the seeming anomalies. 



This case is very instructive as showing how far the 

 distortion and secondary union of parts may obscure perfectly 

 simple homologies. 



In the eel the cerebellum may be characterized as a simple 

 depressed variety with small volvula. In striking contrast to 

 the gizzard shad, the modifications are such as could most 

 easily be referred to the effects of pressure from above. The 

 volvula is exceptionally small. The fourth nerve enters 

 directly behind the optic lobes, decussating in a very obvious 

 and uncomplicated valve. The dorsal lamina of the volvula 

 passes into the tectum opticum with but a short velum 

 cerebelli anterior. Caudad of the trochlearis there is a slight 

 pocket and corresponding revolution cephalad. Then for 

 some distance the cerebellum is a simple thickened dorsal 

 wall of the fourth ventricle which, nevertheless, has the 

 appearance of having been thrust ventrally into that cavity, 

 with the result of thrusting the ventricular gray layer of the 

 cerebellum laterad and dorsad, so that it is thicker on the 

 lateral than the ventral aspects and projects on either side as 

 a slight protuberance on the lateral margin of the dorsal 

 surface. The caudad portion of the cerebellum is a sac-like 



