Section V, 

 The Cerebellom. 



The cer.^bsllaii of Mustelas is of large size in conaparison 

 rtith the adjiicsnt brain-ssgmsats. In the aJalt animal, it is 

 of sufficient longitaiinal extension to overhang the larger 

 part of the midbrain in front, ani also rnuuh of the oblongata 

 behini (?ig.l,cb.). The base is only a third as great, howev- 

 or, indicating the smaller prototype from /»hich the organ has 

 been evolvel. In fact, this cerebellar occupies an interne- 

 iiate position in the phylogenetic scale, standing midflay be- 

 tween the simple plate-like cerebellar of the cyclostOTie, the 

 dipnoan, or amphibian, and the solid mass flith radiating lam- 

 inae characteristic of the mammal. It is essentially a great 

 bulbous dilatation of the dorsal side of the neural tube, the 

 ;»all of ^hich has been thrown into folds as the process of 

 growth thrust the vesicle against the unyielding and more slow- 

 ly expanding cranium. So, instead of a solid central mass of 

 nerve-fibres coverei with layers of gray matter, we find in 

 Mustelus a hollow organ, the fourth ventricle extending free- 

 ly into it and ramifying through its several folds. The folds, 



therefore, are simply doublings in the cerebellar wall, and 



(63) 



