120 JOURNAL OF COMPARATIVE NEUROLOGY. 
clearly demonstrates the overshadowing importance of the pis- 
cine ear asa peripheral organ of equilibration. In fact, his 
latest research ('98) makes it clear that the great ear of Mustelus 
is not a true auditory organ at all, but that its several parts are 
to be interpreted from the standpoint of the equilibrium sense. 
The large nerve-tracts from the ear to the cerebellum are, there- 
fore, definitely significant, being the connecting fibres between 
the central mechanism and its most important peripheral organ. 
It also appears that not only does the ontogeny of the ear show 
its relationship to the lateral line organs, but the functions in 
the two instances are comparable as well. 
An analysis of the habits of Mustelus will, it appears ‘to 
me, go far toward explaining the disproportionately great devel- 
opment of both the ear and the cerebellum which we find the 
animal to have. This shark, although a comparatively small 
representative of the group, is, withal, a restless hunter of the 
seas, ever urged onward by an appetite which, apparently, has © 
no bounds. Continully suspended in a fluid medium, and com- 
pelled to balance itself at every turn, the animal requires a 
precise mechanism of equilibration. This is to be found, in 
the main, in both an ear and a cerebellum developed to a degree 
out of all proportion to the scale occupied by the creature as © 
_ a whole. 
6. Evolution of the Cerebellum. 
Evidence of the origin of one brain-structure from another 
is usually of the most meagre value, but it appears as though 
it were now possible to weave together a few scattered threads 
in the evolution of the cerebellum. SCHAPER (’94) has called 
attention to certain facts in the embryology of the teleostean 
cerebellum which may be taken as the starting point. The cere- 
bellum arises in ontogeny, not asa brain-segment having the 
full value of the others, but as a paired thickening in the 
parietal wall of the neural tube at the anterior end of the ob- 
longata. These thickenings grow upward and meet each other 
in the median line. ScHAPER has since extended his studies 
to all classes of vertebrates, and he finds (’99) that the anterior 
