192 J. B. JOHNSTON, 
respects the development of the cerebellum in Acipenser must be 
the same as in Teleosts, where it has been so admirably described 
by SCHAPER. The theoretical considerations which I have to present 
here are from the phylogenetic point of view and are based on the 
structure of the adult brain. 
In trying to picture the primitive condition of the acusticum; 
we have to think of it as containing the large cells (not PURKINJE), 
granules, and cells of the II type. In front of the choroid roof 
of the IV ventricle there would be a strong commissure formed 
of the neurites of the granule cells, which connected the acustica 
of the two sides. This dorsal commissure of the acustica would 
then constitute the Anlage of the cerebellum. A few cells added 
to this commissure by migration from the acustica would produce 
a cerebellum of the type found at present in such forms as Proto- 
pterus (BURCKHARDT, 92) and the Urodeles (Fisx, 95). It is to 
be considered further that the acusticum in the early Vertebrates 
was very much smaller than in the present fishes, and presented as 
simple an internal structure as that of the dorsal horn of the cord. 
The history of the acusticum and cerebellum in Vertebrates is 
governed by two things: first, the history of the cutaneous sense 
organs of the head (including the ear) and the cranial nerves sup- 
plying them; and second, the history of the special - senses of sight 
and smell and the fibre tracts serving to bring the centers for these 
special senses into relation with the motor apparatus of the base of 
the brain and cord. 
The increase in number and differentiation of the cutaneous 
sense organs resulted in an increase in the size of the acusticum 
and its commissure, the cerebellum. At the same time certain 
central tracts which entered the cephalic end of the acusticum ex- 
ercised a greater influence on its growth and differentiation. From 
the optic center, the tectum, an important tract entered the region 
destined to become the cerebellum. With it came also tracts from 
the lobi inferiores, which are important nuclei in the descending 
paths from the olfactory and optic centers. Both the tectum and the 
lobi were connected directly with the base of the medulla, by the 
tractus tecto-bulbaris and tractus lobo-bulbaris, respectively. The 
function of the cerebellum toward these centers and their tracts 
must be one of coürdination only. The cerebellum contains no 
primary motor elements, but at no place in the brain are so many 
tracts of such different character brought together. The cerebellum 
