Houser, The Neurones of a Selachtan. 107 
structure, not derivable from the cord directly. It is the pri- 
mary centre for the acustico-lateral system, components of the 
VII, VIII, and X nerves. There are present, neurones of the 
molecular, granular, and PuRKINJE types, identical with those 
of the cerebellum. Terminations occur in the molecular layer, 
and near the neurones of PurkKINJE. The PURKINJE neurones 
send their axones ventrally in the arcuate bundles. The cere- 
bellar crest is morphologically continuous with the cerebellum. 
There is ground for believing that further investigation will 
derive the acusticum from dorsal cornu structures. 
Supporting elements are present in the oblongata in great 
numbers. There is traceable a wide range of developmental 
forms of both neuroglia and ependyma. 
SECTION V. THE CEREBELLUM. 
The cerebellum of Mustelus is of large size in comparison 
with the adjacent brain-segments. In the adult animal, it is of 
sufficient longitudinal extension to overhang the larger part of 
the midbrain in front, and also much of the oblongata behind 
(Fig. 1, cb.). The base is only a third as great, however, indi- 
cating the smaller prototype from which the organ has been 
evolved. In fact, this cerebellum occupies an intermediate 
position in the phylogenetic scale, standing midway between 
the simple plate-like cerebellum of the cyclostome, the dipnoan, 
or amphibian, and the solid mass with radiating laminae char- 
acteristic of the mammal. It is essentially a great bulbous 
dilatation of the dorsal side of the neural tube, the wall of 
which has been thrown into folds as the process of growth 
thrust the vesicle against the unyielding and more slowly ex- 
panding cranium. So, instead of a solid central mass of nerve- 
fibres covered with layers of gray matter, we find in Mustelus 
-ahollow organ, the fourth ventricle extending freely into it 
and ramifying through its several folds. The folds, therefore, 
are simply doublings in the cerebellar wall, and are fundament- 
ally different from the solid laminae familiar to us in higher 
vertebrates. 
The arrangement of the structural elements of the cere- 
