The brain of Acipenser. 201 
in Acipenser differ from those in Teleosts in that their neurites enter 
the middle fibre zone, and, so far as I can judge, comport them- 
selves as do the other fibres of that zone. The fact that they enter 
the superficial fibre zone in Teleosts is not sufficient evidence that 
they go to the optic nerve. It is possible that in Teleosts as in Aci- 
penser many fibres which go to the medulla or the lobus inferior, 
run in the superficial zone. It seems to me more probable that 
these cells have the same relations as do the cells D in the tectum 
of Acipenser. I believe that their peculiar form is to be accounted 
for by the conditions of their growth. Two factors have probably 
combined to prevent these cells from gaining a robust growth. First, 
the regions of the dorsal and ventral fissures are areas of retarded 
growth. The presence of slender cells of a primitive type near the 
middle line in the lobus inferior and the fore brain, as well as in 
the tectum, suggests that the restricted growth is due to some ex- 
ternal influence retarding the growth of the nerve cells, such as 
mechanical pressure or a high degree of rigidity on the part of the 
glia. A second factor which may have operated in the same direction 
is the presence of the large-celled nucleus along the whole length 
of the tectum in the lowest Vertebrates (Petromyzon, F. MAYER, 97). 
The enormous cells of this nucleus must have crowded the other 
cells and prevented large growth. In fishes in which the large- 
celled nucleus has receded to the cephalic part of the tectum, the 
cells of the torus are still small as the result of inheritance. In 
higher Vertebrates the torus is wanting, although it is represented 
by a rudiment (an ependymal thickening) in Reptiles and possibly 
in all Vertebrates (RaBL-RUCKHARD, ’87). The absence of the 
peculiar cells of the torus in the higher Vertebrates is not consistent 
with Saua’s view that their neurites become centrifugal fibres to the 
retina, since these fibres exist in higher Vertebrates. My results in 
Acipenser make it doubtful whether SALA has properly identified the 
cells which give rise to the centrifugal fibres of the optic nerve. 
Torus semicircularis. — These ridges are considerably 
larger in Acipenser rubicundus than in A. ruthenus, as described by 
GORONOWITSCH. EDINGER (96a) has made a comparative study of 
this region and finds a nucleus common to Teleosts, Amphibians, 
Reptiles, and birds, to which he gives the name nucleus lateralis 
mesencephali. From this arises a large lateral longitudinal bundle 
which traverses the medulla and probably enters the cord. I have 
Suggested (page 105) that the cells of this nucleus are to be classed 
