The brain of Acipenser. 199 
superficial zone of fibres, and, although the cell body is somewhat 
farther from the cavity in birds than in fishes, the cells are always 
vertically placed and the degree of similarity between these cells in 
all classes of Vertebrates is remarkable. According to S. Ramon, 
the neurites of these cells go out in the optic nerve to end freely 
in the retina. The presence of cells of the same description in 
all Vertebrates is evidence for the existence of the efferent fibres 
iu all. 
The cells D in Acipenser send their neurites (except those 
crossing in the dorsal commissure?) beyond the tectum to other 
parts of the brain. These are the neurites which form a large super- 
ficial bundle at the lateral border of the tectum and form the tractus 
tecto-lobaris and tractus tecto-bulbaris (page 125—127), These cells 
present considerable variety of form in Acipenser, but I have classed 
them all together upon the basis of the disposition of the neurites 
and dendrites. Upon the same basis, I believe, most or all of the 
cells in all Vertebrates whose neurites leave the tectum to other 
parts of the brain, may be brought into one class and homologized 
with the cells D in Acipenser. In all animals as far as investigated 
the most of these cells are stellate and there is a tendency for the 
dendrites to be directed toward a common middle zone in the tectum- 
The dendrites of the cells near the cavity ascend to this zone, while 
those of the cells near the periphery descend to about the same level. 
In the case of vertical fusiform cells, those near the cavity have the 
peripheral process better developed, while those near the periphery 
have the internal process larger. These cells may all have been 
derived from the germ zone near the cavity and migrated to their 
respective positions. In every case the position of the in-coming 
fibres with which the cells are in relation has determined the position 
and development of their dendrites. The neurites probably gain 
their definitive positions earlier than do the cells. Consequently the 
neurites of centrally situated cells run outward to their fibre layer, 
while the cells nearer the periphery send their neurites inward to 
the same layer. In higher Vertebrates the variety of form as well 
as the number of these celis is greatly increased. The greater 
variety of form requires no special explanation, and the increased 
number is readily accounted for by the development of the cerebral 
tracts. It is probable that the fibres to the occipital lobe come from 
some of the cells of the general type now under discussion. 
Fusiform cells disposed horizontally, cells E in Acipenser, are 
