202 J. B. JOHNSTON, 
with the cells D of the tectum. I have found no cells of the II type, 
and the neurites of all the cells go to the medulla in the tractus 
tecto-bulbaris, the greater number running in the crossed tract 
through the ansulate commissure. The gross relations in Acipenser 
indicate that the torus semicircularis is an in-folding or thickening 
at the base of the tectum. As regards its minute structure, its cells 
correspond to only a part of the cells of the tectum, and I have 
not traced optic fibres into it. It may be that it receives only 
secondary sensory tracts and is to be compared with that part of 
the tectum which is in relation with these tracts (middle zone). In 
higher Vertebrates this group of cells becomes farther separated 
from the tectum proper and is to be regarded as a separate nucleus. 
This is opposed to EDINGER’s interpretation which derives this 
nucleus from the central grey matter of the mid brain. 
b) Fibre Tracts connected with the Tectum. 
1. Centripetal Tracts. 
Tractus opticus. — The optic tracts in Acipenser cross 
completely in the chiasma. This is the usual condition in the Verte- 
brates below the birds (EDINGER, ’96b, p. 137) and my description 
is confirmed by the recent work of Fritz (99) on Amphibia. Optic 
fibres have been found ending in the corpus geniculatum by BEL- 
LONCI (’88?, quoted by EDINGER, ’99), in Rana by Gaupp (97), in 
Rana, Lacerta and Emys by P. Ramon (94), in Chamaeleon by 
P. Ramon (96, quoted by EDINGER, 99), in Reptiles by EDINGER 
(99), in the pigeon by EDINGER WALLENBERG (99), and in 
Mammals by various authors. EDINGER states that he has found 
optic fibres ending in no thalamus nuclei except the corpus geni- 
culatum and corpus ectomammillare (in Reptiles, 99). Endings have 
been found in the nucleus anterior in Rana by P. Ramon (94) and 
by Gaupp (97). The latter is the only thalamus nucleus in which 
optic fibres end in Acipenser. The great body of the optic tract 
passes on to the tectum, where its manner of ending is similar to 
that described by P. Ramon (94) in Amphibians and Reptiles, and by 
S. Ramon (91) in birds. A rudimentary corpus quadrigeminum 
posterius has recently been described by Gaurp for the frog (97), 
and according to EDINGER (96b, p. 107) is present in all Verte- 
brates. There is in Acipenser a special group of cells in this position, 
but the wall is thin and the cells are crowded and difficult to study. 
