124 JOURNAL OF COMPARATIVE NEUROLOGY. 
sary by the development of lateral eyes in the vertebrate 
phylum. The tectum embraces the central expansions and asso- 
ciated connections of the nerve-fibres having their origin as 
axones of the retinal neurones. Certain neurones of the tectum 
may, also, send their axones outward into the optic nerve. The 
whole complex is, in fine, the primitive visual centre. 
The tectum of Mustelus does not approach the extreme 
degree of differentiation which Ramon y Caja (89, ’g1) has 
described from the optic lobes of birds, but it is practicable 
to distinguish three zones of structural elements: the layers 
of the superficial, the middle, and the deeper neurones, respect- 
ively (Fig. 21, 5.7., m.”., d.n.). VAN GEHUCHTEN ('94) has 
described three zones from the optic lobe of the teleost, but 
these are not exactly equivalent to the layers noted here. His 
couche moléculaire appears to include both my superficial and 
middle layers; his couche granuleuse corresponds in position to 
my deeper layer; while his couche des cellules épendymaires is an 
inner zone which apparently does not include the central gray 
matter. 
a. Termination of the Opticus. Str. Medullare Profundum.— 
A great bundle of fibres may be traced dorso-posteriorly from 
each angle of the chiasma to the optic lobe, where it becomes 
dissolved through spreading over the surface. A section shows 
that the fibres in the outer zone lie parallel with the surface 
during some little part of the length of their course, and that 
they then pass downward into deeper parts of the tectum. 
Fine branches are given off in the regions occupied by the 
neurones of the middle and deeper layers, and many termina- 
tions appear to occur here. A certain number of fibres, how- 
ever, pass to ever deeper levels, tending to become collected — 
into bundles, and they finally blend into the stratum medullare 
profundum. 
The stratum medullare profundum is a conspicuous feature 
of the deepest part of the tectum mesencephali. At the crown 
of the arch which the optic lobe presents in section, the fibres 
are seen as great horizontal bundles interrupted at almost per- 
fectly regular intervals by small groups of vertical fibres. The 
ee |, ee ee See ee ee ee a ee ee eee 
