78. OSBORN. [Vou. II. 
through the fibres of the extreme lateral portions of the me- 
dulla, entering the cerebellum, cé/¢; then we observe large 
bundles of fibres from the lateral regions of the medulla, 
mst, ascending and spreading over the outer surface of the mes- 
encephalon, Figs. 24 and 30. In sections slightly internal to 
these, the central gray of the mesencephalon comes into 
view, with rows of cells and bands of fibres alternating, and the 
continuation of the same medullary tract is observed spreading 
over the outer surface of the diencephalon in precisely the same 
manner, dst. These direct diencephalic and mesence- 
phalic sensory tracts are beautifully shown in vertical sec- 
tions of the brain of Amphzuma, in which these segments are 
very slightly differentiated from each other. In succeeding. 
sections, still approaching the median line, the direction of the 
fibres is reversed; from the postero-lateral region of the mesen- 
cephalon and the lateral region of the diencephalon, the main 
trend of the fibres is downwards and forwards, mst! and dsz.! 
The latter fibres pass directly forwards into the basal portion 
of the prosencephalon. The former, ms¢’, turn downwards, but 
their forward continuation into the prosencephalon cannot be so 
distinctly followed. The simplicity of these ascending and 
descending systems in the Uvodela is interfered with in the 
Anura by the expansion of the optic lobes, but the arrangement 
is the same.} 
In transverse sections of the optic thalami, the dorsal portion 
of the cerebral peduncles is composed of a compact round bun- © 
dle of fibres, (Osborn, ’84, Fig. 8,2). The origin of this is 
somewhat uncertain. From the fact that it is first differen- 
tiated in the anterior portion of the mesencephalon and becomes 
more distinct in the thalami, we may infer that this bundle is 
composed of the prosencephalic sensory tracts formed 
from these segmenits: 
In Rana, in which the corpus callosum and-anterior commis- 
sure are somewhat separated, a portion of this bundle seems to 
pass between them (Fig. 29). There is room for error here in 
the fact that the basal fore-brain bundle from the medulla is 
reinforced by fibres both from the ascending diencephalic and 
mesencephalic tracts and from the infundibular tract. 
1 See Képpen, op. cit., Taf. III., Figs. 27, 28. 
