82 OSBORN. [VoL. II. 
bedded in the central gray of the thalamus, and not clearly 
differentiated as a distinct nucleus from the surrounding cells. 
The third tract, II. ¢ 3, enters the hemispheres directly. This 
observation, although made in both the Uvodela and Anura, re- 
quires to be confirmed. The optic tracts have other sources 
of origin ;1 the above are the main centres contributing the prin- 
cipal portion of the nerve, and are the only ones which can be 
clearly made out by the carmine method.? ! 
In Rana the roof of the optic lobes is divided into eight 
distinct layers, the fibres of the superior portion of the optic 
tract entering the outermost layer, 1 Fig. 26, and ramifying 
to the interior cells. The inferior portion of the optic tract 
enters the second fibre layer, 3. The tracts surround the whole 
circumference of each of the lobes. 
The relations of the encephalic tracts in the general architec- 
ture of the amphibian brain is shown in Fig. 21, in which only 
the well-determined tracts are introduced. The designation 
of the tracts expresses my present views; as I have already 
stated, the efferent or afferent character of the main tracts is 
not by any means settled. 
COMPARISON OF THE DIPNOAN AND AMPHIBIAN BRAIN.? 
The recent memoir of Fulliquet upon the brain of Protopterus 
is the first contribution to the histology of the Dipnoan brain, 
and enables us to make some interesting comparisons. In the 
first place, as observed by this author and others, the general 
external resemblance between the Dipnoan and Urodele brains 
is very striking. The principal external features in common 
are as follows: the olfactory lobes are not well distinguished 
from the hemispheres ; this is even more marked in Protopterus 
than in the Urodela. The mesencephalon of Protopterus passes 
imperceptibly into the diencephalon, as in Amphiuma, only a 
faint lateral constriction between these segments being evident.* 
1 See the papers of Bellonci, Blaschko and Edinger. 
2 In my paper upon AZenofoma (84, p. 267, Fig. 8), I described a portion of the 
optic nerve as non-decussating. I am now inclined to consider these supposed 
uncrossed tracts, 6, as the mesencephalic tracts which cross at a different level, or the 
basal optic root of Edinger. The sections at this point are deceptive. 
8 See also Appendix, Note 4. 
4 In distinguishing between these segments internally, Fulliquet has failed to take 
