PROSENCEPHALON OF SPINAX AS A TYPE OF ELASMOBRANCH FORE-BRAIN. 497 
cortico-medialis, namely, more longitudinally or obliquely. The course of the tractus 
medianus, as I have found it in the prosencephalon of Spinax, is that of fibres 
rising in the ventral part of the primordium hippocampi just on the dorsal side 
of the zona limitans medialis. which run ventrally on either side lateral to the 
median line and medial to the fibres of the tractus cortico-medialis, and finally 
proceed to the medial fore-brain bundle. The following additional points have, 
however, to be noticed. I have definitely traced a few fibres from the formatio 
pallialis lying external and fairly far lateral to the saucer-shaped tuberculum 
olfactorium, which pass to the more medially situated tractus medianus. I 
hold that the tractus cortico-medialis is also connected by fibres with the more 
external, lateral olfactory tracts. At this region of the brain (that in which the 
posterior ends of the tractus medianus and tractus cortico-medialis are situated) 
one has a great convergence of different tracts which pass directly or indirectly 
into the medial fore-brain bundle or the anterior commissure ; such tracts are 
the tractus cortico-medialis, tractus olfacto-hypothalamicus lateralis and medialis, 
and the tractus medianus. These tracts are in connection by fibres passing 
here and there from one to the other. I have noted these connections in detail 
during my examination of the sections, but for the sake of brevity must here 
omit them. 
Basal Fore-brain Bundle (fig. 11).— From the work of authorities on the structure 
of the brain of Teleostean and Ganoid fishes, it has been recognised that the basal 
fore-brain bundle consists of (a) a median portion, the tractus strio-thalamicus, 
Edinger, etc. (anterior commissure, medial fore-brain bundle, and tractus olfacto- 
hypothalamicus medialis of Johnston), and ( b ) of a lateral portion, the tractus 
olfacto-hypothalamicus lateralis. Johnston has noted the presence of “ a third 
portion of the basal fore-brain bundle which has hitherto been overlooked.” He 
terms this part the lateral fore-brain bundle, but admits that at the region of the 
optic chiasma there is no clear distinction between lateral and medial fore-brain 
bundles. The lateral fore-brain bundle is, according to Johnston’s views, similar 
to the basal fore-brain bundle described by Gaupp in Amphibia. The tractus 
thalamo-corticalis, which is said to make up the greater part of the lateral fore-brain 
bundle in Selachians, is partly seen in figs. 11 and 12, but owing to the limits of space 
I must here omit its discussion. 
Corpus callosum . — Johnston has described and figured on the lateral and dorsal 
surface of the prosencephalon medium of Scyllium canicula a set of fine fibres, of 
which he writes, “ the commissure of the somatic areas in the lamina supra- 
neuroporica resembles in every way the corpus callosum, with which the writer 
believes it to be homologous.” In fig. 9 one sees dorsally to the ventricle several 
fibres running more or less vertically to the margin, and ventrally to these other 
fibres passing parallel with the circumference. The more dorsal of these fibres may 
perhaps correspond to part of the corpus callosum, and the more ventral to part 
