THE TELENCEPHALON IN CYCLOSTOMES 361 
fibers. The pathway by which the fornix columns run in all 
higher classes is occupied by a broad bundle of fibers connecting 
the medial olfactory nucleus with the primordium hippo¢ampi 
(see above). If any fibers descend through this to reach the 
hypothalamus, they would represent the fornix columns. We 
must await further investigations upon these points. 
We may now summarize the evidence for the interpretation of 
this ‘praethalamus’ or ‘dorso-median ridge,’ which has been 
assumed in using the name primordium hippocampi. 
(a) Its caudal end is just in front of the eminentia thalami to 
which the velum transversum is attached. It is therefore wholly 
within the telencephalon. 
(b) It contains highly developed and specialized cells of a type 
which is characteristic of the primordium hippocampi in selachians 
ganoids, teleosts and amphibians. 
(c) It is bounded by a ventricular sulcus which agrees closely 
in position with the sulcus limitans hippocampi of selachians, 
ganoids and teleosts. 
(d) Along the line of this sulcus there is a sudden change from 
the characteristic cells to cells of very different formin the thalamus, 
epithalamus and hemisphere. This abrupt change of structure is 
comparable to the zona limitans of fishes and amphibians. 
(e) It is traversed by a part of the tractus olfacto-habenularis 
as in ganoids and teleosts. 
(f) It has true commissural fibers passing through the superior 
commissure as in fishes and amphibians(commissura pallii pos- 
terior). 
(g) It receives from in front fibers of the olfactory tract, direct 
and crossed, comparable to those in selachians and in part to 
those of ganoids and amphibians. 
(h) It receives a tractus pallii ascending from the hypothalamus 
as inallfishes. The center is therefore to be regarded as anolfacto- 
gustatory correlation center. 
(i) It appears probable that there is a tertiary olfactory tract 
ending in this body (tractus olfacto-corticalis). 
Admitting such uncertainty as exists in the present state of our 
knowledge regarding the posterior pallial commissure and the 
tertiary olfactory connections, we have here a body of evidence 
