364 J. B. JOHNSTON 
striatal area through the foramen into the caudal portion of the 
hemisphere. Somewhat more than the rostral half of the hemis- 
phere is occupied by formatio bulbaris. The further evagination 
would enlarge the caudal part of the hemisphere until it became 
larger than the rostral part. At the same time the hemisphere 
is drawn forward by: the elongation of the rostrum, the olfactory 
bulb becomes separated form the caudal part of the hemisphere 
and a longer or shorter tractus olfactorius or olfactory peduncle is — 
established. 
The relations of the primordium hippocampi to the roof of the 
enlarging hemisphere are especially interesting. We have pointed 
out that there is an abrupt change of structure between the prim- 
ordium and the roof of the hemisphere which may be called a zona 
limitans. As the primordium hippocampi pushes out into the- 
roof of the hemisphere this zona limitans would be placed suc- 
cessively farther and farther laterad in this roof. In selachians 
(11 a) this zona limitans runs from the olfactory peduncle back- 
ward along the dorso-lateral wall of the lateral ventricle. For 
a long time in the phylogeny there remains a portion of the primor- 
dium hippocampi in the telencephalon medium extending along 
the dorsal border of this region to the point of attachement of the 
velum transversum to the eminentia thalami. In this position, 
where the whole of the primordium is located in cyclostomes, 
there exists in Chimaera (’10 d) and selachians (’11 a) a slender 
ridge of gray matter accompanied by the fibers of the posterior 
pallial commissure. In ganoids and teleosts (’11 b), where the 
evagination of the hemispheres has been arrested, the primordium 
has retained in the caudal part of the telencephalon essentially 
the relation which it holds in cyclostomes and has taken part in 
the eversion of the forebrain. In amphibians the evagination has 
gone farther than in selachians, the primordium hippocampi occu- 
pying the roof portion of the hemisphere and the zona limitans 
being carried farther down in the lateral wall. There is still left 
in the telencephalon medium, however, a small part of the primor- 
dium hippocampi corresponding to that in selachians, although 
shorter (Johnston, 06). Herrick (’10, p. 476). has endeévored to 
show that this structure belongs to the eminentia thalami. It 
