368 J. B. JOHNSTON 
medial olfactory nucleus by a sulcus limitans hippocampi. Along 
the line of this sulcus is a sudden change of structure which may 
be compared with the zona limitans of fishes and amphibians. 
In vertebrates above cyclostomes the primordium hippocampi 
is gradually evaginated into the hemisphere, the process being 
complete in the reptiles. It is only as the hippocampus is carried 
into the medial border of the hemisphere that the tela chorioidea 
comes to form the roof of the interventricular foramen and to 
extend into the roof of the hemisphere. 
The expansion of the hemisphere carries both the primordium 
hippocampi and the medial olfactory nucleus rostrad beyond the 
lamina terminalis where they form the medial hemisphere wall. 
The sulcus limitans hippocampi which separates these two nuclei 
in the telencephalon medium in petromyzonts becomes the sulcus 
limitans medialis hippocampi in the hemisphere of selachians, 
amphibians and reptiles. 
The relation of the primordium hippocampi to the lamina supra- 
neuroporica in which the anterior pallial commissure lies is funda- 
mental in vertebrates and must be taken into account in seeking 
the interpretation of higher brains. 
The reader is referred to the extended discussion of forebrain 
morphology contained in the writer’s paper on selachians (711 a, 
p. 37, especially pp. 47-52). The features of the evolution of the 
forebrain upon which the cyclostome brain throws light especially 
are briefly the following. 
(a) The hypertrophy, rising dorsad and eversion of the olfacto- 
gustatory correlation center. The same thing occurs in the vis- 
ceral receptive column in the medulla oblongata of many fishes 
and the nucleus habenulae commonly. 
(b) The presence originally of the whole olfactory central appara- 
tus in the wall of the medial ventricle. Hemispheres are formed 
by the evagination first of formatio bulbaris, then secondary 
olfactory centers, and last the visceral and somatic correlating 
centers which furnish the materials for the development of the 
cortical structures. | 
(¢c) From a condition like that of cyclostomes the selachian 
telencephalon has been derived by the evagination chiefly of the 
