370 J. B. JOHNSTON 
Caudal to this middle point of the forebrain the visceral recep- 
tive column forms the olfacto-gustatory correlation center or 
primordium hippocampi. From the study of other fishes (11 a 
and ’11 b) I have shown reason to think that the somatic correla- 
tion center, or primordium of the somatic cortex has been formed 
by eversion and migration of neuroblasts from the dorsal border of 
the telencephalon in this caudal part. 
The cephalic part of the forebrain comes to be ventrally placed 
owing to the flexure, and is constituted chiefly or wholly of the 
medial and lateral secondary olfactory centers and preoptic 
nucleus. The formatio bulbaris arises from tissue nearest the 
olfactory nerve root at the middle part of the dorsal column and 
in higher forms is carried out upon a peduncle. In some petromy- 
zonts it actually retains its relation to the median ventricle. 
An understanding of the fundamental morphological and func- 
tional relations in the forebrain must be built upon the central 
fact of the forebrain flexure. The conception of a curved axis of 
the telencephalon ending at the preoptic recess, and of the olfac- 
tory nerve entering the dorsal border at the height of the curve 
must never be lost sight of. These are fundamental facts, not 
hypotheses, and all forebrain morphology becomes confused unless 
these facts are held firmly in mind. 
In forms above the cyclostomes the rostrum elongates, the olfac- 
tory sae shifts forward, the bulbar formation follows it and becomes 
pedunculated. At the same time the evagination of hemispheres 
progresses rapidly and the hemispheres elongate. Now the ceph- 
alic and caudal portions of the visceral receptive column are 
sharply flexed on one another in the form of a letter U (figs. 40, 
41). The olfactory tract enters the curve of the U. The cephalic 
limb, containing the secondary olfactory centers is ventrally placed 
and ends at the preoptic recess. The caudal limb (primordium 
hippocampi) is dorsally placed and ends at the eminentia thalami. 
The two limbs are separated by the interventricular foramen and 
by the medial and lateral zona limitans as already explained. 
Owing to the eversion and migration of the neuroblasts of the 
somatic correlating center down upon the lateral surface of the 
telencephalon this primordium of the general cortex comes to lie 
