OLFACTORY CENTERS IN TELEOSTS 241 
lower vertebrates shows, in addition to the membranous median 
plates in the roof and floor, four longitudinal columns or laminae 
on each side, viz., the epithalamus, pars dorsalis thalami, pars 
ventralis thalami and hypothalamus (fig. 128). The last two 
contain motor correlation tissue, with somatic and visceral ele- 
ments, respectively, predominating. 
In the primordial vertebrate these four columns probably 
extended forward into the telencephalon without fundamental 
change. In all existing vertebrate types variable amounts of this 
telencephalic tissue are evaginated to form the cerebral hemi- 
spheres. The olfactory bulb clearly formed the initial center of 
evagination. In cyclostomes the hemisphere is composed of 
olfactory bulb, with part of the secondary olfactory nucleus (these 
coming from the telencephalic extension of the pars dorsalis 
thalami), and a very small corpus striatum, this being an extension 
of the pars ventralis thalami. In the lower elasmobranchs the 
olfactory bulb is fully evaginated and the telencephalon medium 
greatly elongated, with great thickening and a very slight evagina- 
tion of its rostral end. In the higher sharks the telencephalon 
medium is shortened in correlation with an increase in the thicken- 
ing of the tissue about the lamina terminalis and the further eva- 
gination in this region of the secondary olfactory centers. 
The Dipnoi show a very different line of specialization. The 
olfactory bulbs are in all cases fully evaginated. The telen- 
cephalon is not greatly elongated (except in adult Ceratodus) and 
its lateral walls are uniformly thickened and more or less com- 
pletely evaginated to form the cerebral hemispheres, whose form 
and structure, especially in the case of Lepidosiren, are very close 
to those of Amphibia. 
The morphology of the amphibian cerebral hemisphere has 
been fully discussed in the paper cited (Herrick, ’10 b), the author 
showing that it is naturally divided into four parts (exclusive of 
the olfactory bulb), viz., (1) pars dorso-medialis (primordium 
hippocampi), (2) pars dorso-lateralis (primordium of the pyri- 
form lobe), (3) pars ventro-lateralis (primordium of the corpus 
striatum) and (4) pars ventro-medialis (precommissural body and 
septum). He shows further that these four parts are the telen- 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 22, NO. 3 
