242 RALPH EDWARD SHELDON 
cephalic extensions respectively of (1) the epithalamus, (2) the pars 
dorsalis thalami, (3) the pars ventralis thalami and (4) the hypo- 
thalamus and tissues surrounding the preoptic recess. The cere- 
bral hemispheres of amniote vertebrates are modifications of this 
fundamental pattern. 
The teleostean forebrain conforms neither to the selachian nor 
to the dipnoan and amphibian type. Further analysis of the series 
of ganoidean types and of the ontogeny of the teleosts will doubt- 
less shed light upon the steps by which the teleostean pecularities 
have been acquired. The study of the form and fiber connections 
of the adult brain, together with the available data bearing on 
its phylogeny and ontogeny, suggests the following interpretation. 
It is evident that the teleostean olfactory bulbs are completely 
evaginated and that they have earried out with them a small 
amount of secondary olfactory tissue, the nucleus olfactorius ante- 
rior. The remainder of the telencephalon remains unevaginated 
as the telencephalon medium, which is, moreover, considerably 
elongated. The failure of any considerable part of the telen- 
cephalon, except the olfactory bulbs, to evaginate laterally is the 
basis of its difference from that of the Dipnoi and Amphibia. 
The fact that the increase in its tissue takes place uniformly 
throughout its length or somewhat more at its caudal end instead 
of at its rostral end is the basis of its difference from the elasmo- 
branchs. 
The increase in the mass of the telencephalon occurs under 
the influence of two chief factors: (1) olfactory impulses coming 
in by way of the olfactory bulbs, (2) non-olfactory sensory im- 
pulses coming in for correlation purposes from the thalamus and 
hypothalamus. The correlation sought in the lower forms was 
exclusively with the olfactory apparatus; olfacto-somatic in the 
case of the thalamic tracts, and olfacto-visceral in the case of the 
hypothalamic tracts. In higher vertebrates the non-olfactory 
systems effect correlations inter-se thus giving rise to the neopal- 
lium; but little, if any of this sort of correlation occurs in fishes. 
In the teleostean brain, as has been pointed out earlier, the 
arrangement of the telo-diencephalic centers in the form of 
longitudinal columns, is plainly evident. At the rostral end of 
