. OLFACTORY CENTERS IN TELEOSTS 199 
the caudal limits of the forebrain include the velum transversum | 
and the optic chiasma. The pedunculi thalami, the praethalamus 
of C. L. Herrick, are included in the telencephalon, and their cen- 
ters have already been described (nucleus preopticus and nucleus 
entopeduncularis). 
Most writers on the brains of fishes have, however, included 
these structures in the diencephalon; in fact even under the inter- 
pretation here followed, the pars parvocellularis posterior of the 
nucleus preopticus extends into the diencephalon, since it reaches 
caudally to the level of the fibrae ansulatae. 
b. Gross morphology 
The diencephalon in the carp is of the typical teleostean type. 
Immediately caudal to the velum transversum, the diatela is 
thrown into a convoluted folded epithelial sac, extending ros- 
trally over the membranous pallium of the hemispheres, forming 
the saccus dorsalis, post-velar arch, or Zirbelpolster (figs. 68, 73). 
This is an extremely vascular structure, formed by the covering of 
pla mater and a lining, continuous with the ependyma of the third 
ventricle. Arising immediately caudal to the saccus dorsalis, 
with the caudal wall of the one practically adherent to the rostral 
wall of the other, is the epiphysis or pineal body. This is a small 
elongated tubular organ extending rostrally, suspended in the 
folds of the dcrsal sac. Its epithelium, while an extension of that 
of the ependyma, is glandular in type. Lying embedded in the 
membranous wall between the dorsal sac and the epiphysis, is 
found the commissura habenularum, or commissura superior. At 
the caudal base of the epiphysis is found the commissura posterior, 
between it and the tectum opticum. 
The diencephalon is commonly subdivided into epithalamus, 
_ hypothalamus and thalamus. The latter has been divided by 
_C. J. Herrick (10), followimg Ramén y Cajal, into pars dorsalis 
(sensory correlation centers) and pars ventralis (motor correlation 
centers). The epithalamus of the carp is distinct; the other parts 
are so confused that further embryological study will probably be 
necessary to effect this separation; and the assignment of the differ- 
