Functional factors in ihc morphology of the forebrain of fishes 151 



the área olfacto-somatica has begun to differentiate as an olfacto-striatal 

 correlation center within the lateral olfactory área, but it has advanced 

 an almost imperceptible distance in this direction. 



Summary. — In the telencephalon of Acipenser the primitive form of 

 the neural tube is but little modified. The olfactory bulbs are evaginated 

 to form rudimentary cerebral hemispheres and in the remainder of this 

 división of the neural tube (telencephalon médium; the lateral walls are 

 only moderately thickened. Xearly all parts of the telencephalon médium 



Fig. 6. — Acipenser. Schema of the centers of the telencephalon and dience- 

 phalon, as sean in medial section of the brain. Compare figure 2. The primary 

 olfactory center (bulbus olfactorius) is marked with broken horizontal lines. The 

 dorsal zone (área olfactoria dorsalis and habenula) is marked with oblique lines, 

 and the ventral zone (área olfactoria medialis, nucleus preopticus and hypothala- 

 mus) by oblir4UC lines at an angle to the last. The intermediate zone lai-ea olfacto- 

 somatica, área somática and thalamus) is marked by vertical lines as far forward 

 as direct thalamic connections are supposed to extend. The rostral border of 

 this field is undetermined. The position of the área olfactoria lateralis, which is 

 the rostral member of this intermediate series, is not indicated. 



are reached by fibers of the tractus olfactorius and these parts comprise 

 the área olfactoria. 



The diencephalon has three main divisions: (l) epithalamus and (2) 

 hypothalamus, w hich are dominated by descending olfactory tracts,and (3) 

 thalamus proper, which is a non-olfactory somatic sensori-motor correlation 

 center. The telencephalon médium internally is differentiated into three 

 longitudinal cell-columns or zones which converge rostrally into the bulbus 

 olfactorius and whose characteristic internal structure is largely determi- 

 nad by the nature of their physiological connections with the diencepha- 

 lon. The topographic relations of telencephalic and diencephalic centers 

 are illustrated in the accompanying schema (fig. 6). The dorsal and 

 ventral telencephalic zones are related with epithalamus and hypothalamus 



