144 C. Judson Herrick 



the lateral walls of .the primitive neural tube; (2) the lateral eversión of 

 the dorsal parts of the massive lateral walls; (3) inversión of the dorsal 

 borders of the lateral walls and the rostral boundary of the tube (lamina 

 terminalis), thus dividing the rostral end of the cavity of the tube into 

 paired lateral ventricles; (4) local out-pouching or evagination of the late- 

 ral walls, thus producing hoUow cerebral hemispheres. 



AU of these four methods of increasing the bulk of the telencephalon 

 are found in the brains of lower vertebrates, and in some cases two or 

 more of them may be seen in the same brain, as will be illustrated 

 beyond. In the search for functional factors correlated with these mor- 

 phological changes we shall examine the arrangement of conduction path- 

 ways and centers of correlation in a selected number of types of lower 

 brains, taking as our first illustration the forebrain of the generalized ga- 

 noid fish, Acipenser. 



Acipenser. 



The brain of the sturgeon, Acipenser, illustrates in very simple and 

 primitive form three of the types of forebrain differentiation to which 

 reference has just been made, viz., local thickening, eversión and evagina- 

 tion. In this fish at the extreme rostral end of the neural tube each 

 lateral wall has eyaginated laterally and then forward beyond the terminal 

 píate (lamina terminalis) to form a hollow olfactory bulb (fig. 1). The 

 olfactory ventricle communicates with the common ventricle íventriculus 

 imparj by an interventricular foramen and is, in fact, a lateral ventricle. 

 The olfactory bulb is, therefore, a true cerebral hemisphere, though of 

 very small dimensions and limited functionally to the olfactory receptive 

 center. 



Behind the olfactory bulb the primitive endbrain, or telencephalon 

 médium, is extensive and the lateral wall is thickened (figs. 4, 5). Its 

 dorsal border adjacent to the membranous roof (taenia) is at its posterior 

 end very slightly everted (fig. 5). The thickened región contains the 

 centers of correlation between the olfactory system and various other 

 sensory systems whose nerve fibers ascend from lower levéis. These 

 olfactory correlation centers are in Acipenser part of the telencephalon, 

 or endbrain, but they are merely local thickenings of its walls. They 

 are not parts of the evaginated cerebral hemispheres as in higher brains. 



