352 Journal of Compakative Neurology. 



found in the vertebrate series," should be understood in some 

 other than the phylogenetic sense probably, for even a more 

 highly differentiated brain in w^hich the primitive simplicity 

 of the prosencephalic vesicle is preserved may be believed to 

 indicate a more primitive origin. (') 



In order to get an idea of the histological differentiation 

 of the axial lobe, we may first examine a horizontal section 

 somewhat dorsad of the anterior commissure (Plate XXIV, 

 Fig. 2). The anterior commissure fibres are situated near 

 the median margin and are spreading out dorso-laterad in the 

 mesaxial lobe. The actual crossing of the fibres takes place 

 in the praethalamus or a projection from its dorsal aspect. 

 The fibres of the anterior commissure proper pass laterad 

 into the central lobe, beyond which they were not followed. 

 The central lobe is occupied with the numerous bundles of 

 the peduncles around which are grouped niduli of large 

 pyramidal cells such as are commonly found in motor areas 

 of higher animals. The processes are long and subdivide. 

 The apical process is usually peripherad. The nuclei are 

 small and densely stained. The entire cell stains more 

 deeply than other adjacent elements. A spur of such cells 

 follows the callosal fibres toward the meson (Fig. 2, Plate 

 XXV). The whole cephalo-lateral aspect is occupied by 

 the lateral, or parietal lobe, the cells of which are of the 

 aesthesodic variety. From the frontal part of the mesaxial 

 lobe it is separated by a distinct frontal fissure, as it is from 

 the cuneus by the Sylvian fissure. From the central lobe it 

 is separated by a thin tract passing obliquely dorso-cephalad 

 and laterad, i.e., radiating toward the periphery. From this 

 tract, which extends from the frontal fissure cephalad to the 



I It is interestimg to see that Owen has arrived at a conclusion similar to the above 

 on the basis of external form and gross dissection, as may be gathered from page 286 of 

 the first volume of his "Comparative Anatomy," especially the following par.-igraphs : 

 " It is interesting to perceive on the superficies of the solid prosencephalon in many fishes 

 the foreshadowing of the convolutions, which are not fully established until an advanced 

 mammalian grade is attained. The prosencephalon of the fish is far from being a minia- 

 ture model, but it may be regarded as the potential representative of the complex cerebral 

 hemispheres of man." 



