MORPHOLOGY OF THE FOREBRAIN 499 



The olfactory bulb was undoubtedly the site of the initial tel- 

 encephalic evagination, but afterwards secondary olfactory tissue 

 and correlation tissue of all four laminae of the rostral end of the 

 neural tube were involved in this evagination and then further 

 differentiated in situ. 



Primitive^ the evaginated cerebral hemisphere was simply a 

 primary and secondary olfactory center. In very early phylo- 

 genetic stages ascending fibers entered this secondary olfactory 

 center from the pars dorsalis thalami for olfacto-tactile correla- 

 tion, etc., and from the hypothalamus for olfacto-visceral correla- 

 tions; and as we ascend the phylogenetic series this non-olfactory 

 correlation tissue assumes relatively greater importance. So far 

 as this tissue serves simple stereotyped reflexes it is developed 

 in the ventral part of the hemisphere — the visceral centers medi- 

 ally and the somatic centers laterally. The olfactory component 

 of the latter center plays progressively a smaller part in higher 

 animals until this tissue becomes the true corpus striatum. While 

 the ventral parts of the hemisphere are therefore favorably sit- 

 uated to serve simple, rapid stereotyped reflexes, the dorsal parts 

 of the hemisphere (pars pallialis, Gaupp,) not being in direct 

 connection with the corresponding diencephalic and lower regions 

 of the brain in forms above fishes, are not well adapted for these 

 rapid responses but rather for the slower and more complex dis- 

 criminative reactions and (in higher animals) intelligent acts. 

 Thus the two dorsal parts are in the course of the phylogeny grad- 

 ually transformed from secondary olfactory nuclei into true cor- 

 tex cerebri. Both dorsal parts continue throughout the phylo- 

 geny to receive some olfactory fibers, but these are much more 

 numerous in the dorso-lateral part, which forms the pyriform 

 lobe. Accordingly the latter in all mammals is more like the pri- 

 mordial secondary olfactory tissue than are the other parts of the 

 pars pallialis. 



Since the dorso-medial part of the hemisphere is to a less ex- 

 tent under the direct domination of any single one of the func- 

 tional systems which enter into the cerebral hemisphere, in it the 

 higher correlation tissue was first developed. The preponderat- 

 ing element at first in this pallial correlating apparatus was un- 



