336 NERVOUS SYSTEM OF VERTEBRATES. 



dorso-mesial margin. The neopallial commissure at first ran 

 in the primitive forebrain commissure (anterior commissure) 

 and later followed the movements of the hippocampal commissure 

 upward to the dorsal edge of the lamina terminalis. The long- 

 itudinal tracts of the neopallium pass up and down through the 

 corpus striatum lateral to and independent from the olfactory 

 tracts and to quite different destinations (Fig. 166). It is impossi- 

 ble to believe in the face of all that we know of the evolution of 

 structure in the central nervous system that the neopallium has 

 appeared spontaneously as a new formation in the brain of primi- 

 tive mammals. It has arisen by the greater development of 

 some structure previously existing in the vertebrate brain. That 

 it should have arisen by modification of some part of the olfactory 

 centers is inconsistent with all that has gone before in the present 

 volume. The neopallium may be wholly absent from the brains 

 of existing members of the classes below mammals but those 

 brains have by no means been studied exhaustively enough to 

 warrant such a statement. Indications of the existence of a 

 homologue of the neopallium are to be looked for in the form of 

 tracts connecting somatic sensory centers with centers in the 

 forebrain isolated from the olfactory apparatus. A tract is 

 certainly present in fishes running from the tectum opticum to the 

 forebrain, but the center has not been isolated, perhaps because of 

 its small size in all submammalian classes. The existence of a 

 cutaneous nerve connected with the forebrain in many fishes 

 (nervus terminalis) gives encouragement to the expectation that 

 a forerunner of the neopallium may be found in lower vertebrates 

 in the form of a segment of the somatic sensory column in the 

 forebrain. It is to be expected that further study of amphibia 

 and reptiles from this point of view will reveal the rudiment of the 

 neopallium, whose rapid development in primitive mammals 

 led to the dominance of this class of vertebrates. 



DEMONSTRATION OR LABORATORY WORK. 



i. Upon careful dissections of the forebrain of a selachian, a bony 

 fish, an amphibian and a mammal verify the general morphological 

 relations described in this chapter. 



