78 Evolution and Distribution of Fishes 



least these are deeply divided crosswise into an upper an- 

 terior and a lower posterior pair. Such a condition may well 

 have been common to many extinct nemerteans, and it sug- 

 gests the probable primitive origin of the ancestral fore and 

 mid brain of vertebrates. The ventral brain-lobes that, 

 on separation of the sheath-notochord from the proboscis- 

 pituitary body, seem to have risen upward so as to be 

 behind the dorsal masses, could then become the hind 

 brain, while the strong commissure joining them suggests 

 origin of the pons varolii of the vertebrate brain. 



The above then suggests that the brain masses in higher 

 nemerteans consist of an anterior and superior pair of lobes, 

 and of a posterior and inferior pair of lobes, all united 

 by commissures. Further, the superior pair may — as in 

 Eupolia — undergo subdivision into what would then become 

 an anterior and a median pair. Now in relation to the 

 evolving vertebrate brain Graham Kerr {43) has made 

 some highly important observations. For on the basis of his 

 study of the dipnoan brain he concludes that the vertebrate 

 brain primitively consists of two brain masses, an anterior 

 and a posterior. And in relation to the paired lobes of 

 nemerteans his study of the three genera of living Dipnoi 

 causes him to conclude "that the hemisphere region is 

 primitively paired," and so is "against the more generally 

 accepted view that the hemispheres are to be looked on 

 as the terminal region of the brain, as a 'telencephalon' in 

 the sense of His." The above views he has further 

 elaborated in his recent volume on "Vertebrata" (^^:85). 



The brain substance of metanemerteans and of fishes 

 consists of zones of gray and of white matter, the gray sub- 

 stance being specially rich in large ganglionic cells. 



The primitive beginnings of the spinal cord in fishes 

 have been variously outlined and interpreted by different 

 workers. Hubrecht first clearly set forth a possible ex- 

 planation, and connection evolutionarily, with nemerteans. 

 In the latter two main cords run continuously backward 

 from the ventral brain-lobes as the lateral nerves. These 

 may vary in position from lateral to slightly latero-ventral 

 or latero-dorsal. In the marine genus Langia however they 

 are markedly dorso-lateral. But in metanemerteans toward 



