OEIGIN OF THE CEREBEAL HEMISPHERES 441 



separate organs. Observation of the behavior of cyclostomes 

 shows that this is, in fact, their type of action system. 



This histological pattern of the cyclostome brain is in sharp 

 contrast with that of most of the true fishes and especially of the 

 teleosts, where the various functional systems of neurons are 

 segregated into as well-defined nuclei and fiber tracts as are those 

 of the brain stem of mammals. The histological pattern of the 

 cyclostome brain may without question be regarded as typical 

 of the primordial vertebrate ancestor. It is unspecialized, 

 plastic, and capable of differentiation in any direction. 



It may be assumed that, if the gnathostome vertebrates arose 

 from some primitive extinct type of cyclostome (as is the current 

 belief), the cerebral histological pattern of this ancestral form 

 was not more highly differentiated than is that of modern cyclo- 

 stomes. The history of the evolution of the brain from such an 

 ancestral form, through the primitive ganoids to the Amphibia, 

 cannot at present be written with satisfactory assurance. More 

 detailed knowledge of the internal structure of the brains of the 

 most generalized existing ganoids (Polypterus and its allies) 

 would probably contribute important evidence, and this is a 

 most inviting field for future research. Johnston's detailed 

 study of the internal structure of the brain of Acipenser ('01) 

 gives helpful insight into a nervous system but little more highly 

 organized. 



STRUCTURE AND DEVELOPMENT OF THE AMPHIBIAN HEMISPHERE 



Fortunately, the Amphibia themselves can supply the missing 

 hnks in their own phylogenesis with a high degree of probabihty. 

 Modern Amphibia, it is true, are aberrant forms and some are 

 probably retrograde. But the larvae of all species are very 

 similar and they come to functional development earlier in the 

 ontogeny. than do most other vertebrates, thus permitting direct 

 observation of the changes in histological pattern of actively 

 functioning reflex systems from the simplest possible form to the 

 considerable complexity of the adult anuran brain (Herrick 

 and Coghill, '15). Moreover, the adults of the various urodele 



