ANATOMY OF A CYCLOSTOME BRAIN • 655 



DISCUSSION 



The observations here reported were du-ected primarily toward 

 the elucidation of certain facts regarding the external form and 

 ventricular sculpturing of the brain of Ichthyomyzon in relation 

 to the underlying deep structures, in the hope that these data 

 may ultimately be of service in the interpretation of the mor- 

 phogenetic factors which have operated in the evolution of the 

 definitive form of the vertebrate brain. The brief reference to the 

 brain of this species made by Herrick ('10) and the more thor- 

 ough examination made by Johnston ('12) brought out some dif- 

 ferences both of observation and interpretation which require 

 control, and to these points attention will next be directed. 



In the first place, as to terminology, we have here endeavored 

 to select names for all parts, whose morphological significance 

 is not quite definitely established, which are as objective and 

 free from interpretative implication as possible. These topo- 

 graphic designations, such as lobus medius thalami, lobus sub- 

 hippocampalis, etc., are of course purely provisional descriptive 

 terms to be abandoned whenever definite homologies with higher 

 brains can be determined. It is not improbable that for some 

 of these parts such homologies can never be established; for some 

 of the structures here enumerated are doubtless expressions of 

 cenogenetic functional differentiation, rather than vestiges of pri- 

 mary segmental or other primitive morphological relations, and 

 identity of structural patterji in such functional adaptations is 

 not to be expected when aberrant members of the phylogenetic 

 series are directly compared. 



A fundamental problem in such studies is, accordingly, to dif- 

 ferentiate between such structures as the sulcus limitans of His, 

 which mark very ancient and primitive relations, and functional 

 adaptations of more recent origin and limited occurrence. Ob- 

 viously the first step is the determination of the exact form and 

 functional connections of all the parts in question in a large series 

 of vertebrate types and developmental stages before such gen- 

 eralizations can safely be attempted. And our knowledge of 

 the internal structure of the diencephalon of cyclostomes (and 



