492 C. JUDSON HERRICK 



THE SUBDIVISION OF THE PROSENCEPHALON 



Johnston suggests ('09, p. 533) a revision of the BNA subdivision 

 of the cerebrum. Adopting the point of view of his revision, I 

 make the following comments upon his proposals. In discussing 

 the general morphology of the telencephalon, he says ('09, p 519) 

 "The term hemisphere is applied in the BNA to each half of the 

 telencephalon. It would therefore include the right or left half 

 of all that lies in front of a plane passing behind the interventri- 

 cular foramina and the chiasma-ridge." 



It is trie that in the BNA tables the term hemisphere is applied 

 to each half of the telencephalon as that term is there defined; but 

 by treating the hemisphere as the lateral half of the telencephalon 

 as differently defined in this paper it seems to me that Professor 

 Johnston has introduced unnecessary and unfortunate confusion. 



In an earlier paper ('08a), I adopted and defended the boundary 

 between the diencephalon and the telencephalon as defined in 

 the BNA tables; but Johnston in the paper cited has made it 

 plain that a full knowledge of the comparative embryology of 

 these parts requires a revision of the BNA tables in this respect 

 to accord more nearly with the original usage of His. That is, the 

 telencephalon is not to be regarded as a secondary derivative of 

 the primary segmental neural tube, but as the terminal segment 

 of the tube with its secondary derivatives. Accordingly, the 

 lamina terminalis, preoptic recess and adjacent parts, instead of 

 being assigned to the diencephalon, are regarded as belonging to 

 the telencephalon. 



But in the interest of conformity to past usage, as well as ana- 

 tomical fitness, the term hemisphere should be limited to the 

 secondarily evaginated derivative of the primary neural tube and 

 sharply distinguished from the primary terminal segment of the 

 neural tube from which the evaginated portion is derived. That 

 is, the hemisphere should be defined exactly as in the BNA tables; 

 and when we include more in the telencephalon than the hemi- 

 spheres the added tissue should be given an appropriate name of 

 its own without disturbing the existing definition of the hemi- 

 spheres. 



