l82 



C. íudson Herrick 



transversum. This is probably the primitive arrangement. The strong 

 tracts to the habenula from the área dorsaHs and its derivatives in higher 

 vertebrates are preserved throught the vertébrate phylum. 



As already indicated, the development of olfacto-somatic correlation 

 centers in the intermedíate zone seems to have tended to emancipate both 

 the habenula and the telencephaUc part of the dor- 

 sal zone from direct somatic sensory influence. The 

 ascending and descending connection between the 

 área olfactoria dorsaHs and the hypothalamus through 

 the tractus pallii is evidently very primitive, for it is 

 already well developed in cyclostomes, and this plays 

 a dominant 7'óle in the differentiation of the área ol- 

 factoria dorsalis of fishes, as Johnston has shown. 



This dorsal área has been called primordium hip- 

 pocampi by Jonhston and primordium pallii by 

 Holmgren. It is evident that the hippocampus is 

 related to it. According to Holmgren it gives rise 

 to the lobus pyriformis and the general cortex also. 

 Eversión and inversiojí in the brains of fishes. — 

 The problems of the morphologic relationships of 

 the parts of the área dorsalis in various groups of 

 fishes and of the representation of these in the eva- 

 ginated cerebral hemispheres of Dipnoi and Amphi- 

 bia offer great difficulties. 



The history of the embryological development 

 and the internal structure show that in Amia and in 

 teleosts generally the process of eversión has been 

 frequently misunderstood. In Polypterus (^^"aldsch- 

 midt, 1887, Holmgren, 1 922) the thin dorsal part of the wall of the 

 telencephalon médium apparently has been simply folded laterally as a 

 whole, and it was assumed by Mrs. Gage (1S93) and by several others 

 subsequently that the same principie has been followed in Teleostomi. 

 This view may be presented diagrammatically by figures 26 and 27 which 

 represent transverse sections immediately in front of the commissura an- 

 terior with the parts designated by the same conventional marking as in 

 figure 25. 



Figure 26 is from a very young ganoid fish. Figure 2~ represents 



26 



Fig. 26. — Diagram- 

 matic transverse 

 section through the 

 brain of a very 

 young ganoid fish, 

 immediately in 

 front of the commis- 

 sura anterior, illus- 

 trating the arrange- 

 ment of parts of the 

 telencephalon mé- 

 dium and the posi- 

 tions of the medial 

 forebrain bundle 

 and tractus palHi. 

 I, 2, and 3 lie in the 

 área olfactoria dor- 

 salis, 4 lies in the 

 área olfactoria late- 

 ralis.and área olfac- 

 to-somatica, 5 lies 

 in the área olfacto- 

 ria medialis. 



