152 C. Judson Herrick 



respectively. Ascending nonolfactory fibers enter both zones chiefly 

 from the hypothalamus. These bring gustatory, visceral and other func- 

 tional systems iiito relation with the área olfactoria. Both zones discharge 

 into epithalamus and hypothalamus. The intermedíate zone lies opposite 

 the thalamus and its differentiation has been effected largely under the 

 influence of ascending córrelation fibers which have grown forward from 

 the optic and somesthetic centers of the thalamus into the área olfactoria 

 lateralis. Thus aróse an olíacto-somatic zone, purely olfactory at the 

 rostral end, purely somatic at the diencephalic end, and of mixed charac- 

 ter between. The área somática of the telencephalon (or at least its 

 diencephalic border) apparently is of the same physiological type as the 

 somatic thalamus (Johnston). The composition of these longitudinal zones 

 of Acipenser may be expressed as foUows: 



Telencephalon 



Diencephalon 



1 Área olfactoria dorsalis, stria medullaris. 

 . ) Arf 



Habenula. 



TI 11 ir ) Área olfactoria lateralis, área olfacto-somatica, área 



llUlbUS olí.' ,. Tl,-.lnr>,,,c: 



j somática 1 1 halamus. 



f Área olfactOiMa medialis, nucleus preopticus i Hypothalamus. 



The arrangement here expressed is a physiological adaptation, pro- 

 bably of later phylogenetic origin than primitive metamerism, zones of 

 His, and similar very ancient vertébrate characters. It is cenogenetic 

 as compared with the palingenetic characters last mentioned and was 

 probably superposed upon them. 



Petromyzon. 



The forebrain of the petromyzonts is in some respects more primitive 

 than are those of any of the true fishes: in other respects it is specialized 

 in aberrant directions. The comprehensive researches of Johnston have 

 established the most important morphological features of these brains, 

 but many details remain obscure and the homologies of some of the parts 

 can be determined only after further studies of the internal structure. 



The brain of the North American lake lamprey, Ichthyomyzon conco- 

 lor, as described by Herrick and Obenchain (1913) is a favorable type 

 for the purpose of this brief review, for its external form is especially 



