Functional factors in the morpholog}' of the forebrain of fishes 187 



Acanthias and described in detail with photograpliic illustrations the dis- 

 tribution of these superficial cellular laminae which he regards as true 

 cortical primordia. He draws a clear distinction between basal and pallial 

 or cortical regions, and defines the pallium as the dorsal part of the tel- 

 encephalon separated from subpallial parts by a cell-free zona limitans. 

 In Acanthias embryos from 4 to I 5 cm. in length he finds three áreas 

 of superficial gray matter in the pallial región, which he terms hippo- 

 campal pallium, general pallium, and pyriform pallium (fig. ig). These 

 áreas become less distinct in the adult brain; but corresponding re- 

 gions are identified in most other types of fishes and he is of the opi- 

 nión that they are the primordia of the cortical áreas bearing the same 

 ñames in reptiles and mammals. In teleosts the región termed in this 

 paper pars lateralis of the área olfactoria dorsalis (figs. 23, 24) is regar- 

 ded as primordial hippocampal cortex, the pars dorso-lateralis as pri- 

 mordial general cortex, and the pars dorso-medialis as primordial pyri- 

 form cortex. 



The pallial áreas of these embryos of Acanthias all lie within the área 

 olfactoria dorsalis. That this región contains the primordium from which 

 the hippocampus of higher brains is developed I think there can be no 

 doubt. But the further interpretation of the embryonic «cortical» sub- 

 divisions within this área cannot safely be made on the basis of cellular 

 pattern in embryonic stages alone; it must wait upon fuller knowledge of 

 the fate of these subdivisions in the adult and their functional connections. 

 In this review, therefore, a conservative course has been follwed. In all 

 fishes the dorsal área is undoubtedly an olfactory nucleus, and it is here 

 so designated. 



In the present state oí our knowledge it is better to regard the área 

 olfactoria dorsalis of all groups of fishes which are considered in this con- 

 tribution as a unit, without attempting to recognize within it primordia of 

 definite cortical áreas of higher brains. In the more highly specialized 

 fishes its shape varíes in dillerent species depending on the space requi- 

 rements of this and adjacent parts; and the internal structure shows local 

 differentiations whose patterns vary with the nature and topographic re- 

 lations of the functional connections with other parts. This implies some 

 plástic rearrangement of the material concomitant with changes in the 

 functional connections, rather than a rigid mosaic of morphological 

 pattern. 



