118 The American Naturalist. [February, 



others from the cephalic portions of the cerebrum. No indi- 

 cation of the eallosum was seen in Scaphirynehus." 



In the Drum, however, the eallosum is relatively large and 

 conspicuous. It lies far cephalad and somewhat ventrad (mor- 

 phologically dorso-cephalad) of the anterior commissure. It is 

 directly associated with a large nidulus of pyramidal cells, 

 which occupies a central position in the axial lobe in the direct 

 line of the pyramid fibres produced. This central lobe is the 

 unmistakable homologue of the motor areas occupying the 

 cephalic cortical regions of higher vertebrates, and is therefore 

 properly associated with the eallosum. The eallosum lies 

 dorsad of and adjacent to the line where the pallium adheres 

 to the axial lobe, it is therefore just where it would be ex- 

 pected upon the hypothesis that the structures otherwise found 

 in the cortex had been driven from the latter by its conversion 

 into the pallium, or, better, had failed to grow out into the pal- 

 li vim when it was formed. 



Remembering that the growth of the cerebrum has been 

 largely dorso-caudad, such a retarding as here supposed would 

 leave the eallosum where we find it at the eephalic juncture of 

 pallium and axial lobe. Since the interventricular cortical 

 lobe is suppressed in teleosts, it follows that the eallosum and 

 anterior commissure, which are collocated by accident, rather 

 than relationship in Reptilia and Amphibia, are here widely 

 separated. These facts are entirely in agreement with the 

 highly philosophical theory of the commissures proposed by 

 Osborn, though in the present case, Osborn supposed (an 

 opinion in which the writer at first shared) that the eallosum 

 of fishes is contained in the anterior commissure group. The 

 considerations above mentioned show that in the absence of the 

 mesal walls of the cortex a collocation the two commissural 

 systems, except by great axial shortening of the brain, would 

 be impossible. 



As to the general question, whether it is proper to suppose 

 that the cell structures normally found in the cortex are derived 

 from the axial lobe and may be retained there by conversion 

 of the cortex to a cell-less pallium, I refer, first, to the data 

 of embryology ; second, to my own observations upon young 



