Johnston, 77!^' Brain of Pctromyzon. 7 1 



dency is seen in the recognition of a cerebral cortex with several 

 appropriate tracts where nothing of the sort exists. Aside 

 from a predisposition to find these structures, there is a certain 

 suggestion of their presence to be found in the gross structure 

 of the Petromyzon brain. This consists in the presence of lat- 

 eral expansions of the fore brain containing cavities analogous in 

 position to the lateral ventricles of higher vertebrates. Upon 

 the dorsal wall of these apparent lateral ventricles, then, Mayer 

 ('97) and Studnicka ('98) have found a so-called cortex. The 

 proof that this is in no sense a true cortex, given in this and 

 the previous paper ('oi a), makes it necessary to find some other 

 explanation for the "lateral ventricles." As already suggested 

 (p. 39). these are probably due to simple mechanical agencies. 

 The displacement of the hypophysis and olfactory pit upon the 

 dorsal surface of the head by the growth of the oral funnel is 

 well known. That this organ has at the same time compressed 

 and altered the form of the brain is not so evident, but a little 

 attention shows it to be very probable. The olfactory pit is 

 closely pressed against the olfactory lobe, so that the olfactory 

 nerve is of no appreciable length. The lobe itself is short and 

 wide and in surface view does not appear to be so distinctly 

 paired as in other fishes. Internal examination shows, how- 

 ever, that the two lobes are widely separated throughout the 

 greater part of their length by structures of very different na- 

 ture, — dorsally by the epistriatum, ventrally by the lamina ter- 

 minalis. The lamina contains the anterior commissure as usual, 

 except that here it is divided into two distinct commissures, 

 the cephalic one of which is related to the olfactory lobes alone 

 and is situated near the cephalic end of the lobes. These facts 

 indicate that the olfactory lobes have been compressed and 

 pushed backward and outward by pressure from the olfactory 

 pit. This has brought the lobes to lie at the sides of the axial 

 portion of the fore brain and they have in turn crowded the 

 olfactory areas outward and backward until they form the great 

 lateral expansions to which the name "hemispheres" has been 

 erroneously given. The whole fore brain at the same time has 

 been somewhat bent upward as is indicated by the position of 



