70 Journal of Comparative Neurology. 



shown that even those cells which most resemble the granules 

 had neurites. The explanation of this condition was that the 

 lobe in Acipenser is relatively primitive and that these slightly 

 differentiated cells are the material from which the highly differ- 

 entiated elements of the higher olfactory lobe have developed. 

 This was supported by reference to the lobe of Rana and of 

 reptiles, in which similar cells arc found ('oi c, p. 90, 172). 



The olfactory lobe in Petromyzon gives the fullest confir- 

 mation of this interpretation. The existence of numerous cells 

 at all levels of the olfactory lobe which are in relation with ol- 

 factory fibers in the glomeruli, the grade of differentiation of 

 these cells being still lower than in Acipenser, the presence of 

 somewhat larger cells near the glomeruli with the characteristics 

 of mitral cells poorly developed, and the common destination 

 of the neurites of all these cells give conclusive evidence that 

 the olfactory lobe in the fishes is in the condition of a mass of 

 slightly differentiated cells, from which the true mitral cells are 

 the first to become differentiated Later appear the small and 

 superficial ' 'Pmselzellen. " It seems certain also that the granules 

 are represented by the stellate, spindle, and granule cells, which 

 are functional nerve cells in the fish, amphibian, and reptilian (?) 

 brain. 



b. The fore brain. 



The identification of the parts of the fore brain with their 

 characteristic connections brings Petromyzon into line with 

 other fishes and, taken with similar findings in other parts of 

 the brain, goes to show that the fundamental or reflex apparatus 

 is already well developed in the ancestors of existing fishes. 

 Probably very little of the mid, 'tween, and fore brain structure 

 exists in Amphioxus, indicating the very great gap between 

 the points at which Amphio.xus and Petromyzon branched off 

 from the phyletic tree. The somewhat complete development 

 of the reflex apparatus, including the olfactory area, striatum, 

 and epistriatum of the fore brain, in Petromyzon, does not at 

 all warrant, however, the tendency shown by F. Mayer and 

 others to read back into this primitive brain mjch of the com- 

 plexity of the higher vertebrate brain. The result of this ten- 



