•j^ Journal of Comparative Neurology. 



development it is meant to include size and complexity together 

 with the disposition of the dendrites with reference to functional 

 efficiency. For example : the functional efficiency of a stellate 

 cell in the tectum and of an epistriatum cell may be equally 

 g^reat, although the latter has developed along special lines and 

 has not always migrated from the central cavity ; but the effi- 

 ciency of either of these is much greater than that of the cells 

 with central processes and poorly developed dendrites in any 

 center. 



It occasionally happens that cells are found holding the 

 same relations to the external surface of the brain as the cells 

 of the central grey hold to the cavity. Examples of this are 

 found in the superficial horizontal and other superficial cells in 

 the tectum of Acipenser and Petromyzon. In these cases the 

 same principles hold, since these cells were primitively in con- 

 tact with the external surface. In the case of the superficial 

 cells of the fore brain which constitute the cortex (e. g., in 

 Acipenser) it is a question whether they have undergone further 

 changes than any of those above described, or are to be com- 

 pared with the primitively superficial cells of the tectum. 



It would appear from a comparative view of the choroid 

 plexuses in the roof of the brain that they are more extensive 

 in the lower forms. In Petromyzon the dorsal wall is non-ner- 

 vous throughout its whole extent except where commissures 

 occur, and it is only in the cerebellum that the commissure is 

 accompanied by a thick wall of cells in the median line. The 

 primitive vertebrate brain seems to have had a choroid roof 

 throughout its whole extent except for the commissures. 



V. Summary of Results. 



The study of the brain of Petromyzon has brought out the 

 following new facts and theoretical conclusions : 



1. The viscero-motor and somatic motor nuclei of the 

 medulla are imperfectly differentiated. 



2. The Mullerian fibers are the neurites of giant cells 

 lying in the central grey (ventral and lateral motor columns) of the 

 medulla and mid brain which have the same general characters 



