74 Journal of Comparative Neurology. 



brains of other fishes, and in some cases this amounts to ahriost 

 a complete absence of differentiation among the elements con- 

 stituting the center. This simplicity is especially evident in the 

 cerebellum, lobus vagi, tectum, hypothalamus, lobus olfactor 

 ius. as described in the foregoing pages. 



Aside from this simplicity of nerve centers viewed from 

 the standpoint of function, or considered as apparatus, many 

 parts of the brain show primitive or embryonic characteristics 

 in the form and position of the individual cells. In lower ver- 

 tebrate brains the cells are commonly aggregated near the cen- 

 tral cavities and the outer zone is chiefly made up of dendrites 

 and fiber tracts This is true to a greater degree of the brains of 

 embryos or very y(jung specimL-ns. When these aggregations 

 of cells are examined they are found in embryos to be masses 

 of similar cells with slight differentiation ; and in adults the 

 greatest differentiation, as indicated by marked structural char- 

 acters, is usually found in those cells which are farthest removed 

 from the central aggregations. In this respect the brain of Pe- 

 tromyzon is to be compared with the embryo rather than with 

 the adult, although the higher differentiation of the cells farther 

 from the cavity is well illustrated. 



The cells of the most primitive form stand near the central 

 cavity, sometimes between the ependymal cells, and have a 

 central process which extends down in the ependyma and helps 

 to line the internal surface of the brain. There are no basal 

 dendrites and only one peripheral dendrite which shows few and 

 simple branches extending toward tac surface through the fiber 

 zone. The neurite arises from the dendrite or one of its chief 

 branches. Such cells are found in Petromyzon in the corpus 

 mammillare, where they constitute the whole of the grey matter ; 

 and in the acusticum, lobus vagi, cerebellum, central grey of 

 the thalanius and mid brain, tectum, inferior lobes, nucleus 

 thaehiae, striatum, and olfiictory lobes, where they constitute a 

 more or less important part of the structure. Such cells are 

 always relatively small and they are most numerous in those 

 parts of the brain which have not reached a large growth. 



The study of the cells in the brain of Acipenser and Pe- 



