Johnston, The Brain of Petroniyzon. 75 



tromyzon shows that growth and differentiations of cells in the 

 various centers takes place by several kinds of changes, of 

 which the following are chief: 



(i) Migration farther from the central cavity. 



(2) Elongation of the central process, followed by its loss 

 or modification into a dendrite. 



(3) Growth in volume of the cell body and dendrites. 



(4) Increasing complexity of the dendritic branching and 

 of the number of dendrites given off from the cell body, with 

 a tendency in the more highly developed centers to a speciali- 

 zation in the form and arrangement of the dendrites. 



(5) A tendency in slightly differentiated centers for the dis- 

 position of the dendrites to be determined by the direction of 

 the fiber tracts among which they lie. 



The migration of the cell with consequent elongation of 

 the central process is beautifully illustrated in the inferior lobes 

 of Petromyzon, and in the nucleus thaeniae of Acipenser. The 

 latter case also illustrates the modification of the central process 

 into a dendrite, the cell body in this especial case becoming 

 bipolar with two long dendrites ('01 c, Fig. N). The bipolar 

 cells of the tectum of Petromyzon probably have a similar his- 

 tory. In both these cases the neurite is given off from the end 

 or near the end of the original (peripheral) dendrite. The 

 growth and increase in number of dendrites is accompanied in 

 some cases by migration of the cell body, and in some cases not. 

 In the former case the cells frequently become multipolar or 

 stellate, passing through numerous intermediate gradations, as 

 in the tectum and olfactory lobe of both Acipenser and Petro- 

 myzon. In the latter case the cdls are likely to become spindle 

 shaped or pyramidal with few or small basal processes and with 

 the peripheral processes greatly developed in special forms, as 

 in the vertical cells of the tectum in Acipenser and in the epi- 

 striatum cells in both Acipenser and Petromyzon, Finally, it is 

 very noticeable in those centers, such as the tectum and olfac- 

 tory lobe, in which occur central grey masses with cells scat- 

 tered in the peripheral fiber zone, that the cell is more highly 

 developed the further it is removed from the central cavity. By 



