Johnston, The Brain of Petromyzon. 21 



which extends forward somewhat into the aqueduct. Beneath 

 it there is a small and inconspicuous commissure formed of very 

 fine fibers which collect in the median acusticum nuclei just as 

 they come together in the cerebellum. The fibers of this small 

 commissure enter the fiber layer after crossing. The two com- 

 missures are probably homologous with the commissure of fine 

 fibers in the moltfcularj layer of Acipenser which arise from 

 the granule cells and enter the cerebellar crest of the other side. 

 The cells in the cerebellum are almost identical with those 

 in the dorso-median nucleus of the acusticum. There are rela- 

 tively large cells (10-18 X 20-32 //) which form a layer adjoin- 

 ing the cavity and send their dendrites out into the fiber layer. 

 As may be seen from Figs, i i and 22, these cells closely re- 

 semble cells similarly placed in the acusticum. Other cells 

 measuring 12— 16 X 16-24/^ are found in the outer part of the 

 cell layer (Fig. 12 left). The neurites of many of these cells 

 are very readily traced in sagittal sections (Fig. 22). Both the 

 cells adjoining the cavity and those lying in the outer part of 

 the granular layer give off neurites of medium thickness from 

 the base of one of the dendrites. The neurites run ventrally 

 and slightly forward, form a loose bundle and enter the wall of 

 the mid brain, where they descend to the ansulate commissure. 

 I believe that they cross here and run to the tectum, as do some 

 of the arcuate fibers in the brain of Acipenser. Other cells 

 lying in the ventro lateral part of the cerebellum near the junc- 

 tion with the acusticum, send their neurites more directly ven- 

 trad to cross to the opposite side with the arcuate fibers from 

 the acusticum. Thus the internal arcuate fibers from the acus- 

 ticum and cerebellum form a continuous series extending for- 

 ward to the ansulate commissure, and there is so gradual a tran- 

 sition from the fibers of the acusticum to those of the cerebel- 

 lum that the two can not be distinguished unless they are traced 

 to their cells of origin. 



In all parts of the granular layer there are found very small 

 cells (6—9 X 8—12 fi) whose dendrites are longer and straighter 

 than those of the small cells in the acusticum but do not extend 

 b yon J the limits of the granular layer (Fig. 12). Although 



