THE CEREBELLUM OF AMBLYSTOMA 275 



case of those passing forward into the midbrain is there any def- 

 inite grouping of fibers to form a bundle. This bundle (fig. 16, 

 br.conj.) is loosely arranged and has relatively few myelinated 

 fibers. Those present are of small diameter. The unmyelinated 

 fibers are more numerous, so that the bundle is more evident in 

 favorable Cajal preparations than in those stained by the Weig- 

 ert method. The majority of fibers which enter into this bundle 

 appear to arise from the cells composing the group which has 

 been described as the primitive nucleus dentatus. Some appear 

 to come from the molecular layer, but could nob be traced for 

 any distance within this layer. These may possibly represent 

 axones of the Purkinje cells, which, as described above, pass into 

 the molecular zone to be lost within it among the numerous 

 fibers there present. No other efferent cerebellar tracts were 

 recognized, if present. 



The afferent fibers which terminate within the cerebellum are 

 definitely grouped into tracts of greater or less size. They are 

 for the most part composed of myelinated fibers. 



The tractus spinocerebellaris ventralis is the largest of the affer- 

 ent tracts. It lies dorsally and laterally of the bulbar lemniscus, 

 and is closely associated, as pointed out by Herrick in both Nec- 

 turus and larval Amblystoma, with the tractus spinotectalis. 

 Just before reaching the level of the superficial origin of the V 

 nerve, the two tracts separate to some extent, but not entirely. 

 They continue forward as far as the ventral and anterior portion 

 of the corpus cerebelli (fig. 16, tr.sp.cb.v., tr.sp.t.). Here the 

 ventral spinocerebellar tract is completely separated from the 

 mixed bundle and turns dorsally and medially to pass into the 

 body of the cerebellum (figs. 16 and 21, tr.sp.cb.v.). Its fibers 

 remain distinct from those of the cerebellar commissure for some 

 distance, then it becomes part of the commissural bundle; but 

 in sagittal sections the tract, somewhat reduced in size, is seen 

 to separate again from the commissure on its dorsal side (fig. 16). 

 Its fibers pass among the cells of the granular layer a few at a 

 time as the median plane is approached, but many cross to the 

 opposite side of the cerebellum. The bundle is therefore not 

 greatly reduced in size as it also receives fibers from the corre- 

 sponding tract of the opposite side. 



