174 THE BRAIN OF THE TIGER SALAMANDER 



median mass receives fibers from the mesencephalic tectum and (in 

 lower-vertebrates) also from the hypothalamus. The tecto-cerebellar 

 tract may be activated from the mesencephalic V nucleus and from 

 terminals of the optic and lemniscus systems. 



The very rudimentary cerebellum of Myxine has been the subject 

 of much controversy. Holmgren ('46, p. 54) described in the embryo 

 two possible primordia, one in the auricle of the rhombencephalon 

 and one at the posterior end of the mesencephalic tectum. Larsell's 

 comprehensive study ('47, '47a) of this region of cyclostomes has 

 clarified the problem. Myxine has the most primitive cerebellum 

 known, and, so far as its differentiation has gone, it conforms with 

 the typical vertebrate pattern. 



The spino-tectal terminals in the inferior colliculus are primitively 

 contiguous with the vestibular and lateralis terminals in the auricle, 

 and from the wedlock of these two systems the cerebellum was born. 

 The primordial inferior colliculus, accordingly, was primitively con- 

 cerned with proprioception. Upon this foundation the cochlear sys- 

 tem of higher animals was built in much the same way that the bulb- 

 ar cochlear nuclei emerged from the lateral-line nuclei of the medulla 

 oblongata (p. 138). It is significant that in Amblystoma the primor- 

 dial inferior colliculus contains dense collections of cells of the mesen- 

 cephalic V nucleus, that fibers of the mesencephalic root of the V 

 nerve penetrate cerebellar tissue, and that cells of the mesencephalic 

 V nucleus are found sparsely scattered in the cerebellar gray (p. 140). 

 It seems probable that the mesencephalic V nucleus was first differ- 

 entiated at the posterior end of the tectum and that from this focus 

 its cells spread forward into the optic tectum and, in smaller number, 

 backward into the cerebellum. 



In the gray of both the median body of the cerebellum and the 

 lateral auricle some of the neurons are specialized as Purkinje cells, 

 in contrast with smaller cells, which are precursors of the cerebellar 

 granules. The largest and best developed Purkinje cells are loosely 

 arranged at the outer border of the central gray, but they are not 

 delaminated from the granular layers as in mammals. If cortex is de- 

 fined as laminated superficial gray, Amblystoma has no cerebellar 

 cortex, though its primordium is clearly evident. The Purkinje cells, 

 though of simple form, are quite distinctive. From them and from the 

 nucleus cerebelli, efferent fibers stream downward, forward, and 

 backward as cerebello-tegmental fibers, and one large fascicle of 



