58 Journal of Comparative Neurology. 



Except for one fact the cerebellum must be regarded as only a 

 dorsal fusion or bridge between the two acustica. That fact is 

 that a small bundle of fibers from the inferior lobes enters the 

 cerebellum. This small bundle marks the beginning of differ- 

 entiation of the cerebellum into a coordinating center, but its 

 effect has not become appreciable in the structure of the cere- 

 bellum in Petromyzon. The very primitive condition of the 

 cerebellum is well shown by the fact that no other tracts enter 

 it from in front. 



Emphasis has been laid on the continuity of the dorsal 

 horn and acusticum, and the close similarity of the cells in the 

 two is important. In this respect Petromyzon adds strong evi- 

 dence at the point of the argument which necessarily has the 

 least support in Acipenser or any higher brain. The separation 

 of the medullary centers from the cord becomes greater in 

 higher forms, and here in Petromyzon we have an approach to 

 primitive conditions. The presence of a cerebellar crest and a 

 lobus lineae lateralis throws a new light on the early condition 

 of the cutaneous centers. It shows that the conditions in Aci- 

 penser and the selachians (?) are much more primitive than in 

 the teleosts, not simply divergent. In the teleosts the cuta- 

 neous centers have become relatively reduced, the lobus lineae' 

 lateralis being absent, although the cerebellum has reached a 

 higher development as a coordinating center. The presence of 

 a molecular layer and cerebellar crest while the cerebellum is 

 still a cutaneous center and not to any degree a coordinating 

 center, suggests that the separation into granular and molecular 

 layers is the earliest differention which appears in the somatic 

 sensory centers. The molecular layer is made up chiefly of the 

 neurites of granule cells which are characteristic connective ele- 

 ments in the cutaneous centers of all vertebrates. The gteat 

 development of the molecular layer is connected with the exer- 

 cise of the function of coordination. Only the front end of the 

 cutaneous centers (cerebellum) takes on this function secondarily 

 and in this region the Purkinje cells develop. In lower fishea 

 (selachians, ganoids) the effect is felt in the adjoining part of 

 the acusticum and Purkinje cells are developed there also. In 



