6o Journal of Comparative Neurology. 



functionelles Organ auftritt. " Both these conclusions go too 

 far in reading highly developed structures back into a primitive 

 brain. The present paper shows that the cerebellum of Petro- 

 myzon is far from being a full equivalent of the cerebellum even 

 of other fishes. The structure of the brain of Petromyzon, so 

 far from indicating that the cerebellum is one of the oldest brain 

 centers, shows quite the reverse ; namely, that the cerebellum 

 with the functions and relations characteristic of that organ in 

 higher vertebrates, is scarcely represented at all in Petromyzon 

 and is but poorly developed in any of the fishes. Johannes 

 MILLER was much nearer the truth in pointing to theacusticum 

 as the representative of the cerebellum of other fishes, for the 

 cerebellum in Petromyzon is little more than the fused front 

 end of the special cutaneous part of the dorsal horn of the 

 medulla. 



This interpretation readily explains Schaper's ('99 b) diffi- 

 culty in finding a caudal limit to the cerebellum in fishes. The 

 cerebellum being the direct continuation of the acusticum, it 

 has no sharp caudal limit. 



Houser ('01) describes in Mustelus a general cutaneous 

 nucleus and acusticum clearly separated by a deep groove. 

 Both contain Purkinje and molecular cells comparable with 

 those in the cerebellum, and both are covered superficially by 

 the cerebellar crest. He considers the general cutaneous nu- 

 cleus as the direct continuation of the dorsal horn of the cord 

 and the acusticum as a later derivative of the general cutaneous 

 nucleus. The cerebellum has arisen as a fused outgrowth of 

 the acustica. The Purkinje cells are poorly developed as com- 

 pared with those of Acipenser and seem to show similar grada- 

 tions between the general cutaneous nucleus, acusticum, and 

 cerebellum. 



The medulla of Mustelus as described by Houser presents 

 some unexpected peculiarities : the large size of the general 

 cutaneous nucleus as compared with the acusticum, the absence 

 of a lobus liueae lateralis above the acusticum, the presence of 

 Purkinje cells in the general cutaneous, nucleus and the exten- 

 sion of the cerebellar crest over this nucleus as well as the 



