Johnston, The Brain of Petroinyzo7i. 59 



the dorsal horns Purkinje cells are never developed for lack of 

 a great number of granule cells and a large molecular layer. 



The lobus lineae lateralis has little significance further than 

 to give additional proof that this nucleus in selachians and ga- 

 noids is part of the acusticum. In Petromyzon it is more 

 closly connected with the acusticum, while the cerebellar crest 

 lies on the ectal surface of the lobe. This may be regarded as 

 the primitive relation ; in selachians and ganoids the crest came 

 to lie between the lobe and acusticum by means of an infolding 

 of the wall along the line of the crest. In higher forms both 

 crest and lobe have disappeared. 



The argument for the unity of the cutaneous centers of the 

 cord and medulla could scarcely receive stronger support than 

 has been derived from the comparative study of the brains of 

 Acipenser and Petromyzon. The result has been to show that 

 the centers for general and special cutaneous sensation present 

 apparently much less differentiation and separation in fishes 

 than exists between the peripheral organs themselves. The 

 evidence is, I think, conclusive that these centers have been 

 developed from a primitive dorsal horn of the cord-medulla in 

 response to the development of the special cutaneous sense 

 organs of the head. 



Johannes MCller (quoted by Schaper '99) thought that 

 he saw in the lateral walls of the medulla (acusticum ?) of Pe- 

 tromyzon the equivalent of the lateral part of the cerebellum 

 of other fishes. 



Schaper ('99a) shows, that the cerebellum. of Petromyzon 

 consists of granular and molecular layers and contains large 

 cells which he thinks are true Purkinje cells, although he has 

 not demonstrated the characteristic spiny dendrites. He con- 

 cludes that the cerebellum is to be considered " als \bVi\^ gleich- 

 wertiges Organ in die Reihe der Kleinhirne der iibrigen Verte- 

 braten."' He further says : "Fiir die phylogenetische Betrach- 

 tung endlich ergiebt sich hieraus, dass, wie Edinger schon bei 

 friiherer Gelegenheit vermuthet hat, das Kleinhirn in der That 

 ciner der altesten Hirnabschnitte zu sein scheint und jedenfalls 

 schon im friihesten Beginn des Wirbelthierlebens als specifisch 



