Johnston, The Brain of Petroinyzon. 57 



cells are connected with the ordinary large cells by so many 

 transitional forms as to remove all doubt of their orii^in. 



(5) The fact that the secondary connections are the same 

 for the dorsal horns and acusticum. 



Similar evidence is found in the brain of Petron-yzcn : 

 (i) Continuity of the dorsal horns, acusticum, and cere- 

 bellum, there being even in this more primitive form, a slightly 

 developed molecular layer and cerebellar crest which bear essen- 

 tially the same relations to the granular layer, acusticum and 

 lobus lineae lateralis as in Acipenser. The continuity of the 

 dorsal horns with the special centers strengthens the argument 

 very greatly, as it is what should be expected in a lower brain. 

 The association is so close here that the nucleus funiculi is not 

 well developed and the acusticum does not become a separate 

 nucleus until a point in front of the IX root. 



(2) The ending of general cutaneous fibers in the nucleus 

 funiculi, nucleus trigemini spinalis, acusticum (?), and cerebel- 

 lum ; of lateral line fibers in the acusticum and cerebellum; and 

 of VIII fibers in all three centers. 



(3) The identity of the nerve elements in all the centers 

 upon a lower plane than in Acipenser. In Petromyzon there 

 are large cells and granules, while no II type cells have b«en 

 found. 



(4) The PuKKiNjE cells have not yet been differentiated. 

 The cells in the cerebellum which correspond to the Pukkinje 

 cells of higher forms are exactly like, certain large cells which form 

 the most conspicuous elements in the acusticum. These large cells 

 in the cerebellum and acusticum have the same relation to the 

 molecular layer and cerebellar crest respectively. Further, the 

 primitive Purkinje cells in the cerebellum send their neurites 

 as internal arcuate fibers to the tectum as do the large cells in 

 the acusticum. This leads to the next point. 



(5) While in Acipenser only the dorsal horns and acusti- 

 cum were shown to have the same secondary connections, this 

 is clearly true in Petromyzon for the cerebellum also. 



(6) The cerebellum in Petromyzon has reached a stage of 

 differentiation only very slightly above that of the acusticum. 



