NEAL: NEKVOUS SYSTEM IN SQUALUS ACANTHIAS. 



223 



wall. Dohrn considers it as the punctum saliens of the evidence given 

 by him, that ganglion cells and ganglia which may be traced to the 

 adult are to be found in the course of the oculomotorius before this 

 comes into connection with the mesocephalic ganglion, and concludes 

 that such ganglion cells can have had no other source than the ventral 

 horn of the midbrain. He thus takes the view of Balfour, Marshall, 

 Kupffer, and others, that this ventral nerve is formed as a chain of 

 medullary cells, in opposition to the views of His ('89), Kolliker ('92), 

 von Lenhossek ('92), and others, that ventral nerves are formed from 

 processes of " neuroblast " cells in the ventral horn of the medullary 

 tube. 



Figure H. 



Miss Plat<^'91) comes to fundamentally different conclusions from 

 those of Dohrn ('91). She finds that the oculomotorius appears first as 

 a single cell proliferated from the mesocephalic (ciliary) ganglion toward 

 the base of the midbrain, with which it at first has no connection. Ob- 

 servations on Squalus, Eaja, Pristiurus, and Torpedo convince her that 

 the oculomotorius develops after the type of a sensor nerve [?] by a pro- 

 liferation of ganglion cells toward the brain wall. Mitrophanow's ('93) 



Fig. H. Left face of a parasagittal section through the right half of the same 

 embryo as that represented in Figures F and G, showing the oculomotorius in an 

 early stage of development (62 somites). X 447. The relation of the nerve fibre 

 with an axis-cylinder process from the neuroblast cell x seems clear, ax-cyl., axis- 

 cylinder process ; oc-mot., oculomotorius ; x, neuroblast cell. 



