Editorial, Problems of Comparative NeiiroJogv. 103 



indicated by Spencer and Marshall, and His, that, in Selachii 

 at least, the dorsal nerve roots grow from the spinal ganglion 

 into the medullary tube, and that this process does not take 

 place till the peripheral growth of the nerve has been con- 

 siderably extended. In the cranial nerves motor fibres do 

 pass from the region of the lateral columns into the ganglion 

 ridge at a period earlier than that of the central growth of 

 the sensory fibres. These motor fibres find their way to the 

 muscles of the visceral arches. These fibres are always on 

 the inner side of the ganglion. 



The ganglionic ridge originates from an outgrowth of the 

 dorsal part of the medullary tube where the closure is effected. 

 That portion of the ganglionic ridge which is not trans- 

 formed into actual ganglia is destroyed. Dohrn suggests 

 that the ridge is simply a primitive condition of the ganglia 

 and that they are only apparently nonsegmented at the begin- 

 ning, but wherever, as in the neck, they are not too much 

 crowded the ganglia are obviously distinct, though connected, 

 from the first. 



In the attempt to separate the trigeminus and facialis 

 acusticus from the vagus and glossopharyngeal, Rabl dis- 

 tinctly terms the hypoglossus the ventral root of the neuro- 

 mere, of which the vagus is the dorsal root. Dohrn, how- 

 ever, quotes the observations of Balfour, Wijhe, Froriep and 

 Ostraumoft\ as well as his own, in support of the view that 

 the hypoglossus contains all that remains of several spinal 

 nerves. The discovery of two or three ganglia in early 

 embryos of selachians attached to fibres of the hypoglossal 

 and the rapid disappearance of these ganglia without the 

 formation of sensory fibres may be regarded as strong evi- 

 dence of the development of the motor roots of XII from 

 the vagus neuromere. The trigeminus, facialis, glosso- 

 pharyngeal . and vagus all contribute to the formation of 

 organs of the lateral line, while no spinal nerves sustain any 

 such relation. 



That no motor fibres enter any of the spinal ganglia, as 



