No. I.] THE CRANIAL NERVES OF AMPHIBIA. 209 



For example, the lateral-line system is obviously different, 

 whatever may have been its origin, from such cutaneous nerves 

 as the Trigeminus, yet in Gaskell's nomenclature they are 

 both simply classed together as ganglionated afferent somatic 

 nerves. In fact, Gaskell's system also ignores the differences 

 between these nerves and those of the special sense organs, for 

 these latter would also fall under the above category. The chief 

 defects in Gaskell's work appear to arise from the fact that it 

 has been confined to the highest vertebrate types, that it is not 

 comparative. Nevertheless, it is upon lines of work approach- 

 ing those of Gaskell that, in my opinion, the most fruitful 

 result^ will be obtained. 



It is also evident that an exact determination of the compo- 

 nent parts of the nerves is a necessary preliminary step to deal- 

 ing with questions of segmentation. This fact has already been 

 recognized to a certain extent, as is evidenced by the numerous 

 attempts to find the corresponding ventral roots for dorsal 

 roots, and vice versa. In these attempts, however, many other 

 differences in the character of the nerves have been ignored. 

 His, Van Wijhe, Gaskell, and others have demonstrated the 

 presence of two sets of motor nerves, and it is evident that 

 further differences, especially in the sensory nerves, must also 

 be taken into account. For example, in the tadpole, the Tri- 

 geminus proper, the ''dorsal VII" and the "fasciculus com- 

 munis " root of the VII, are mainly sensory yet all different ; 

 either all three must be included in a complete " segmental 

 nerve " or, if one is omitted, it must be shown why. That some 

 particular component may disappear in certain cases is evident, 

 and the cause is then to be sought in some peripheral change. 



In the study of segmentation from the standpoint of the 

 neuromeres, as developed by Beraneck, Orr, McClure, Waters, 

 Piatt, and others, these qualitative differences should likewise 

 be taken into account. When the results of these two lines of 

 work shall have been brought into correlation with each other, 

 a better insight will be afforded, on the one hand, into the 

 significance and value of the neuromeres, and, on the other 

 hand, into the transpositions and other changes undergone by 

 the various components. 



