MOTOR NUCLEI IN PHYLOGENY 387 



by but a very slight interval from the motor IX-X column 

 (v. Herrick, 1. c, fig. 1). 



Nerve VI 



In R. catesbeana three very fine roots emerge in series from 

 the ventral periphery of the brain stem, approximately midway 

 between the exit levels of the motor IX and VII roots. Three 

 rootlets of this nerve were also distinguished by Kappers in R. 

 esculenta but Rothig has only indicated two rootlets of this 

 nerve in his reconstructions of R. fusca and Bufo (fig. 9). The 

 superficial attachment of the abducens rootlets appears to lie 

 nearer to the exit level of the motor VII root in R. catesbeana 

 than in any of the other anurans thus far recorded. In Van der 

 Horst's reconstruction, however, the three abducens rootlets 

 are represented in a position very similar to that which they 

 occupy in R. esculenta. As I am at present unable to verify 

 my records by an examination of the original material, this 

 point must remain in doubt but some individual variation in the 

 superficial origin of this nerve would not be surprising. Gage 

 (23) long ago noted that the superficial origin of the abducens 

 nerve appeared to be variable in different amphibians. 



The nucleus from which the abducens rootlets arise is very 

 diffusely arranged, and is with difficulty distinguished from the 

 neighboring coordination elements of the motor tegmental 

 nucleus. For this reason the outline of the nucleus in figure 

 9 A represents only the approximate area in which its cells are 

 scattered. In his reconstructions Kappers has already drawn 

 attention to this point (33). 



The cells of the abducens nucleus are arranged around the 

 fasciculus longitudinalis medialis and are intimately associated 

 with its fibers. The nucleus is also traversed by many fine and 

 coarse medullated fibers from the homolateral acustic area. A 

 similar intimate association of the acustic area and the abducens 

 nucleus is evident in urodeles as well as in all anurans, and in 

 the larval Amblystoma Herrick has described the course of a 

 special fascicle from the area acustico-lateralis via the fasciculus 

 longitudinalis medialis to the abducens nucleus. 



