2 26 Journal of Comparative Neurology and Psychology. 



be entirely justified by the facts. The above described condi- 

 tions seem to indicate clearly that the ancestors of Petromyzon 

 had in the branchial region one complete nerve in each segment 

 of the same constitution as the present glossopharyngeus. It 

 appears that the visceral sensory and motor fibers of the caudal 

 branchial segments, instead of continuing to reach the brain by 

 way of the dorsal roots proper to their segments, have progress- 

 ively changed their course so as to run to the brain through the 

 root of the next more cephalic segment. This process may 

 have begun as the result of the expansion of the gill sacs and 

 growth of branchial arch muscles which served to crowd the ven- 

 tral rami of the nerves at the caudal end of the gill region. 

 When the shifting had brought the penultimate gill back to the 

 level of the ultimate nerve root the fibers of the ultimate ventral 

 rami shifted their course to the root of the penultimate nerve. 

 The dorsal ramus was unaffected by these conditions and hence 

 remained independent. As the process went on progressively 

 the branchial and pharyngeal rami became united by the com- 

 mon branchio-intestinal trunk which enters the brain by way of 

 the vagus root. In Bdellostoma (35, 80) it seems that the 

 process has gone one segment farther and the rami of the whole 

 branchial region behind the ear have been collected into a single 

 root which may be supposed to occupy the place of the glosso- 

 pharyngeus. The writer sees no reason for the supposition 

 made by Koltzoff that Nn. IX and X in Petromyzon may 

 have arisen by the division of such a root. 



It seems to the writer that this interpretation of the vagus 

 as a collector, which is so obviously applicable to the cyclo- 

 stomes, offers the most simple and satisfactory solution of the 

 vexed question of the vagus region in gnathostomes. Without 

 going into a detailed examination of the literature, it is evident 

 that facts directly supporting this interpretation are not wanting 

 in gnathostomes themselves. So-called dorsal hypoglossal roots, 

 either as ganglionic rudiments or as fully developed roots and 

 ganglia, have been described by numerous workers (30, 31, 

 100, 26, 27, 59, 12). Some of these ganglia undoubtedly lie 

 in segments whose branchial portions are supplied by vagus 



