MOTOR NUCLEI IN PHYLOGENY 385 



medulla. Rostrally its limit is not sharply defined but it cer- 

 tainly terminates some distance caudal of the exit level of its 

 first emergent rootlet. Caudally the motor X nucleus extends 

 well below the calamus but in this region its continuity is inter- 

 rupted and it is represented by scattered cell clusters which may 

 be traced backwards to the end of the second cervical segment. 

 In figure 9 A the rostral and caudal limits of the motor X nucleus 

 have not been definitely marked on account of these peculiari- 

 ties. The relations of the motor X nucleus at the exit level of 

 the first motor rootlet of the second spinal nerve are indicated 

 in figure 2. At the level indicated in figure 1, two cells appar- 

 ently belonging to this nucleus can be distinguished just dorsal 

 to the ventro-lateral cell group of the anterior horn. 



Between the motor IX and X nuclei a very definite hiatus 

 occurs. In transverse section the former nucleus occupies a 

 position exactly analogous to that of the caudally placed motor 

 X nucleus. The motor IX nucleus extends approximately 

 from the exit level of its own motor root, caudally to the level 

 of emergence of the first vagus motor rootlet. The emergent 

 motor IX fibers pass laterad and make their exit below the corre- 

 sponding sensory root (cf. figs. 3 and 9 A). 



The hiatus between the motor IX and X nuclei in R. cates- 

 beana has also been recorded by Van der Horst (1. c, fig. LXXXI). 

 In R. esculenta on the other hand, these two nuclei appear to 

 be continuous with one another (Kappers). According to 

 Rothig the latter condition also obtains in R. fusca and in 

 Bufo (ci. fig. 9). 



It is of interest to note at this point that in the 38 mm. larva 

 of Amblystoma tigjinum, Herrick has shown that the nucleus 

 ambiguus (motor IX-X nucleus) is placed in relation to its 

 emergent roots in a position practically identical with that 

 obtaining in R. esculenta, R. fusca and Bufo (v. Herrick, 29, 



fig.l). 



Nerve VII 



The motor VII nucleus in R. catesbeana is situated on the 

 periphery of the periependymal gray and extends partly over 

 the level of its root exit and for a short distance caudal to this 



