MOTOR NUCLEI IN PHYLOGENY 65 



level of the second cervical segment. In the middle third of 

 its extent the nucleus becomes enlarged by the addition of a 

 number of cells along its ventrolateral periphery. Caudally 

 this nucleus forms a small but well-marked cell column which 

 can be traced through the first cervical segment. At this level 

 its continuity is interrupted and it becomes represented by scat- 

 tered cell clusters so that its exact caudal limit is ill defined. 

 These relations are illustrated diagrammatically in the recon- 

 struction chart, figure 11, C. 



The fibers arising in the motor X nucleus pass first dorsad 

 and then laterad in a curved course and emerge through the 

 radix spinalis trigemini upon the dorsolateral periphery of the 

 medulla.^ The caudal rootlets of this series emerge on a slightly 

 more ventral plane than those described by Lubosch (34) in 

 Testudo. 



The arrangement of the elements of the motor vagus nucleus 

 in Damonia closely corresponds to that obtaining in Chelone. 

 In both these forms the motor vagus nucleus in the middle third 

 of its extent is characterized by the presence of an accumulation 

 of motor perikaryons on its ventrolateral periphery. The broad 

 base of this flap-like cellular appendage is throughout its extent 

 in direct continuity with the dorsal motor vagus column, and 

 no subdivision into dorsal and ventral moieties occurs in these 

 forms (fig. 11, C and D). 



A more specialized condition obtains in Varanus and Alligator, 

 where the differentiation of a lateral (ventrolateral) motor vagus 

 cell group is clearly evident (fig. 12, A and C). 



The apparently simple arrangement of the motor vagus column 

 characteristic of Boa is possibly the direct result of the loss of 

 certain motor vagus elements as a consequence of ophidian 

 specialization, as Kappers has already noted (fig. 12, B). This 

 question will be discussed subsequently. 



The motor vagus column extends rostrally to the same relative 

 level in all the reptiles thus far examined, irrespective of the 



* Fibers from some of the rostral vagus rootlets in Damonia terminate in the 

 radix spinalis trigemini in a manner similar to that noted by Kappers in Chelone, 

 Alligator, and Varanus (32, 33). In Anolis, on the other hand, Willard found no 

 cutaneous components in any of the nerves between V and the third spinal (43). 



