MOTOR NUCLEI IN PHYLOGENY 95 



quent to the loss of the ophidian shoulder-girdle. Since, how- 

 ever, among the Lacertilia, Willard has found no trace of the 

 accessory nerve in such a form a3 Anolis, it is possible that this 

 nerve may have been absent in the saurian type from which 

 ophidians were derived. 



The rostral extent of the hypoglossal nucleus is singularly 

 uniform among reptiles and in its overlap of the vagus motor 

 area presents a marked advance over the more primitive condition 

 obtaining in amphibians. These changes in the mutual rela- 

 tions of the hypoglossal and motor vagus areas have been shown 

 to be the result of an actual displacement rostrad of the former 

 nucleus evidently under the reflex influence of the VII-IX taste 

 center and the nucleus of termination of the ramus lingualis 

 V which appears in phylogeny for the first time in reptiles. 



A considerable range of variation is to be observed in the 

 topographical relations of the motor V nucleus in reptiles. In 

 Damonia the motor V nucleus has to a great extent retained its 

 primitive dorsal position on the level of its root exit. In Boa 

 and Varanus, on the other hand, the motor V nucleus is almost 

 wholly ventral (peripheral) in its position, while in Alligator an 

 intermediate condition may be recognized. The degree of 

 ventral (peripheral) displacement of the motor V nucleus appears 

 thus to vary directly with the complexity of the development of 

 the V jaw musculature. In other words, the motor V nucleus 

 is most ventrally (peripherally) placed in streptostylic reptiles 

 and, further, it is probable that in these forms it constitutes a 

 recent character independently acquired. 



Similar examples of analogous or convergent evolution are to 

 be observed among reptiles in the variations in the arrangement 

 and specialization of the eye-muscle nuclei. Thus in Boa and 

 Varanus the abducens root and nuclear pattern resembles that 

 constantly found in many mammals and birds, but differs 

 markedly from that obtaining among crocodilians, chelonians, 

 and amphibians examined. Again, in the disposition of the 

 trochlear nucleus and. the intrinsic specialization of the oculo- 

 motor nucleus, Varanus, Alligator, and Chamaeleon exhibit 

 certain avian characteristics. 



