MOTOR NUCLEI IN PHYLOGENY 381 



Of the urodele amphibians, reconstruction charts of the motor 

 roots and nuclei have been made in the following forms: Triton 

 (Molge) vulgaris and Siren lacertina (Kappers, 35); Crypto- 

 branchus japonicus and Necturus maculatus (Rothig, 50). Of 

 these, the first three (Triton, Siren and Cryptobranchus) are 

 reproduced here in figure 10, p. 395. 



MOTOR ROOTS AND NUCLEI IN RANA CATESBEANA 

 Second spinal nerve (hypoglossus) 



The motor nucleus ol the second spinal nerve (first of the 

 adult spinal series) in Eana catesbeana shows evidence of con- 

 siderable specialization and is very definitely divisible into two 

 sub-nuclei, viz., (a) a dorso-medial cell group, and (b), a ventro- 

 lateral cell group. 



The latter cell group is directly continuous caudally with the 

 motor cell columns of the anterior horn, of which it represents 

 the unmodified rostral extremity. The ventro-lateral cell group 

 does not extend rostrad as far as the exit level of the first motor 

 rootlet of the second spinal nerve. 



On the other hand, the dorso-medial cell group, though its 

 caudal end lies in the gray reticular matrix of the somatic motor 

 column of the cord, is throughout its extent easily distinguished 

 from the other elements of this column. Its large multipolar 

 cells form a compact nucleus which extends from slightly below 

 the exit level of the last motor rootlet of the second spinal nerve, 

 to a point some distance rostrad of the exit level of the first 

 motor rootlet of this series (figs. 1 and 2). In figure 9 A the 

 sagittal extent of this nucleus is indicated diagrammatically. 



The dorso-medial cell group described here without doubt 

 corresponds to the nucleus centralis of Stieda (52). The nucleus 

 gives rise to certain motor fibers which are evidently restricted 

 in their emergence to the ventral rootlets of the second spinal 

 nerve, as Gaupp long ago considered probable (26). 



From a study of central relations alone, it is not possible to 

 decide whether the most rostral of the motor rootlets noted 

 above represent remnants of the ventral root of the first spinal 



