MOTOR NUCLEI IN PHYLOGENY 



391 



The oculomotor nucleus in R. catesbeana is a large well 

 developed structure which extends over the level of its root 

 exit and for some distance rostrad of this. The motor peri- 

 karyons are placed for the most part dorso-mediad of the bundles 

 of the fasciculus longitudinalis medialis, though many of them 

 may be seen lying between separated bundles of this tract 



Vel.m.s 



Fig. 6 Rana catesbeana. Transverse section through the brain stem at the 

 level of the trochlear root exit. Abbreviations: Coll., coUiculus; G.i., interpe- 

 duncular ganglion; N.IV., trochlear nerve; Tr. II., posterior root of optic tract; 

 Vel.m.s., superior medullary velum. Other abbreviations as before. 



(v. fig. 8). The nucleus extends laterally for some distance 

 and a distinct tendency toward subdivision into medial and 

 lateral cell groups is evident. This phenomenon has already 

 been noted by Kappers in R. esculenta (35). 



The oculomotor fibers emerge in numerous fascicles which pass 

 ventrad and slightly laterad through the base of the midbrain. 

 Some of the most caudal oculomotor fibers appear to arise from 

 the heterolateral nucleus, but this relation was not demonstrated 

 with certainty. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



