MOTOR NUCLEI IN PHYLOGENY 393 



In general it may be said that the relations above described 

 in R. catesbeana are characteristic of all anurans yet examined. 

 In modification of this statement, however, it must be noted 

 that the size of the trochlear nucleus among these forms is sub- 

 ject to some variation, although in this its relation to the ocu- 

 lomotor nucleus remains practically unaffected. 



The condition of the oculomotor and trochlear nuclei and roots 

 in urodeles presents a marked contrast to that obtaining in 

 anurans. In the adult Triton examined by Kappers (35), 

 though the small nuclei and roots in question present relations 

 essentially similar to those in anurans, yet the nuclear differ- 

 entiation in this animal was much inferior to that in Rana. 

 Further, Van der Horst found that the trochlear nucleus in 

 Molge cristata was placed far behind the oculomotor nucleus 

 and lay about midway between the exit levels of the oculomotor 

 and trochlear roots, in the Sylvian gray. 



In Siren, Kappers (1. c.) could discover neither trochlear root 

 nor nucleus, though subsequently Norris (46) has been able to 

 trace the peripheral course of this much reduced nerve in all his 

 specimens. In Necturus and Cryptobranchus, Rothig (50) was 

 unable to distinguish with certainty the trochlear nucleus, 

 though he identified the nerve at its point of emergence in both 

 these animals. 



In Siren (Kappers), Cryptobranchus and Necturus (Rothig), 

 the oculomotor nucleus is very small and poorly differentiated 

 from the neighboring periependymal gray, and it lies on the 

 level of its root exit. In Siren the oculomotor root emerges 

 but a very short distance rostrad of the exit level of the motor 

 V root. In general, among the urodeles examined the distance 

 between the rostral border of the emergent oculomotor root and 

 that of the motor V, tends to be reduced when compared with 

 anurans (cf. figs. 9 and 10). 



