388 DAVIDSON BLACK 



In all adult amphibians examined the abducens nucleus occu- 

 pies a position in close association with the elements of the peri- 

 ependymal gray. In Amblystoma at least, this is a larval char- 

 acter (Herrick, 1. c.) and there is no reason to suppose that 

 the nucleus in question is more ventrally situated at any time 

 in other amphibian larvae. Thus, though the adult topo- 

 graphical relations of the amphibian abducens nucleus closely 

 resemble those of selachians (cf. figs. 9, 10 and 11), yet the onto- 

 genetic history of the nucleus appears to differ in these two verte- 

 brate groups; for Kappers h^s already pointed out in this con- 

 nection that for a time during the development of selachian 

 larvae the abducens nucleus in these animals is quite ventral 

 in its position (33). 



ISJote on the nucleus olivaris superior 



In R. catesbeana a circumscribed nuclear mass appears in 

 the' ventral area of the reticular formation of the medulla and 

 extends in this position over the area included between the exit 

 levels of the motor IX and VII roots (figs. 3, 4 and 9 A). In 

 anurans this nucleus has been considered as the representative 

 of the superior olive of higher forms by Gaupp (26), Kappers 

 (33) and others. The nucleus in question has been represented 

 in the reconstruction charts in all forms in which it occurs in 

 order to increase the number of landmarks for comparison. 



The connections of this nucleus are as yet imperfectly under- 

 stood, but Kappers has pointed out that it is a derivative of 

 the acustic area which secondarily becomes displaced ventrally. 

 It is significant also to note that the area in which the anuran 

 superior olive becomes differentiated, corresponds exactly to 

 that in which Herrick's tractus octavo-tectalis et thalamicus 

 (bulbar lemniscus) courses in the larval Amblystoma brain. 



Of further interest is the fact that the superior olive is not 

 differentiated as a definitely circumscribed nuclear mass in any 

 vertebrate in which the lateral line organs are functionally 

 developed in the adult. Thus among amphibians the superior 

 olive is present only in anurans. 



