MOTOR NUCLEI IN PHYLOGENY 403 



Visceral motor nuclei 



In amphibians all the viscero-motor nuclei (V, VII, IX and 

 X) are dor.sally placed mthin the brain stem. A dorsal posi- 

 tion is also characteristic of these nuclei in petromyzonts, se- 

 lachians, ganoids and according to Van der Horst (54) in dip- 

 noans also (cf. figs. 9, 10 and 11). This condition as already 

 noted (10), is to be considered a primitive feature in the motor 

 nuclear pattern of these forms. 



In respect to the grouping of the viscero-motor nuclei in 

 their relation to one another and to their emergent roots, two 

 major types of nuclear pattern evidently obtain among adult 

 amphibians. These types are apparently of ordinal importance, 

 for in all anurans examined the motor VII nucleus is much 

 more closely related to the motor V nucleus than to the motor 

 IX-X complex, while in all adult urodoles examined the reverse 

 is the case and the motor VII nucleus is placed far caudad of 

 the motor V nucleus and forms the rostral component of the 

 caudal viscero-motor column. In the case of the larval Ambly- 

 stoma, however, it has been already noted that the motor VII 

 nucleus, although situated immediately rostrad of the motor 

 IX-X nucleus, is not directly continuous with the latter (Her- 

 rick, I.e., fig. 1). 



In considering the causes underlying the arrangement of the 

 motor nuclei in cyclostomes and fishes, it became evident that 

 the most important influ'ences determining the relations of the 

 motor VII-IX-X nuclei were exerted respectively by the visceral 

 effector mechanism of the jaws and branchial region and by the 

 visceral sensory centers. It will be of interest therefore to 

 inquire whether these factors exert an equally important in- 

 fluence upon the viscero-motor nuclei in amphibians also. 



Kappers (35) has already noted the striking similarity of the 

 motor VII-IX-X nuclear pattern in Siren and Triton to that 

 obtaining among sharks. He was inclined to ascribe the simi- 

 larity of pattern in Siren and sharks to the fact that, since both 

 are gill respiring forms without opercula, the caudal position of 

 the motor VII nucleus and its fusion with the motor nuclei of 



