386 DAVIDSON BLACK 



point. This small nucleus lies below the poorly defined sulcus 

 limitans in a situation similar to that of the more caudally 

 placed motor IX nucleus (figs. 4 and 9 A). 



The motor VII fascicles first pass slightly rostrad from their 

 nucleus of origin and then curve latero-ventrad to emerge be- 

 low the radix spinalis trigemini. These relations are indicated 

 in figure 4, in which the arrangement of the acustic and the 

 sensory VII roots are also to be seen. 



It is of interest to note that the acustic fibers enter the brain 

 stem for the most part dorsad of the sensory VII root as Strong 

 (53) has shown to be the case in the tadpole stage. In the 

 larval Amblystoma on the other hand the reverse is the case 

 and the two VIII roots enter the medulla chiefly ventral to the 

 visceral sensory VII (Herrick, 29), while in the adult Ambly- 

 stoma the visceral sensory VII enters the medulla midway be- 

 tween the dorsal and ventral VIII roots (Coghill, 16). 



The relations of the motor VII nucleus described above closely 

 approximate those of this nucleus in R. esculenta, R. fusca and 

 Bufo, though in the last mentioned animal the motor VII nu- 

 cleus lies wholly caudad of its root exit level. On the other 

 hand, in all adult urodeles examined the motor VII nucleus 

 forms the rostral end of a cell column which includes within its 

 limits all the motor perikaryons of the VII-IX-X nerves (figs. 

 9 and 10). 



The arrangement ot the motor VII-IX-X muclei in adult 

 urodeles is thus strikingly similar to the arrangement of these 

 nuclei in selachians, ganoids and dipnoans (cf. figs. 10 and 11). 

 On this account the members of each of these animal groups 

 are characterized by the presence of a relatively long ascending 

 emergent motor VII root whose relations in Necturus were first 

 adequately described by Kingsbury (40). In contrast to this, 

 the situation of the motor VII nucleus approximately on the 

 level of its root exit as in adult anurans, is not paralleled else- 

 where among the Ichthyopsida except in cyclostomes. 



In this connection it is important to note that the motor VII 

 nucleus in the 38 mm. larva of Amblystoma tigrinum, though 

 situated approximately on the level of its root exit, is separated 



