402 DAVIDSON BLACK 



fine internal arcuate fibers from the communis area. Thus, 

 though many arcuate fibers reach this nucleus from other sources 

 (especially from the acustic area of the medulla), it would ap- 

 pear probable that its dorsal position is chiefly determined 

 through the influence exerted upon it by the tecto-bulbar con- 

 stituents of the fasciculus longitudinalis medialis and by arcuate 

 efferents from the homolateral fasciculus solitarius. 



In addition to the specialization of a hypoglossal nucleus in 

 the most rostral segment of the cer\'ical motor column in the 

 frog, evidence is also to be adduced indicating a rostral dis- 

 placement of the whole somatic motor column in opisthoglossal 

 anurans as compared with urodeles. 



In opisthoglossal anurans, the first spinal nerve of the adult 

 is in reality the second nerve of the series, the first having been 

 lost at metamorphosis (v. Fiirbringer, I.e.). In urodeles, how- 

 ever, this is not the case and the first nerve of the larva is re- 

 tained in the adult, while in not a few of these animals the last 

 spino-occipital nerve is also functional in the adult. 



Notwithstanding the reduction of the rostral cervical nerves, 

 the first motor cervical rootlet in the adult opisthoglossal anu- 

 rans examined emerges either on the same level as the first motor 

 cervical rootlet in the urodeles charted, or even on a more ros- 

 tral plane. Thus, in Triton where the last spino-occipital nerve 

 is frequently retained in the adult, the distance between the 

 exit level of the first cervical motor rootlet and that of the motor 

 VII root is practically the same as this distance in R. catesbeana 

 and R. esculenta, and even greater than the corresponding 

 distance in Bufo (v. figs. 9 and 10). 



It is evident therefore that the reduction of the spino-occipital 

 and occipito-spinal elements in the ontogeny of anurans is 

 accompanied by a rostral migration of the whole cervical motor 

 column, together with a rostral displacement of its emergent 

 motor roots, so that in this respect these amphibians form no 

 exception to the general rule observed elsewhere in other ichthy- 

 opsidans examined (10). 



