MOTOR NUCLEI IN PHYLOGENY 401 



nucleus especiajly concerned in the innervation of the intrinsic 

 tongue musculature. According to this interpretation the dorso- 

 medial cell group would consist of motor perikaryons whose 

 axones innervate the protractor, rotator and retractor muscles 

 of the tongue, together with certain coordination neurones, 

 and the whole complex would be essentially homologous with 

 the hypoglossus nucleus of higher forms. 



Though the position of the hypoglossal nucleus in Rana is in 

 many respects similar to that of the hypoglossal nucleus of 

 higher forms, the factors which determine its. location would 

 appea,r to differ radically in the two cases. 



Kappers (34) has already pointed out that the ontogenetic 

 dorsal migration of the hypoglossal nucleus in mammals 

 is probably to a great extent due to the fact that in these 

 forms the tongue has become the chief organ bearing taste 

 buds, and for this reason its reflexes are largely dominated 

 by sensory impulses from the IX-X terminal nuclei. Thus in 

 mammals the ontogenetic migration of the hypoglossal nucleus 

 is in the direction of the most important center acting upon it 

 reflexly, in accordance with the first concept of nem-obiotaxis 

 (v. 10 and 39). 



Since the function of the taste organs upon the frog's tongue 

 manifestly cannot be exercised until the food reaches this 

 organ, the influence of taste impressions in initiating the pre- 

 hensile tongue reflex in this animal need not be considered. On 

 the other hand, the whole complex series of movements by which 

 the frog catches his prey (of which the reflex action of the in- 

 trinsic tongue musculature is an integral part) is almost ex- 

 clusively initiated as the result of visual impressions (v. Yerkes, 

 62, 63). Further, the protractor and retractor action of the 

 tongue musculature is brought into play in the closing phase of 

 every complete respiratory cycle in the frog, so that these 

 muscles act in a coordinate manner with the other muscles of 

 the respiratory mechanism (vide infra). The reflex action of 

 this mechanism is to a great extent initiated by afferent im- 

 pulses from the mucosa of the bucco-pharyngeal area (v. Bag- 

 lioni, 5) and the hypoglossal nucleus of the frog receives many 



