MOTOR NUCLEI IN PHYLOGENY 423 



facts, the arrangement of the cerebral motor nuclei in this 

 animal is undoubtedly a specialized one (10). In the case of 

 Rana however, conditions of an apparently opposite nature 

 exist. The anuran type has certainly been evolved compar- 

 atively recently in vertebrate phylogeny, and the Ranidae 

 represent the highest stage in the evolution of the Anura (22), 

 but within the brain stem in Rana a motor nuclear pattern 

 obtains which on first examination would seem to be much 

 more primitive than the motor nuclear pattern in selachians. 



The fundamental difference between the anuran and urodele 

 visceral motor nuclear pattern is essentially due to the different 

 position of the motor VII nucleus in the two groups. In Rana 

 during ontogeny this nucleus apparently retraces the path 

 along which it travelled in phylogeny. The neurobiotactic 

 influences which determine the phylogenetic migrations of the 

 motor VII nucleus have been considered in detail by Kappers 

 (I.e.), but unfortunately with regard to the factors which are 

 directly responsible for the rostral migration of this nucleus 

 during development in Rana but little can be said as yet. The 

 observations of Herrick (I.e.) on the mesencephalic V root in 

 Amblystoma make it plain that many fibers from this source 

 establish connections with the motor VII nucleus in this form 

 and probably also in other urodeles. If similar connections 

 obtain in Rana, where descending mesencephalic root fibers are 

 known to exist (55, 56), the rostral migration of the motor VII 

 nucleus in this form may well have taken place along this path. 



Not only does the general arrangement of the anuran motor 

 nuclear pattern differ from that of urodeles but also in certain 

 anuran motor nuclei a higher degree of intrinsic specialization 

 has been acquired than is e\ddent in the corresponding nuclei 

 of urodeles. Especially is this true in the rostral part of the 

 somatic motor column of the cord, so that in Rana two very 

 definite cell groups are to be distinguished here, from which 

 arise fibers emerging by the most rostral cervical motor roots. 

 One of these cell groups has been identified as the homologue 

 of the hypoglossal nucleus of higher forms, and the dorsal posi- 

 tion of this cell group is most probably the result of neurobio- 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



