422 DAVIDSON BLACK 



are normally focussed for distance (Beer, 6, 7, 8, 9), it is not 

 improbable that the chief original cause of the amphibian ocu- 

 loanotor nuclear specialization may be seen in the readjustment 

 of the mechanism of accommodation which must have taken place 

 in phylogeny during the evolution of the amphibian type. 



Finally it should be observed that, as Kappers has pointed 

 out, in the arrangement of the elements of the anuran ocu- 

 lomotor nucleus a first stage in the evolution of the more highly 

 specialized sauropsidan condition may be recognized (35). 



CONCLUSION 



In the foregoing pages attention has been drawn to the strik- 

 ing correlation that exists between the arrangement of the 

 visceral motor nuclei in amphibians and the functional develop- 

 ment of the musculature which they innervate. It has been 

 shown that in correlation with the more primitive arrangement of 

 the branchial musculature in urodeles, the visceral motor nuclear 

 pattern in these animals conforms in all essentials to the type 

 of nuclear arrangement characteristic of selachians and crossop- 

 terygians (cf. figs. 10 and 11). 



On the other hand, the evidence here produced makes it 

 strongly probable that the apparently primitive arrangement of 

 \dsceral motor nuclei obtaining in anurans is in reality the re- 

 sult of nuclear rearrangement completed late in ontogeny, and 

 that the anuran nuclear prostadium must closely approximate 

 the permanent urodele condition. It thus emerges that in 

 correspondence with the specialized development of their 

 branchial skeleton and musculature, the anuran visceral motor 

 nuclear pattern is a specialized one which however has come to 

 resemble that of petromyzonts as the result of analogous or 

 convergent evolution (cf. figs. 9 and 11 A). 



In the present connection it will be of interest to compare 

 the conditions which obtain in two such widely separated forms 

 as Rana and Bdellostoma. In the general organization of 

 Bdellostoma many primitive as well as regressive characters are 

 evident, and the animal has manifestly been derived from a 

 type very low in vertebrate phylogeny. Notwithstanding these 



