92 DAVIDSON BLACK 



and birds, but which differs from the pattern characteristic of 

 the more ancient reptihan forms (chelonians and crocodilians) 

 and of amphibians. This would appear to be but another 

 example of analogous or convergent evolution (5, p. 422). 



Nerves III and IV. In respect to the relation of their oculo- 

 motor and trochlear nuclei and roots the reptiles examined are 

 divisible into two groups, viz., those showing distinct evidence 

 of relatively high specialization of the elements of this complex 

 and those in which such evidence is lacking (vide supra). To 

 the former group belong Alligator, Chamaeleon, Varanus, and 

 probably Sphenodon; to the latter group belong Boa, Damonia, 

 Chelone, and possibly Anolis. 



The specialized features to which allusion is made are as 

 follows: the apposition or close approximation of oculomotor 

 and trochlear nuclei (i.e., the trochlear nucleus is rostrally placed) ; 

 the exit of the trochlear root on a level with its nucleus; the differ- 

 entiation of the oculomotor nucleus to form distinct cell groups 

 (i.e., dorsolateral and ventromesial cell groups). All these 

 features are in varying degrees characteristic of the animals 

 mentioned above in the first group, but are not present among 

 those of the second group. 



The contrast between the grouping of these forms on the basis 

 of the motor V nuclear pattern and that based on the arrange- 

 ment of the elements of the oculomotor and trochlear nuclei 

 is marked. In the former case some definite relation was ap- 

 parent between the degree of specialization of the effector 

 mechanism of the jaws and that of the motor V nucleus. In 

 the present instance, however, with the possible exception of 

 Chamaeleon,^ all the reptilian types examined exhibit a singular 

 uniformity in the development of their extrinsic oculomotor 

 effectors. 



Among amphibians a subdivision of the oculomotor nucleus 

 into medial and lateral moieties occurs apparently for the first 

 time in phylogeny in Rana (and probably in most opisthoglossal 



^ The eyes of chameleons are unique among reptiles in the freedom and inde- 

 pendence of their movement, though vision in these animals as in other reptiles 

 is monocular (Gadow, 18, p. 569). 



