MOTOR NUCLEI IN PHYLOGENY 421 



rostrad of its root exit recalls the relations obtaining in all gan- 

 oids and most teleosts (cf. figs. 9, 10 and 11). A more primitive 

 arrangement of the trochlear nucleus has been recorded in Molge 

 cristata by Van der Horst (I.e., fig. LXXX). Here the nucleus 

 lies a considerable distance caudad of the oculomotor nucleus and 

 about midway between the exit levels of the trochlear and ocu- 

 lomotor roots. 



In describing the oculomotor nucleus in fishes (10) it became 

 evident that in both ganoids and teleosts this complex showed 

 distinct indications of division into dorsal and ventral cell 

 groups. It should also be noted that in petromyzonts a peculiar 

 specialization of the oculomotor nucleus into dorsal and ventral 

 moieties has been described by Huet (31) and Kappers (35), but 

 this nuclear differentiation evidently differs from that obtaining 

 among the true fishes. 



Attention has already been drawn to the tendency among 

 anurans toward subdivision of the oculomotor nucleus into sub- 

 groups, but in contrast to the conditions obtaining in other 

 ichthyopsidans, the components of the anuran oculomotor 

 complex are arranged to form medial and lateral moieties. 



The differentiation of the oculomotor complex in ganoids and 

 teleosts away from the condition obtaining in sharks has prob- 

 ably been correlated with specialization of intrinsic effectors 

 in the eyes of the former animals (10), and this differentiation 

 appears to have reached its maximum in modern teleosts (e.g., 

 Pleuronectidae, Lophidae). The relatively simple undifferen- 

 tiated condition of this nucleus which obtains in selachians as 

 well as in dipnoans and crossopterygians (Van der Horst, I.e.) 

 indicates, however, that the arrangement of the oculomotor 

 elements so characteristic of modern ganoids and teleosts can- 

 not be regarded in any sense as representing a prostadium of the 

 amphibian condition. 



It would thus seem probable that the factors which deter- 

 mined the anuran oculomotor cell arrangement must have ap- 

 peared comparatively late in phylogeny. The eyes of most 

 fishes are normally focussed for near objects when at rest, and 

 since the eyes of most amphibians, reptiles, birds and mammals 



