304 



the nncleiis lias shifted in a frontal direction and the whole of it 

 has become more compact and shorter and is found in its totality 

 on the level of the VII root. It is equally possible that the consi- 

 derable elongation of the trochlear nucleus in my specimens of 

 Rana and Bufo is a result of the originally more caudal position of 

 the nucleus. In my Rana and Bufo the frontal shifting would thus 

 be in its first stage: the elongation in the direction of the III^' nucleus, 

 while in higher animals the whole of the IV nucleus has attained 

 the more frontal position and consequently t!ie nucleus has become 

 more compact and shorter again, having concentrated directly behind 

 the III^' nucleus. From this standpoint considered, the condition which 

 Kappers found to be present in his Rana has to be explained as a 

 case in which the shifting has reached already its second and last 

 stage, the concentration of the whole IV nucleus directly behind 

 the oculomotor one^) (see fig. 8j. 



In Necturus and Cryptobranchus I have not been able to determine 

 the topography of the trochlear iiucl., though the root of this nerve was 

 fairly well developed and could be traced a good distance in the 

 cerebrum. 



It is possible however that a 

 small group of cells lying later- 

 ally (not ventrally as in anura) 

 to the ventricle has to be consi- 

 dered as such (fig. 11). The fibres 

 of the trochlear nerve can be 

 followed easily in its vicinity. 

 The possibility of this is sus- 

 tained by the fact that in the 

 original condition (found in Pe- 

 tromyzon) the nucleus has a posi- 

 tion dorsal to the ventricle (see 

 fig. 1), the position under the 

 ventricle having to be considered 

 as a secondary one and a transition between them is e. g. the location 

 as found in Varanus ((Kappers 1. c. fig. 56, p. 61) where a more 

 lateral position is found. 



Till. // fr^ntaéirK/:>nf7t^ 



1) The possibility that the difference in caudal extension between the trochlear 

 nucleus in the Rana described by Kappers and my specimen might be due to 

 inaccurate observation is excluded, because I have examined myself Kapper's 

 Rana, which gave me the conviction that his topographic -map is perfectly right 

 and that the trochlear nucleus in his specimen is indeed much more compact 



