274 



C. U. Ariüns Kappers 



rabie layer of granules. Investigations by RotJiig and Black proved 

 however the VI nucleus to be lying here also next to the fase. longi- 

 tud, posterior, mostly in a caudal position sometimes reaching as far as 

 the level of the IX root, thus showing a great resemblance to sharks 

 (figure 6). 



In Reptilia other conditions occur. In Tiirtles and Crocodiles (fig. 4), 

 the nucleus in its dorsal position begins to move in a forward direction, 

 which is demonstrated by the frontal stretching of its cell column (fig. 6) 

 only the posterior pole of the nucleus keeping its caudal position. 



"HIE 



X dn\c- 



Oliva sop 



Oliva sup 



Fig. 5. — -Dorsal position of the abducens nucleus in a bird (Casuaris) 

 next to the f. 1. p. 



In land-reptiles (fig. 6, Varanus), birds (fig. 5 and 6), and vtammals 

 the posterior part of the nucleus also shifts in a forward direction, which 

 causes a shorter and more compact nucleus abducens, lying on, or even 

 partly before the level of the VII root projection. (The position entirely 

 in front of the entrance level of the motor VII root in man fig. 6 is chiefly 

 due to a backward shifting of this root.) 



This successive frontal shifting of the VI nucleus in dorsal position, 

 is most evident when we compare together the shark, frog, turtle, vara- 

 nus and mammals.(fig. 6). 



