1037 



cord. Tlie two frontal rootlets are thin and their cells remain ver}^ 

 dorsal, more as in Selachii than in Teleosls. 



On the frontal level of the s[)ino-0('ei|)i(al coltimii a quantity of 

 large reticular cells is found, which we may compare with the 

 nucleus reticularis inferior in Rays. (Comp. van H(>EVpn.L). As in these 

 animals a central inferior rcticulai- nucleus (lyiu"- in the i-aphe, as 

 occurs in all the higher vertebrates froui Reptiles to Man) hardly 

 occurs here: the cells keep a lateral position. Considering the differences 

 which Amia as well as Acipenser exhibits when compared with 

 Teleosts, we find as the most striking the less ventral position of the 

 oculomotor and abducens nucleus and the absence of a division of 

 the latter into two chief cell groups. A frontal isolation or ventral 

 displacement of the VII nucleus does not occur. 



All of these characteristics are easily understood if we compare 

 the sensory systems and their reflex paths in these animals. 



Concerning the less ventral extension or position of the III and 

 VI nucleus, it may suftice to remark that the ventral tecto-bulbar 

 tracts are not nearly so well developed in Acipenser (and Aniia) as 

 in bony fishes. The tectum opticum itself is relatively smaller (com- 

 pared with the underlying midbrain and thalamus) tiian in most ot 

 the, bony fishes and reminds us more of the condition found in sharks. 

 No doubt this smaller development of the ventral tecto-bulbar paths 

 is the reason of the less ventral migration of the eye muscle-nuclei 

 mentioned above. 



This fact may at the same time explain why the abducens nucleus 

 keeps a relatively caudal position. 



We know that a liypertro[)hy of the ventral tecto-bulbar tracts 

 is not only correlated with a very ventral abducens nucleus, but 

 equally causes a more frontal position, at least of its frontal division 

 as is specially shown in Pleuronèctidae '). 



Concerning the position of the facial nucleus in these Ganoïds 

 and its resemblance in this respect with Selachii the following ex- 

 planation must be given : 



In his excellent paper on Acipenser (I.e. p. 31) Johnston remarks 

 that from the sensoi'y IX — X lobi (in which also the sensory VII 

 root finishes) a secondaij ascending fibres-tract runs along the de- 

 scending V. According to his description, this tract (Mayser's: 

 "vagotrigeminale Bahn") is still unmyelinated in Acipenser, which 



1) Compare Kappers, The migrations of the V, VI, and VII nuclei and the 

 concomilating changes iu their root fibres. Verb der Kon. Akad. v. Wetensch. 

 1910, Deel 16, 2de Sectie, and Folia Neurobiol. Erganz. Heft Vol. VI, 1912. 



