neal: nervous system in squalus acanthias. 



221 



early stages of its development, and their conclusions are therefore 

 purely theoretical. Both agree in considering the nerve a derivative of 

 neural-crest cells. Eabl ('89, p. 221) thinks he has some right to bi'ing 

 this portion of the neural crest into genetic connection with these nerves, 

 since the course of the third and fourth nerves in later stages corre- 

 sponds with a portion of the trigeminus Anlage, which I infer from his 

 description to be the " trochlear " portion. He adds, " Feruer darf ich 



Figure F. 



aus einer Reihe von Beobachtungen, die ich nicht bloss an Selachiem, 

 sondern auch an Vogelii und Siiugethieren angestellt habe, schliessen, 

 dass die Oculomotoriuswurzel, die nach dem gesagten Anfangs ebenso 

 wie die des Trochlearis aus der dorsalen Kante des Mittelhirns austreten 

 rausste, aus dieser Lage allmahlich durch die Ausbildung der Pedun- 

 culusbahnen verdiangt und an die veutrale Seite verschoben wird." 



Fig. F. Left face of a parasagittal section through the left half of an embryo 

 with 52 somites, showing the relations of the oculomotorius to enceplialomere II 

 at this stage. X 50. /, //, ///, first, second, and third encephalomeres ; 1, 2, 

 van Wijhe's first and second head cavities ; a, ventral fibre tract ; oc-mot., oculo- 

 motorius ; vn. crd., anterior cardinal vein. 



