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. Rabl (’89, p. 221) thinks he has some right to bring 
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, “ Ferner darf ich 
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Fieurr F. 
aus einer Reihe von Beobachtungen, die ich nicht bloss an Selachiern, 
sondern auch an Vögeln und Sat 
gethieren angestellt habe, schliessen, 
dass die Oculomotoriuswurzel, die nach dem gesagten Anfangs ebenso 
wie die des Trochlearis aus der dorsalen Kante des Mittelhirns austreten 
musste, aus dieser Lage allmählich durch die Ausbildung der Pedun- 
culusbahnen verdrängt und an die ventrale Seite verschoben wird.” 
Fie. F. Left face of a parasagittal section through the left half of an embryo 
with 52 somites, showing the relations of the oculomotorius to encephalomere II 
at this stage. X 50. J, II, III, 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. 
