Quarterly journal of microscopical science . mites. the oculomotor being the premandibular somites nerve, andtlic patheticus, in spite of its curious course, doubtless belongsto the second and mandibular segment. The fourth and fifthsomites since they disintegrate have no ventral roots as such(though the fifth is present in Scyllium). To the sixth BOmitea ventral root can be seen up till about the 10 mm. stage. The abducens has usually been regarded as the nerve i thethird somite and therefore as the ventral root corresponding !the facial. It certainly innervates the external rectus muscle,but
Quarterly journal of microscopical science . mites. the oculomotor being the premandibular somites nerve, andtlic patheticus, in spite of its curious course, doubtless belongsto the second and mandibular segment. The fourth and fifthsomites since they disintegrate have no ventral roots as such(though the fifth is present in Scyllium). To the sixth BOmitea ventral root can be seen up till about the 10 mm. stage. The abducens has usually been regarded as the nerve i thethird somite and therefore as the ventral root corresponding !the facial. It certainly innervates the external rectus muscle,but Neal (18) states that in Squalus this muscle is of compositeorigin, consisting of elements derived from the mandibular Bfi will as the hyoid somite. 470 G. RYLANDS DE BEER The abducens is held to arise from the neural tube by manyroots ; according to Neal four, corresponding to Van Wijhessomites 3, 4, 5, and 6. I have been able to discern three rootswhich in a 23 mm. embryo arise not very far behind the facial Text-fig. Ventral view of an embryo 23 mm. long to show the origin of theabducens nerve. (Text-fig. 12). In earlier stages their origin appears to be slightlymore posterior. It has been suggested that the hypoglossusand abducens roots form a continuous series, implying that theabducens is a compound nerve derived from elements belonging HEAD OF SQUALUS 471 to three or more segments, but even going by topographicalrelations alone it is not unreasonable to regard the abducensas being the genuine third ventral root. At any rate I do notsee that the condition of the abducens furthers the assumptionthat a gill-slit has been lost. If Neals contention is true thewhole question of the eye muscles innervation and segmenta-tion will require revision. Lastly, one more train of thought has been brought tobear on the supposedly lost organs, and that is the ques-tion of the relation of neuromeres to the other segmentalstructures. Xeal (11) describes seven neuromeres, of which the
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