Quarterly journal of microscopical science . 20 mm. embryo. Text-fig. 10. ,Sio. 60mm. stage. View of the occipital region. Three ventral rootsare included in the skull, V 1-r •. and the next. V 10. joins a dorsalroot to form the first mixed nerve. HEAD OF SQUALUS 467 arch so that it is not included in the cranium. Its fibres jointhose of the spinal ganglion to form the first mixed nerve,branches of which I have traced to the tenth somite. Sincethe eighth somite is the last of the vagus segments the ninthis morphologically the first spinal or post-vagal (in the case ofSqualus included in the sk
Quarterly journal of microscopical science . 20 mm. embryo. Text-fig. 10. ,Sio. 60mm. stage. View of the occipital region. Three ventral rootsare included in the skull, V 1-r •. and the next. V 10. joins a dorsalroot to form the first mixed nerve. HEAD OF SQUALUS 467 arch so that it is not included in the cranium. Its fibres jointhose of the spinal ganglion to form the first mixed nerve,branches of which I have traced to the tenth somite. Sincethe eighth somite is the last of the vagus segments the ninthis morphologically the first spinal or post-vagal (in the case ofSqualus included in the skull), and the tenth, which in Squalusforms the first mixed root, is really the second spinal or dorsal ganglia are present belonging to the seventh,eighth, and ninth somites (Text-fig. 9). In Squalus, therefore,there are nine segments included in the skull. Hoffmann (9),Sewertzoff (17), and others state that there are ten, but since theyadopt Van Wijhes somites, and like him intercalate a somite be-tween those related to the fifth and seventh nerves,
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