A treatise on zoology . alorigin, unsegmented and v;nderlying the central nervous system ; theprolongation of the body beyond the anus in the form of a metameri-cally segmented tail, containing tissues derived from the threegerm-layers; the well-developed coelom, and the separate blood-vascular system. 1 I CEPHALISA TION Among the chief features which denote the great advance instructure of the Craniate vertebrate should first be mentioned thepronounced cephalisation of the anterior segments. Now a typicaltrunk segment of Amphioxus, besides a portion of the nerve-cord,of the notochord, and of
A treatise on zoology . alorigin, unsegmented and v;nderlying the central nervous system ; theprolongation of the body beyond the anus in the form of a metameri-cally segmented tail, containing tissues derived from the threegerm-layers; the well-developed coelom, and the separate blood-vascular system. 1 I CEPHALISA TION Among the chief features which denote the great advance instructure of the Craniate vertebrate should first be mentioned thepronounced cephalisation of the anterior segments. Now a typicaltrunk segment of Amphioxus, besides a portion of the nerve-cord,of the notochord, and of the gut, contains a paired coelomic cavity,and a right and left myotome or muscle segment. Motor fibres aresupplied to these muscles direct from the nerve-cord by the ventral nerve-root (Fig. 1). The separate dorsal nerve-root passesout from the nerve-cord between the myotomes to the surface,supplying sensory fibres to the skin, and probably both sensoryand motor fibres to the viscera (Hatschek [202], Heymans and van Fio. 1. Diagrams to show the relations of the dorsal and ventral nerve-roots in (A) the trunk ofAmphioxus (modified from Hatschek), in (B) the gill-region of a Gnathostonie (cranial nerve),and in (C) the tmnk-region of a Gnathostome. ol, alimentary canal; br, brain ; , dorsalroot; e, eye-muscle ; g, gill-slit; I, mixed nerve to paired limb ; ra, myotome ; , nerve-cord ;p, ventral branch to skin and visceral muscles; , ramus dorsalis ; , ramus ventralis ;s, sympathetic ; , ventral root. Fibres from the dorsal root are black. der Stricht [213], Johnston [248]). But whereas in Amj)hioxusthe mesoblast is clearly segmented to the very tip of the head, andthe myotomes, developed from the second segment backwards,persist with their corresponding nerves throughout the anteriorregion, in the Craniate the segments at the anterior end of thebody are so highly modified, and their limits so obscured, thatthey can only be made out with difficulty in the embryo
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
