. Text-book of embryology. Embryology. II NERVE DEVELOPMENT 105 The His view is concerned primarily with the actual functional nerve-fibres. As regards the primitive sheath (Gray Sheath of Schwann), in which these fibres are enclosed, the His view regards it as being formed by mesen- chyme cells which apply themselves to, and spread out over, the surface of the originally naked nerve-fibre. (2) The Balfour ViEW(Cell -chain theory). —While Schwann(1839) long ago described the multicellular structure of nerve-trunks in the foe- tuses of mammals, it was F. M. Balfour (1876) who really founded the
. Text-book of embryology. Embryology. II NERVE DEVELOPMENT 105 The His view is concerned primarily with the actual functional nerve-fibres. As regards the primitive sheath (Gray Sheath of Schwann), in which these fibres are enclosed, the His view regards it as being formed by mesen- chyme cells which apply themselves to, and spread out over, the surface of the originally naked nerve-fibre. (2) The Balfour ViEW(Cell -chain theory). —While Schwann(1839) long ago described the multicellular structure of nerve-trunks in the foe- tuses of mammals, it was F. M. Balfour (1876) who really founded the view that the nerve - trunk arises in development from a chain of cells. Balfour found in Elasmo- branch embryos that the nerve-trunk was repre- sented by a chain of cells in early stages (Fig. 58, ), and similar observations have been made by subsequent observers. According to this view the whole nerve- trunk is multicellular in origin, the cells not only forming the sheath of the nerve-trunk but also giving rise to the nerve-fibrils which come into existence traversing the cellular strand from end to end. On the question of the origin of the cells which constitute the nerve-rudiment opinions vary. Most supporters of this view have regarded them as having emigrated from the spinal cord { Balfour, van Wijhe, Dohrn): while others (Kolliker) have looked on them as mesenchymatous in nature. Sedgwick took this latter view and as he regarded the mesenchyme as a continuous syncytium, the bridges connecting the cells being primitive—persisting from the. Fig. 58.—Section through the dorsal part of the trunk of a Torpedo embryo. (From Balfour's Embryology.) , dorsal root; g, spinal ganglion; my, myotome; N, noto- chord ; n, nerve-trunk; no, cavity of spinal cord ; , ventral root. the surface of the cell and its processes. We know from the recognized unreliability of the method that the occurrence, or not, of this precipitation is liable to be decided by extremely del
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