Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 47 of the latter. They possess in addition an outer sheath, known as the sheath of Schwann (cerebro-spinal nerves of most vertebrates). In the second form, , in the non-medullated nerve fibres, the me- dullary sheath is absent, the axis cylinder being either naked or sur- rounded by a connective tissue sheath. The axis cylinder here also is connected with a ganglion cell (sympathetic nerves, nerves
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 47 of the latter. They possess in addition an outer sheath, known as the sheath of Schwann (cerebro-spinal nerves of most vertebrates). In the second form, , in the non-medullated nerve fibres, the me- dullary sheath is absent, the axis cylinder being either naked or sur- rounded by a connective tissue sheath. The axis cylinder here also is connected with a ganglion cell (sympathetic nerves, nerves of Cyclostomata and Invertebrates). Very often, however, and this is especially the case with sense nerves, we find that the axis cylinder may break up into very fine nerve fibrillre, and be, so to speak, resolved into its elements. Finally, the nerves of In- vertebrates very often appear as finely striated bundles of fibrilla?, in which, on account of the absence of a sheath, it is not possible to recognise the limits of the individual axis cylinders. Peripherally the sensory nerves become connected with accessory structures (end-or- gans), derived usually from epithelial cells and their cuti- cular products, or rarely from connective tissue substance (tactile organs). The end- organs are therefore for the most part derived from modi- fied epithelial cells (sensory epithelium). Ganglion cells are frequently found inserted in the course of the nerve fibres close to their termination (fig. 39, a, b, c.) Fio. 39.—Rod-shaped sense cells from the olfac- tory organ (after Max Schultze). a, from the frog ; Sz, supporting cell between two ciliated i-od-cells. b, from man. c, from pike. Pro- bable connection between the nerve fibrillaei and the sense cells. INCREASE IN SI2E AND PROGRESSIVE DIFFERENTIATION, DIVISION OF LABOUR AND PERFECTION. The lowest organisms possess neither tissues nor organs formed from cells. The whole organism consists of a single cell. The body of such an
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