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, i.e., 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 animal is composed of protoplasm, and its skin of the 



