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GENERAL PRINCIPLES OF ZOOLOGY 



Contractile fibre-cells, on the other hand, are connective-tissue cells, 

 which usually have surrounded themselves with a layer of contractile 

 substance; corresponding to their origin, they have the form of connective- 

 tissue cells, and are spindle-formed or branched; the branches arising from 

 the ends of the cells (fig. 50). The similarity of form renders the distinc- 

 tion between ordinary connective-tissue cells and fibre-cells difficult; 

 if the contractile layer on the surface be slightly developed, the distinction 



FIG. 50. FIG. 51. 



FIG. 50. Contractile fibre-cells, a, of man; b-e, of Beroe (a Ctenophore); b, young 

 fibres; c, branched ends; d, middle portion of a fibre; e, cross-section. 



FIG. 51. Cross-striated primary bundle (after Gegenbaur). a, nuclei; s, a point 

 where the sarcolemma is plainly shown on account of the tearing of the fibres. 



is impossible. To recognize the character of the elements, therefore, 

 we must choose well-defined examples, in which the nucleated mass, the 

 'axial substance,' is sharply marked off from the muscle-mass, the 'cortical 

 layer' (fig. 50, c, d, e). The regular arrangement of the epithelial cells 

 side by side, gives the muscle fibres arising from them a parallel arrange- 

 ment, so that a layer is formed, which becomes folded when much muscle 



