GENERAL HISTOLOGY. 



93 



layer on the surface bo slightly developed, the distinction is im- 

 possible. To recognize the character of the elements, therefore, 

 we must choose well-defined examples, in which the Tininucleated 

 or the muUinucleated mass, the 'axial substance,' is sharply 

 marked ofi from the muscle-mass, the 'cortical layer' (fig. 49', 

 c, d, e). 



I'lu. la. 



Fiu. TjJ. 



Fig. •49.— Contractile flbre-ceUs. «, of man; b-c, of Beroe (a Ctenophorc) ; )), young 

 fibres; c, branched ends ; ti, middle portion of a fibre; f, cross-section. 



Fig. .To.— Cross-striated primary bundle. (After ftegenbaur.) n, nuclei ; s, a point 

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



In vertebrates and arthropods the contractile fibre-cells occur 

 in the vegetative organs as elements of the ' organic musculature ' ; 

 on the other hand we find here the epithelial musculature in tlie 

 cross-striated primary bundles, separated from the epitlielium, 

 and only developmentally referable to the epithelium of the body 

 cavity (fig. -50). A primary bundle is a cylindrical mass, bounded 

 externally by a structtireless envelope, tlie sarcolemma. Its con- 

 tents consist of fine fibrils, wliich, closely jiarallel to one another 

 and pressed closely together, run from one end of the mass to the 



