GENERAL HISTOLOGY. 



93 



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

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

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

 or the multinucleated mass, the < axial substance/ is sharply 

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

 c, d, e). 



FIG. 49. 



FIG. 50. 



FIG. 49. Contractile fibre-cells, a, of man; 1)-e, of Beroe (a Ctenophore); ft, young 

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



FIG. 50. Cross-striated primary bundle. (After Gegenbaur.) w, nuclei ; s, a point 

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



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 the 

 cross-striated primary bundles, separated from the epithelium, 

 and only developmentally referable to the epithelium of the body 

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

 externally by a structureless envelope, the sarcolemma. Its con- 

 tents consist of fine fibrils, which, closely parallel to one another 

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



