112 



GENERAL PRINCIPLES OF ZOOLOGY. 



occur in the vegetal organs as elements of the "organic 

 musculature"; on the other hand we find here the epithe- 

 lial musculature in the cross-striated primitive bundles, 

 separated from the epithelium, and only developmentally 



FIG. 47. 



FlG. 47. Contractile fibre-cells. , of man ; l>-e, of Beroe (a Ctenophore) ; 6, young fibres ; 



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

 FIG. 48. Cross-striated primary bundle. , nuclei ; s, a point where the sarcolemma is 



plainly shown on account of the tearing of the fibres. (After Gegenbaur.) 



still referable to the epithelium of the body-cavity (Fig. 

 48). A primary bundle is a cylindrical mass, bounded 

 externally by a structureless skin, the sarcolemma. Its 

 contents consist of fine fibrils, which, closely parallel to 

 one another and strongly compressed, run from one end of 

 the mass to the other. Each fibril is formed of singly and 

 doubly refractive parts, which alternate with one another 

 in more or less complicated arrangement. Since now the 

 doubly-refracting parts of the fibrils within a bundle lie at 



