VII MUSCLE AND CONNECTIVE-TISSUE 327 



clear streak can be seen in the centre, which in many cases 

 appears as a canal, so that then the whole fibre forms a tube." 

 Between the typical connective-tissue- and muscle-fibres there 

 occur " forms perfectly intermediate, and there is no means 

 of definino' a distinction between the fibres which contain the 

 smallest proportion of contractile substance and those which 

 contain none at all, for even staining with carmine supplies 

 no criterion when only a small quantity of this substance is 

 present. The first unmistakable trace of contractile substance 

 appears in the fibres as a doubly refracting layer on the inner 

 wall of the central cavity, surrounding the axis of the fibre as 

 a delicate envelope. 



"The connective -tissue -fibres traverse the gelatinous 

 substance of the animal from within outwards, as well as 

 from above downwards ; very often, however, in a direction 

 at rio'ht ans^les to that of the muscle-fibres. 



"... In the lower Medusae, and also in the Acraspedota, 

 the gelatinous supporting tissue of the body is not yet 

 traversed by muscle-fibres, but only by connective-tissue- 

 fibres. It seems here not to be itself contractile, or only by 

 the aid of the musculature lying upon it. In the Ctenophora 

 the gelatinous tissue is abundantly traversed by muscle-fibres 

 passing from wall to wall, and it can therefore be contracted 

 in itself. 



" Here, from the cell layer which produced both connective- 

 tissue- and muscle-fibres, the latter have developed in those 

 directions in which the body made the greatest efforts to 

 contract. In those directions in which such effort was made 

 not at all, or only slightly, the evolution went no farther than 

 the formation of connective -tissue -fibres, or at most there 

 arose muscle-fibres which contain but a small proportion of 

 contractile substance. 



" In a direction at right angles to that in which, at a given 

 part of the body, contraction exclusively occurs, the con- 



