jengelMann on contraIdtility and double refraction. S7 



curious rather than an important phenomenon. This point 

 of view was modified when it was shown hy Engehnann 

 himself (Pfliiger's ' Archiv,' Bd. 7) that contraction is ex- 

 clusively connected with the double-refracting layers in 

 striped muscle. The supposition then arose that, as a general 

 law, contraction might be dependent on the presence of 

 double-refracting particles, especially as the boundaries be- 

 tween different forms of movement in protoplasm and in 

 muscle formerly assumed to be so sharply defined had broken 

 down. 



Moreover, it had been demonstrated that in all these cases 

 the molecular mechanism of movement was the same in 

 principle. But a direct empirical proof was still wanting, 

 and the present researches were undertaken with the view 

 of supplying this. 



Two methods of investigation were adopted : 



1st. An examination was made of the most various forms 

 of contractile matter with reference to their behaviour towards 

 polarised light. 



2nd. Observations of individual forms were instituted 

 during the course of their development with a view to dis- 

 cover if contractility and double refraction are always 

 mutually associated. 



1. Muscle Substance of Hydra and Hydractinia. — On ex- 

 amining a living Hydra under a microscope fitted up with 

 crossed Nichol's prisms, the uniform stripe of muscle-fibrils 

 between the ectoderm and endoderm appears clear provided 

 that the long axis of the portion of the animal's body under 

 observation lies in the plane of the field of the microscope, but 

 appears dark when it lies in the directionof one of the planes 

 of polarisation. 



In vertical sections cut through the long axis of the body 

 the layer separating the ectoderm and endoderm does not 

 react to polarised light. These appearances are explained by 

 considering these longitudinal muscle-fibrils to be double- 

 refracting uniaxial elements, whose optic axis is parallel to 

 the long axis of ihe fibrils. If we consider that these ele- 

 ments have a positive double refraction with reference to 

 their long axis, the appearances which are noticed in an 

 observation with a coloured field are also explicable. It is 

 possible that the property of double refraction does not 

 belong exclusively to the muscle -fibrils, but also to the 

 homogeneous supporting lamella which also lies between 

 ectoderm and endoderm. But in our case the extent of 

 double refraction is too great to be ascribed exclusively to 

 the supporting lamella. For this tissue refracts light very 



