40 Pi KIUDi 



The result of a study of the development of cilia in dif- 

 ferent forms of Stentor and Epistilis showed that as soon as 

 the cilia became visible they reacted to polarised light as 

 strongly as they did later, and from the first they were con- 

 tractile. Contractility and double refraction may be said to 

 go hand in hand in point of development. 



In the case of the spermatozoa of winter frogs Valentin 

 noticed that double refraction could be detected in these 

 bodies when examined in half per cent, salt solution, and it 

 may be noted here that the non-contractile head portion of the 

 spermatozoa possessed this peculiarity as well as the con- 

 tractile tail, or indeed in a higher degree than the tail, per- 

 haps chiefly on account of its greater thickness. But there 

 was a remarkable difference between them — the head exhi- 

 bited negative double refraction with reference to the long 

 axis of the spermatozoon, the tail, on the contrary, positive 

 double refraction in agreement with all other contractile 

 elements that have been investigated up to the present time. 



Positive double refraction was also observed in sperma- 

 tozoa from the body-cavity of Chcetogaster vermicular is. 



4. Contractile Protoplasm of Actinosphcsrium EichJiornii. 

 — According to Engelmaun the fact that the protoplasm of 

 Amoebse, colourless blood-corpuscles and many vegetable cells, 

 shows no evidence of double refraction is to be explained by 

 the existence of this protoplasm in very thin layers and by 

 its poorness in stable molecules, as shown by the great dimi- 

 nution in volume which it undergoes under the influence of 

 reagents that abstract water. 



It is also due to the fact that the contractile molecules of 

 this protoplasm are not arranged in definite parallel axes, as 

 in muscle-fibres, but are heaped together apparently in an 

 irregular manner and are susceptible of a change in their 

 position and direction. Even a weak depolarising influence 

 might be expected in this case. 



But this is not the case when the protoplasm is in layers of 

 requisite thickness, and appears as if morphologically polarised. 



Here the smallest optically active particles are arranged 

 with their axes in definite parallel directions, as in a crystal, 

 and their optical effects must accordingly be greater. This 

 hypothesis is not opposed to facts ; a good example of this is 

 seen in Actinospharium Eichhor?iii (Actino^hrys) . When 

 examined in a drop of water with a magnifying power of 100 

 with crossed NichoVs prisms, all the rays appeared bright 

 whose long axes made an oblique angle with both planes of 

 polarisation ; but all the rays whose long axes lay in one of 

 the planes of polarisation appeared completely dark. 



