Proteins and Protoplasmic Structure 25 



tion in areas of morphologically undifferentiated cytoplasm were at 

 first without success. To some extent, this was due to the thinness 

 of the object and the consequent small magnitude of the phase 

 difference. Introduction of more sensitive compensators and the 

 use of higher light intensities has largely removed this difficulty and 

 has made it possible to detect double refraction even in extremely 

 thin strands such as spindle fibers (41). With these refinements in 

 technique, numerous investigators have succeeded in showing the 

 presence of doubly refractive elements in cyptoplasm itself, as well 

 as in the spindle fibers, cilia, and plasma membrane (41, 42) . Inas- 

 much as orientation of anisodiametric particles enhances double 

 refraction, it might be expected that streaming cytoplasm would 

 exhibit more brilliant birefringence than in the quiescent state, just 

 as solutions of myosin or tobacco mosaic virus become bright under 

 crossed nicols when streaming through a tube (9) . A consideration 

 of the magnitude of the shearing stress needed to produce noticeable 

 streaming double refraction in sols under experimental conditions 

 suggests that the low velocities encountered in cyclosis would hardly 

 be enough to cause sufficient orientation of submicroscopic proto- 

 plasmic particles. This seems to be the case, for such effects do not 

 appear to have been reported. 



On the other hand, it is well known that many substances com- 

 posed of rod-shaped particles, e. g., a gelatin gel, can be statistically 

 isotropic and yield the same mean value of the refractive index in 

 all directions, due to random arrangement of the micells. Any force 

 causing orientation of the micells produces birefringence under such 

 conditions. Engelmann (10) pointed out long ago that apparent iso- 

 tropy of cytoplasm is not proof for the absence of doubly refractive 

 micells. Although cytoplasm may appear isotropic, pseudopodia 

 extended from it are often brightly birefringent (41). The birefrin- 

 gence persists for a short distance into the mass of the cytoplasm and 

 then disappears. If the pseudopodium is retracted, the anisotropy 

 fades away. When amoebae are rounding up before encysting, the 

 double refraction is much more definite (41). The spindle arising 

 during mitosis has been reported to be anisotropic in the living con- 

 dition by several observers (41). In these cases it is probable that 

 orientation of micells has been produced by activity. 



As predicted by statistical mechanics, a system tends to assume 

 the most thermodynamically probable state when left to itself. In 

 substances such as rubber, which possess long-chain, flexible mole- 

 cules, the completely extended state is only one among the many 



