IV. VISCOSITY M K A S U R E AI E N T S 115 



that the stage is vertical (and the tube horizontal). In the original 

 use of the gravity method, the German botanist Heilbronn (17) ob- 

 served the rate of fall of starch granules through the protoplasm of 

 root cells in a bean plant. He then compared the rate of fall of the 

 same particles in distilled water. He regarded the ratio of the speed 

 of fall in the two media as a relative measure of the viscosity of the 

 two media. Actually Stokes' law should have been employed. The 

 fall of a starch grain through protoplasm is slower than it is through 

 water, not onty because of the greater viscosity of the protoplasm, 

 but also because of its greater specific gravity. Moreover, because 

 starch grains are relatively large in comparison with the diameter of 

 the tube through which they drop, a correction must be made for the 

 effect of the container wall. In addition, another correction is neces- 

 sary because of the fact that many starch grains are falling at the same 

 time. 



There are two other instances in which the gravity method has 

 been used. In the immature eggs of many animals, there is a large 

 nucleus, the germinal vesicle, and this large nucleus contains a 

 spherical nucleolus. In the egg of the lobster, it was found many 

 years ago that the nucleolus falls under the influence of gravity. 

 Similarly, the nucleolus can be seen to fall in other types of marine 

 eggs. Gray {18) measured the speed of fall in the immature egg of 

 the sea urchin. From this data, on the basis of reasonable assump- 

 tions as to the specific gravity of the nucleolus and the nuclear fluid, 

 Heilbrunn (1) calculated the viscosity of the nuclear fluid. Harris 

 (19) has made accurate photographic measurements of the rate of 

 fall of the nucleolus, and has recalculated the viscosity on the basis of 

 these measurements. Because of the fact that the nucleolus is a 

 single sphere, relatively small in size in comparison with its contain- 

 ing nucleus, Stokes' law can very properly be used for a determination 

 of the viscosity of the nuclear fluid. It is necessary, however, to 

 make assumptions as to the specific gravity both of the nucleolus and 

 of the nuclear fluid. For relative measurements of the viscosity of 

 the nuclear fluid under different conditions, no such assumptions have 

 to be made. All that is necessary to assume is that the specific gravi- 

 ties do not change markedly during the course of the experiment. 

 Also, within the past few months, Rieser {19a) has been able to use 

 the gravity method for isolated muscle fibers. Small oil drops were 

 injected into these fibers. In most cases the injection itself causes 

 injury and the oil drops do not move through the protoplasm under 



