420 L. V. HEILBRTJNN 



it is important that these factors remain constant. During 

 mitosis in the Cumingia egg the granules do not change appre- 

 ciably in size. This constancy is shown in the drawings of 

 Morgan ('lO).^ Perhaps the specific gravity of the granules 

 changes. In the egg there are two sets of granules, one set lighter, 

 the other heavier than the cytoplasm. When the eggs are 

 centrifuged, the lighter granules go to one pole, the heavier 

 granules to the opposite pole. It has been found that whenever 

 the lighter granules move more rapidly, the heavier granules 

 also move more rapidly, and a drop in the speed of the lighter 

 granules is always accompanied by a drop in the speed of the 

 heavier granules. If the change in speed were in any case due 

 to' a change in specific gravity of the granules, w^e would have to 

 assume that an increase in the specific gravity of the lighter 

 granules was in every case accompanied by a corresponding 

 decrease in the specific gravity of the heavier granules, and vice 

 versa. At first sight this appears impossible, but it is not. An 

 entrance of water into the granules would increase the specific 

 gravity of the lighter granules and decrease the specific gravity 

 of, the heavier granules, and an exit of water would have the 

 opposite effect. But such a passage of water would of necessity 

 produce very decided changes in the diameter of the granules 

 and these changes, as we have already noted, do not occur. 



Since, therefore, the diameter of the granules and their specific 

 gravity can be taken as constant, the velocity of the granules 

 affords a measure of the viscosity of the cytoplasm. In measur- 

 ing changes in protoplasmic viscosity, one can either deter- 

 mine the variation in the speed of the granules with a certain 

 amount of centrifugal force or one can determine the amount 

 of force necessary to produce a certain velocity of the granules. 

 In actual practice it is easier to use the first method, especially 

 if a hand centrifuge is employed. With such a centrifuge one 

 acquires the knack of turning very regularly at a given speed 

 but it is somewhat more difficult to change from one speed to 

 another. 



2 It was also shown by careful measurement of granules in living eggs. 



