120 L. V. H E I L B R U N N 



dence that the changes are real, for it would be hard to assume that, 

 whenever the heavy granules became heavier, the light granules be- 

 came lighter to the same degree. 



Although most of the information we now have about the viscosity 

 of protoplasm has been obtained by the centrifuge method, the un- 

 fortunate fact is that relatively few types of cells can be studied by 

 this method. The egg cells of various invertebrates, echinoderms, 

 worms, mollusks, etc., frequently provide excellent material; but not 

 always. Thus, for example, although the protoplasm of the egg of the 

 sea inx'hin Arhacia stratifies beautifully and various other types of 

 sea urchin eggs behave in similar fashion, the eggs of the European 

 sea urchin Paracentrotus are apparently so full of granules that these 

 granules cannot be moved into one part of the cell. When this egg 

 is centrifuged for long times, only a small region of the cell becomes 

 free of granules. It should be remembered that spherical particles 

 of uniform size cannot fill more than three-quarters of a given space; 

 actually 74.05% is the maximum. Presumably some cells have con- 

 centrations of granules that approach this percentage. This seems 

 to be true for the Paramechmi cell. Paramecia may be centrifuged 

 more vigorously for long times, but the granules in the protoplasm 

 do not move. However, viscosity measurements can be made for 

 Paramecium protoplasm by studying the centrifugal movement of 

 food vacuoles. Various types of materials can be fed to the jpara- 

 meciiim, for example, Chinese ink particles, carmine particles, or 

 starch grains. 



Amoeba is a very useful organism for protoplasmic viscosity stud- 

 ies. It is the only type of cell in which studies can be made of the 

 viscosity of both the outer cortex of the cell and the interior proto- 

 plasm. For such studies, two species of amoebae are used. In 

 Amoeba dubia, there is a very thin cortical layer, and when this 

 amoeba is centrifuged, one can study the viscosity of the main mass 

 of the protoplasm. On the other hand, the related species, A. pro- 

 teus, has a relatively thick cortex. When this amoeba is centrifuged, 

 it is easy to study the viscosity of the stiff cortex, but because this 

 cortex is thick, it is not possible to study the interior. By making 

 studies of both types of amoebae, it is possible to o])tain information 

 as to the viscosity of both cortex and interior. In the unfertilized 

 sea urchin egg, the cortex is very thin ; indeed it is only a single layer 

 of granules thick. The viscosity of this cortex is so high that it has 

 not been possible to make viscosity determinations for it. Sometimes 



