I CYTOPLASM 165 



Spherical shape or can be spread at will on the surface of another 

 liquid. In those cases where the cytoplasm forms liquid drops, its 

 surface tension y can be measured (E. N. Harvey, 1936) by observing 

 the cell as a sessile drop flattened by gravity. The relation 



y == g (d - d')r-^F 



is used to calculate y; g is the acceleration due to gravity, (d — d') 

 the difference in density between drop and medium, r the radius of 

 greatest flattening and F a function of the distance a in Fig. 105 a 

 representing the flattening of the drop. For the egg of the mollusc 

 Busjcon canaliculatum a tension of 0.5 dyne/cm is found by this method, 

 while the egg of the salamander TriturHS virescens gives only o.i 

 dyne/cm (Table XXI). 



The eggs of mackerel contain a large oil droplet which can be 

 flattened against the rigid cell membrane when revolving the egg at 

 high speed in the centrifuge microscope of E. N. Harvey. From its 

 shape, an oil/cytoplasm interfacial tension of 0.6 dyne/cm is calculated: 

 if the centrifugal force is increased up to 450 times gravity, this 

 tension does not change, showing that the surface is not elastic. In 

 contact with sea water this oil gives a tension of 7 dyne/cm, a high 

 value which is explained by the rule that the interfacial tension be- 

 tween two immiscible liquids is the difference of the tensions of the 

 two liquids against air. As the surface tension y of water is 72 dyne/cm 

 and that of oils is only about half as much, it is evident that the cell 

 surface cannot be formed of pure lipids, because this would provoke 

 a higher interfacial tension between the surface of a cell and its culture 

 medium. A surface with only o.i dyne/cm tension against the medium 

 cannot be very lipidic; besides the lipids it must contain proteins 

 with a certain affinity for water. 



If the cell does not flatten under its own weight, the flattening can 

 be achieved by compression (E. N. Harvey, 1937): the spherical cell 

 is loaded by a thin beam of gold with micro weights. The weight W 

 divided by the area D of the flattened cell in contact with the beam 

 gives the pressure P, from which the surface tension is calculated by 

 the formula 



