92 FERTILIZATION 



physical state of the cytoplasm, and vice versa. Cole, for example, 

 attempted in 1932 to measure the internal pressure of a sea-urchin 

 egg, and its surface tension, by observing the changes in the shape 

 of the egg when compressed by a small gold beam. Reliable 

 measurements by this method require rather large deformations 

 of the egg. One then measures the resistance to deformation of 

 internal structures, such as the amphiaster, as well as the resist- 

 ance to deformation of the cortex. Mitchison & Swann (1954a, b; 

 1955) invented an instrument, which they call the Cell Elastimeter, 

 though it is commonly known as the 'Sucker', to overcome such 

 difficulties. The instrument consists of a micropipette, filled with 

 water, one end of which is connected to a device for producing 



Pipette 



ci\ 



FIG. 16. — Diagram showing measurements involved in estimating cortical stiff- 

 ness with the Elastimeter. After Mitchison & Swann (1954a). 



negative pressures; the other end, which has a terminal diameter 

 of about 50 /x, is placed against the surface of an egg. When a 

 negative hydrostatic pressure is applied, part of the egg surface 

 bulges into the pipette, Fig. 16. From a knowledge of the diameter 

 of the pipette, the negative pressure applied, and the degree to 

 which the egg bulges into the pipette, an estimate can be made of 

 the tension at the surface or, more accurately, the stiffness, of the 

 cortex. The measurements involved in estimating the degree of 

 bulging are shown in Fig. 1 6. The units for stiffness are dynes. cm~'' 

 per /x deformation. By comparing the observed behaviour of sea- 

 urchin eggs and large-scale models such as balloons, Mitchison 

 & Swann came to two conclusions, apart from quantitative data 

 referred to later ; first, as was to be expected, that the cortex was 

 elastic; and secondly, that its thickness was appreciable. Mitchi- 

 son (1956) has adduced other, optical, evidence in favour of 

 this view. The permeability barrier may still, of course, be fairly 

 thin, the capacitance measurements mentioned in chapter 10 



